Current Experience
Signalling Engineering Manager
- Public Transport Authority
- Sep 2021 – Present
- 3 yrs 2 mos
Highest Level Of Education
Mangalore University
- Bachelor Of Engineering ,Electronics & Communication
- Nov 1995 - Sep 1999
About Me
Signalling Expert for Conventional Fixed Block ,CBTC and ETCS Signalling & Train Control Systems.
Proven Signalling Strategy Consultant.
Unique experience on different aspects of rail signalling ,such as specification ,detailed design ,requirements, detailed design ,installation and T&C.
Through knowledge on international Railway Safety standards such as EN50126,EN50128,EN50129 ,EN62290 IEEE 1474-1-3 and other signalling construction material and EMC standards.
Hands on experience on System engineering, installation and T&C.
Engineering and Project Management Skills on complex railway signalling projects over 100 million USD.
Proven people management and development skills.
Significant railway Signalling Expertise gained from Australia , India, The Netherlands and Singapore, on feasibility stage to detailed design.
Deep rooted knowledge on Signalling Material , Products and International standards.
EMC Knowledge and participated in test at site and lab
Participated in CBI product type approval and EOI docs.
Skills on Interface & Issues Management.
Experience
Public Transport Authority
3 yrs 2 mos
Alstom
5 yrs 9 mos
Regional Centre Installation & Engineering Manager-East Asia
Full-time
Nov 2015 - Aug 2021
Roles & Responsibilities ... Read more
Responsible for Installation &Engineering Management for East Asia Region based at Singapore. Deepu managed a highly diverse Engineering team for projects in Singapore (Thomson East Coast Line ,Circle Line 6 ,North East Line extension ,ITTC ,Simulation Centre) ,Philippines (Manila Light Rail) ,Hanoi Metro etc Senior Expert of Alstom World Class Engineering (WCE) -France.
GE Transportation
3 yrs 3 mos
Engineering Technical Leader
Full-time
Jul 2012 - Oct 2015
Roles & Responsibilities ... Read more
Responsible for the Wayside Application Engineering delivery of the project. Management of Internal and External resources/contractors allocated to the application engineering. Technical Management of Suppliers to ensure all technical requirements are compliant. Responsible for wayside equipment specification and type/compatibility approval. Budget and Schedule owner for Wayside Engineering.
Worley Parsons
1 yr 3 mos
Lead/Interface Engineer
Full-time
Mar 2011 - Jun 2012
Roles & Responsibilities ... Read more
Concept Design. Preparation of Requirements Allocation and Traceability Matrix(RATM). Review of operation requirement Plan.(NorthStrathfield Rail Underpass,Gosford Passing Loop,Epping-Thoprnley Third Track) Review of System Requirement Specification Preparing Scope of works. Signalling circuit design. Review of signalling works(Circuit book review of Glenfield Leppington Rail Line Project for Transport for NSW) Computer based Interlocking application design. Control and Indication panel design. Signalling,Track Insulation and Bonding Plan. Signalling functional specification.(Northern Sydney Freight Corridor Project) Preparation of Design Report.(Northern Sydney Freight Corridor Project)
Novo Rail
1 yr 5 mos
Senior Signal Engineer
Full-time
Sep 2009 - Feb 2011
Roles & Responsibilities ... Read more
Worked for Novo Rail(An alliance between Public Private organisations-Railcorp,Aurecon,Laing o'rourke and ODG).This alliance will deliver a major infrastructure upgrade over the next 5 years.The project cost involves 1.5Billion Australian Dollars...Engaged in complete circuit book design of Sulphide Junction upgrade and re signalling project.Assisted TOC preparation on Glenfiled Project from signalling discipline.Prepared scope of work doc for control panel modification.Developed the signal plan for Woolongong.Liaised with the estimators to prepare the estimation of various projects under the alliance.
UGL
1 yr 11 mos
Signal Engineer
Full-time
Sep 2007 - Aug 2009
Roles & Responsibilities ... Read more
Signalling circuit design,Design Estimation,Computer based interlocking application logic preparation and interface circuit design. Signalling power supply design,Telemetry design are some of the other activities Deepu was engaged in. Engaged in assisting the estimation activities to come up with winning tenders for Adelaide signalling and Western Australia level crossing projects. Attended meeting for Cronulla Line Duplication project with other partners and clients like John holland,TCA ,and Railcorp. Assisted the Victorian team for the SSI data backp and EPROM programming from remote Sydney side. Updated the Microlok configuration and Microlok Application logic for Epping-Chatswood Rail Link Project.
Ansaldo Signal Pty Ltd (Hitachi)
1 yr 4 mos
Ansaldo Signal (Hitachi)
Full-time
Apr 2006 - Aug 2007
Roles & Responsibilities ... Read more
Update Signalling Plan,Control Table ,Signalling Circuits ,Track Insulation and Bonding Plan for upgrade projects. Concept signal design with proposed aspect sequence chart. Detailed Signal Designs. Interface with other design subcontractors like MGB and Maygurney.
Alstom Transport India Pvt Ltd
5 yrs 4 mos
Application Engineer
Full-time
Nov 2000 - Mar 2006
Roles & Responsibilities ... Read more
Signal plan design for metro projects. Application circuit design, Interface design with other sub systems like ATP,ATS etc. Frequency code allocation SIL4 Application data validation for computer based interlocking. Generate test report based on pre defined scenarios to validate the control table,raise anomalies to sub system leader using simbool(Simulation Boolean Equation). Test Script Preparation for ERTMS Project in The Netherlands -Betuwe Route Project. Data Validation for Incheon Rail Road Transport Project, South Korea
Education
Mangalore University
3 yrs 10 mos
Bachelor Of Engineering ,Electronics & Communication
Nov 1995 - Sep 1999Certifications | Licence
Certifications not yet added
Patents | Awards and recognitions | Memberships
Patents or Awards not yet added
Skills
Skills not yet added
Language Proficiency
Languages not yet added
- Posted 4 years ago
CH1 | THE PURPOSE OF SIGNALLING
SIGNALLING BOOK | CHAPTER 1 CONTENTS 1.Introduction 2.The Problems to be Solved 3.Basic Requirements 4.Lineside Signals 5.The Absolute Block System 6.Interlocking of Points and Signals 7.Single Lines 8.Further Developments 1. INTRODUCTION In general, the railway traveller assumes that his journey will be safe. This high standard of safety which is taken for granted is the result of a long history of development. As human errors and deficiencies in safety systems become evident, often as a result of an accident, improvements are made which are then incorporated into new generations of equipment. This is certainly true of railway signalling. It also appears to be a continuing process. We have not yet reached the situation where absolute safety can be assured. It is useful to start by looking back at some of the early history of signalling development. In the early days of railways, trains were few and speeds were low. The risk of a serious collision between two trains was minimal. Better track and more powerful locomotives allowed trains to run faster (requiring greater stopping distances). Railway traffic increased, requiring more and larger trains. The risks thus became greater and some form of control over train movements became necessary. The need for railway signalling had been identified. 2. THE PROBLEMS TO BE SOLVED 2.1 Collision with a Preceding Train When one train follows another on to the same section of line, there is a risk that, if the first train travels more slowly or stops, the second train will run into the rear of the first. Initially, trains were separated using a system of "time interval" working, only permitting a train to leave a station when a prescribed time had elapsed after the departure of the previous train. Although this reduced the risk of collisions, a minimum safe distance between trains could not be guaranteed. However, in the absence of any proper communication between stations, it was the best that could be achieved at that time. 2.2 Conflicting Movements at Junctions Where railway lines cross or converge, there is the risk of two trains arriving simultaneously and both attempting to enter the same portion of track. Some method of regulating the passage of trains over junctions was therefore needed. This should ensure that one train is stopped, if necessary, to give precedence to the other. 2.3 Ensuring that the Correct Route is Set Where facing points are provided to allow a train to take alternative routes, the points must be held in the required position before the train is allowed to proceed and must not be moved until the train has completely passed over the points. Depending on the method of point operation, it may also be necessary to set trailing (i.e. converging) points to avoid damage to them. 2.4 Control of Single Lines Where traffic in both directions must use the same single line, trains must not be allowed to enter the single line from both ends at the same time. Although this could in theory be controlled by working to a strict timetable, problems could still arise if trains were delayed or cancelled. 3. BASIC REQUIREMENTS We therefore have the basic requirements of any railway signalling system. The method of implementation has changed over the years but the purpose remains the same:- To provide a means of communicating instructions to the driver (signals) to enable him to control his train safely according to the track and traffic conditions ahead. To maintain a safe distance between following trains on the same line so that a train cannot collide with a preceding train which has stopped or is running more slowly. To provide interlocking between points and the signals allowing trains to move over them so that conflicting movements are prevented and points are held in the required position until the train has passed over them. To prevent opposing train movements on single lines. All the above requirements place restrictions on train movements, but it is vital that the signalling system will allow trains to run at the frequency demanded by the timetable to meet commercial requirements. This must be done without reduction of safety below an acceptable level. Signalling involves not only the provision of equipment but the adoption of a consistent set of operating rules and communication procedures which can be understood and implemented by all staff responsible for railway operation. 4. LINESIDE SIGNALS It will probably be evident that the decisions regarding the movement of two or more trains over any portion of the railway can only be made by a person on the ground who has sufficient knowledge of the current traffic situation. His decision must be passed on to the driver of each train passing through his area of control. In the early days railways employed policemen whose duties would include the display of hand signals to approaching trains. As the policemen also had many other duties, it soon became impractical for them to be correctly positioned at all times. Fixed signals of various designs, often boards of different shapes and colours, were provided. The policeman could then set these and attend to his other duties. The simplest signals would only tell a driver whether or not he could proceed. From this evolved a standard layout of signals at most small stations; a "home" signal on the approach side controlling entry to the station and a "starting" signal protecting the section of line to the next station. Between these signals, each train would be under the direct control of the policeman. These signals could give only two indications, STOP or PROCEED. They therefore became collectively known as "stop" signals. As line speeds increased, "distant" signals were introduced which gave advance warning of the state of stop signals ahead. A distant signal could be associated with one or more stop signals and would be positioned to give an adequate braking distance to the first stop signal. It could give a CAUTION indication to indicate the need to stop further ahead or a CLEAR indication, assuring the driver that the stop signal(s) ahead were showing a proceed indication. With the addition of distant signals, trains were no longer restricted to a speed at which they could stop within signal sighting distance. It is important to understand the difference between stop and distant signals. A train must never pass a stop signal at danger. A distant signal at caution can be passed but the driver must control his train ready to stop, if necessary, at a stop signal ahead. The earliest signals were "semaphore" signals (i.e. moveable boards). To enable operation at night, these often had oil lamps added. With the advent of reliable electric lamps, the semaphore signal became unnecessary and a light signal could be used by day and night. Red is universally used as the colour for danger while green is the normal colour for proceed or clear. Initially, red was also used for the caution indication of distant signals but many railway administrations changed this to yellow so that there was no doubt that a red light always meant stop. If necessary, stop and distant signals can be positioned at the same point along the track. Alternatively, certain types of signal can display three or more indications to act as both stop and distant signals. 5. THE ABSOLUTE BLOCK SYSTEM Although time-interval working may seem crude, it is important to remember that nothing better was possible until some means of communication was invented. The development of the electric telegraph made the Block System possible. On many railways, time-interval working on double track lines is still the last resort if all communication between signal boxes is lost. 5.1 Block Sections In the Block Signalling system, the line is divided into sections, called "Block Sections". The Block Section commences at the starting signal (the last stop signal) of one signal box, and ends at the outermost home signal (the first stop signal) of the next box. With Absolute Block working, only one train is allowed in the Block Section at a time. The signalman may control movements within "Station Limits" without reference to adjacent signal boxes. The accompanying diagram shows a block section between two signal boxes on a double track railway. To understand the method of working, we will look at the progress of a train on the up line. Signalbox A controls entry to the block section but it is only signalman B who can see a train leaving the section, whether it is complete (usually checked by observation of the tail lamp) and who thus knows whether or not the section is clear. Signalbox B must therefore control the working of the UP line block section. Similarly, signalbox A controls the DOWN line block section. 5.2 Block Bell The signalmen at each end of a block section must be able to communicate with each other. Although a telephone circuit is a practical means of doing this, a bell is normally used to transmit coded messages. It consists of a push switch ("tapper") at one box, operating a single-stroke bell at the adjacent box (normally over the same pair of wires). The use of a bell enforces the use of a standard set of codes for the various messages required to signal a train through the section and imposes a much greater discipline than a telephone, although a telephone may be provided as well, often using the same circuit as the block bell. 5.3 Block Indicator This provides the signalman at the entrance to the section with a continuous visual indication of the state of the section, to reinforce the bell codes. It is operated by the signalman at the exit of the block section. Early block instruments were "two position" displaying only two indications; line clear and line blocked. Later instruments display at least 3 indications. The most usual are:- Line clear Giving permission to the rear signalman to admit a train to the section. Normal or Line Blocked Refusing permission. The signalman at the entrance to the section must maintain his starting signal at danger. Train on Line There is a Train in the block section. 5.4 Method of Working When signalbox A has an UP train approaching to send to box B, the signalman at A will offer it forward to box B, using the appropriate bell code (so that signalman B knows what type of train it is). If the signalman B is unable to accept the train for any reason, he will ignore A's bell, and leave the UP line block indicator at "Normal". If he is able to accept the train, signalman B will repeat the bell code back to box A, and change the indication to "Line Clear". When signalman A sees his block repeater go to "Line Clear", then he can clear his starting signals to admit the train to the section. When the train actually enters the section, signalman A sends the "Train Entering Section" bell code to box B. Signalman B will acknowledge this by repeating the bell code back to A, and turning the block indicator to "Train on Line". When the train leaves the block section at B, the signalman there checks that it is complete by watching for its tail lamp. He then turns his block indicator to "Normal" again. He also sends the "Train out of Section" bell code to A, which A acknowledges by repeating it back. The system is now back to normal, ready for the next train. On multiple track railways, a pair of block instruments as above is required for each line. 5.5 Extra Safeguards The basic three-position block system, as described, relies on the correct sequence of operations for safety. A signalman could forget that he has a train in section and turn the indicator to "line clear", allowing a second train in. A detailed record (the train register) is kept of the actual times of train arrival and departure, and the times at which the bell signals are exchanged. In most places, additional safeguards have been added to the basic system. An electric lock on the starting signal will prevent it being operated unless the block indicator is at line clear. Track circuit occupation may be used to set the block instruments to ''Train on Line" if the signalman forgets to do so. Electric locking may also be used to ensure that signals are operated for one train movement only and replaced to danger before another movement is permitted to approach. Although it is unusual for absolute block working to be installed on any new signalling installation today, there are many railways on which it is in widespread use. The block system, by ensuring that only one train may occupy a section of line at any time, maintains a safe distance between following trains. 6. INTERLOCKING OF POINTS AND SIGNALS On all early railways, points were moved by hand levers alongside the points. They could therefore be moved independently of the signals controlling the movement of trains. A great improvement in safety (as well as efficiency) was possible by connecting the point switches via rodding to a single central control point (the signal box). Similarly the signals could also be operated by wire from levers in the signal box. With the control of points and signals all in one place the levers could be directly interlocked with each other. This had the following benefits:- Signals controlling conflicting routes could not be operated at the same time. A signal could only be operated if all the points were in the correct position. The points could not be moved while a signal reading over them was cleared. In early signalling installations, all point and signal operation, together with any interlocking, was mechanical. Although it was a great technological advancement to be able to control a station from one place, the effort required to operate the levers restricted control of points to within about 300 metres from the signal box and signals up to about 1500 metres. At large stations, more than one signal box would often be necessary. The possibility still existed for a signalman to set the points, clear the signal, the train to proceed and then for the signalman to replace the signal to normal. This could free the locking on the points before the train had completely passed over them. Signalmen's instructions usually required the complete train to pass over the points before the signal was replaced to danger. 7. SINGLE LINES On most single line railways trains are infrequent. It is not normally necessary for two trains to follow each other closely in the same direction. Single lines were therefore treated in the same way as a normal block section with the important extra condition that trains could not be signalled in both directions at the same time. To enforce this condition and also to reassure the driver that he could safely enter the single line, some form of physical token was used as authority to travel over the single line. On the simplest of systems only one token existed. This caused problems whenever the pattern of service differed from alternate trains in each direction. If the timetable required two trains to travel over the single line in the same direction, the driver of the first train would be shown the token (or train staff as it is commonly known) to assure the driver that no other train was on the single line. His authority to enter the single line would however be a written ticket . The following train would convey the train staff. Although workable, this system would cause problems if trains did not work strictly to the timetable. A further improvement was to provide several tokens, but to hold them locked in instruments at either end of the single line. The instruments would be electrically interlocked with each other to prevent more than one token being withdrawn at a time. The one token could however be withdrawn from either instrument. If the single line block equipment fails, many railways employ a member of the operating personnel as a human token. The "pilotman", as he is usually known, will either travel with the train or instruct the driver to pass through the section. No other person may allow a train on to the single line. Operationally, this is the equivalent of the train staff and ticket system described earlier. 8. FURTHER DEVELOPMENTS The main functions of the signalling system had now been defined, although they were to be continuously improved as the available technology developed. All signalling systems would be required to maintain a safe distance between trains, interlock points and signals and thus prevent conflicting movements, and provide the necessary information so that the speed of all trains can be safely controlled. In recent years, the signal engineer has been asked to provide further facilities within the general scope of the signalling system. These include, train information to the operating staff, train information for passengers, detection of defective vehicles, identification of vehicles and the increasing automation of tasks previously carried out by humans. The technology exists to completely operate a railway without human intervention although the level of automation desirable for a particular railway is for that railway administration to decide. Factors such as cost, maintainability, reliability, staffing policy, passenger security and sometimes political considerations must be taken into account. In many cases the final decision on the type of signalling to be provided is outside the direct control of the signal engineer. However, he should always endeavour to provide the best possible information and propose cost-effective solutions to particular problems so that the best decisions can be made.
Read Full Article- Posted 4 years ago
CH2 | BASIC SIGNALLING PRINCIPLES | PART 1
SIGNALLING BOOK | CHAPTER 2 | PART 1 CONTENTS 1. Introduction - In Part 1 2. Signal Aspects - In Part 1 3. Signalling Principles - In Part 2 4. Drawing Standards - In Part 2 5. Interlocking Principles - In Part 2 6. Train Detection & Track Circuit Block - In Part 2 7. Colour Light Signals - In Part 2 8. Control Panels & Other Methods of Operation - In Part 2 9. Colour Light Signalling Controls - In Part 2 1. INTRODUCTION Whatever type of signalling system is provided on a railway, its basic functions will remain the same. Safety must be ensured by preventing trains colliding with each other and locking points over which the train is to pass. The means of achieving these functions may vary from one railway administration to another but a set of rules must be laid down to define:- The positioning of signals The types of signals The aspects to be displayed by the signals and the instructions to be conveyed by those aspects The controls to be applied to the signals The method of controlling points The method of interlocking points with signals The standardisation of human interfaces Many countries have sytems of signalling based on British railway signalling practice. The basic British system is very simple having only a small number of different signal aspects displayed to the driver. The driver is responsible for knowing the route over which he is to pass. The signal engineer must, in turn, provide sufficient information for the driver to safely control the speed of his train and, where necessary, to inform him which route he is to take. Other signalling systems have developed along a different path. The driver is given specific instructions to travel up to or reduce to an indicated speed. Route indications are optional. This will generally require a more complex set of signal aspects. This section will deal mainly with the principles and practices of the State Rail Authority of New South Wales, with reference to other systems where appropriate. 2. SIGNAL ASPECTS There are three principal types of signal, each serving a different purpose:- Main or Running signals control the normal movement of passenger and freight trains on running lines. The great majority of movements will be controlled by main signals. Subsidiary signals, mounted on the same post or structure as running signals, control movements other than for normal running, such as the shunting or coupling of trains. Independent shunting signals, generally similar to the subsidiary signals above, are provided for shunting movements at positions where there is no need for a running signal. We will examine the aspects displayed by each type of signal and the instructions and/or information conveyed by them. 2.1 Main or Running Signals As all early signals were semaphore signals, displaying a light for night time use, the aspects of colour light signals are usually based on the indications of the semaphore signals which they replaced. As most new signalling installations are likely to employ colour light signals, this section will concentrate on colour light signalling only. SRA (Currentlly TfNSW) employs two methods of signalling on main lines; single light and double light. As the name suggests, double light signals will always display at least two lights to the driver. Double light signalling is generally used in the Sydney metropolitan area. Single light signals normally use only one light to convey instructions to the driver, although a second marker light may be illuminated to aid the driver in locating the signal. Single light signalling is mostly used on lines outside the Sydney metropolitan area. Although there are similarities between the two systems, we will deal with each separately. We will then make a comparison with the corresponding aspects displayed by the British system to enable readers to read signalling plans drawn in British style. 2.1.1 Double Light Signalling This is intended for use in areas where signals are closely spaced. Each stop signal is therefore required to carry a distant signal for the signal ahead. To give the driver a consistent indications, each signal carries two separate signal heads. The upper signal head can be considered as the stop signal. It will always be capable of displaying, at least, stop and proceed aspects. The lower signal head can be considered as the distant for the next signal ahead. An additional green light may be provided below the distant. This is used for a "low speed" indication . FIGURE 1 shows the normal running aspects for double light signalling. Four aspects are used for normal running :- STOP is denoted by two red lights, one above the other. Note that the lower signal head will always display a red if the upper signal is at red, even if the signal ahead is showing a proceed aspect. This is important to avoid misleading or confusing the driver. CAUTION is denoted by green over red. In other words, this signal is at "proceed" (top signal head) but the next signal is at "stop" (bottom signal head acts as distant). The caution indication tells the driver to be prepared to stop at the next signal. MEDIUM is a preliminary warning of the need to stop. It is denoted by green over yellow. Signals in urban areas may be closely spaced. The one signal section between the caution and the stop may provide insufficient braking distance for a train travelling at full line speed. The medium indication tells the driver that the next signal is at caution. This implies that he may have to stop at the second signal ahead. CLEAR allows the train to proceed at maximum speed. A clear indication is two green lights. This will tell the driver that there is no need to reduce speed (other than for fixed speed restrictions) before the next signal. All the above indications require the driver to know where the next signal is, to safely control the speed of his train and be able to stop where required. An additional indication is provided on some signal, A LOW SPEED indication, consisting of a small green light below a normal stop aspect tells the driver to proceed at no more than 27km/h towards the next signal. This is generally more restrictive than the caution. The low speed aspect is used when the track is only clear for a very short distance beyond the next signal. Fig 1: DOUBLE LIGHT SIGNALLING - ASPECTS FOR NORMAL RUNNING *Where a low speed indication is provided. Fig 2: DOUBLE LIGHT SIGNALLING - TURNOUT ASPECTS NOTE: A full clear indication is not given for turnouts. Note the difference in the indications given by Multi-light signals at a turnout. The yellow over red indicates "Proceed" at Medium speed through Turnout, next signal at "Stop". The Yellow over Yellow indicates "Proceed at Medium speed through Turnout" the lower Yellow is cautioning the driver to continue at Medium Speed towards the next signal which is indicating either "CAUTION" or "CLEAR" *Where a low speed indication is provided. **In the Sydney and Strathfield resignaled areas this indication represents a 'low speed' with the train stop at stop. In this case the signal in the rear will show a caution indication. Figure 2 shows the indications for double light turnout movements. If there is more than one route from a main signal, the driver must be told whether he is to take the main line or through route or whether he is to take a lower speed diverging turnout. This information is necessary to prevent the driver running through a turnout at too high a speed. The upper signal head is used to display a distinct proceed aspect for a turnout. Instead of the green normally displayed, a yellow light will tell the driver that he is to take the turnout. The proceed aspects for turnouts are:- CAUTION TURNOUT (yellow over red, also described in some operating documents as medium caution) tells the driver to expect the next signal to be at red. MEDIUM TURNOUT (yellow over yellow) tells the driver that the next signal ahead is displaying a proceed aspect. This is the least restrictive aspect for a turnout. There is no equivalent of a clear aspect for trains signalled over a turnout. To give a clearer indication to the driver where several routes are possible from one signal, the main signal aspect may be used instead, in conjunction with a theatre route indicator. The route indicator contains a matrix of small lunar white lights which can be illuminated to display a Jetter or number. Each character will be associated with a distinct route. The route indicator is not illuminated when the signal is at stop. When the signal is required. to clear, the route indicator will illuminate for the appropriate route. LOW SPEED aspects may be used. If a low speed aspect is provided for a turnout route, this is no different in appearance to a low speed for the main line route. This is not a problem as this aspect conveys a specific speed instruction. As the speed is likely to be lower than that of most turnouts, route information is not essential 2.1.2 Single Light Signalling On lines where single light signalling is installed, the spacing of signals may vary widely. Therefore some signals may be combined stop and distant signals (as for double light) but there may also be signals which are stop signals only or distant signals only. The instruction to the driver is therefore generally conveyed by a single light. A second marker light is provided below the main light to aid the driver in locating the signal. The meaning of the signal aspects is equivalent to the double light aspects but the appearance to the driver is different. FIGURE 3 shows the normal running aspects for single light signalling. The appearance of each aspect is as follows:- STOP consists of a red light. The marker light also displays a red, except on some older signals where it is lunar white. CAUTION consists of a single steady yellow light. The marker light is extinguished, except on some older signals where it is lunar white. If the main light should fail the marker light will display a red on stop signals or yellow on distant signals. MEDIUM, where this aspect is necessary, will be a flashing or pulsating yellow light. The marker light will operate as for the caution aspect. CLEAR is a green light. The marker light will operate as for the caution aspect. LOW SPEED aspects may be used in single light signalling where required. An additional small green light is provided below the marker light. The complete low speed aspect will be a main red light over a red marker light with the additional green light illuminated . Fig 3: SINGLE LIGHT SIGNALLING - ASPECTS FOR NORMAL RUNNING Fig 4: SINGLE LIGHT SIGNALLING - TURNOUT ASPECTS The indication displayed by a Home signal for a turnout movement through facing points into a Loop Refuge siding or important siding consists of a band of three yellow lights in a subsidiary light unit (inclined towards the direction of the movement). The Red light is displayed in the Main line signal , as shown. The marker light for the main line signal, contained in the subsiduary light unit will be extinguished when the main line or turnout signal indication is displayed. 4.1 ROUTE INDICATIONS MAIN LINE At locations where more than one turnout is provided one signal indication is some times given and in such cases a route indicator working in conjunction with th e signal is provided, this enables drivers to ascertain the route for which th e signal has been cleared. The route indicator will not show any indication when the signal is at stop, but when the points have been set for the turnout movement a yellow light will appear in the signal in conjunction with the route indication showing a letter to denote the line to which the train will travel, e.g. Figure 4 shows the indications for single light turnout movements. For junction signals, two distinct methods are used according to the situation. For a simple turnout into a loop or siding, a separate turnout signal is provided below the main aspect, incorporating the marker light. For a CAUTION TURNOUT aspect, the main aspect remains at red, the marker light is extinguished and the three yellow lights of the turnout signal are illuminated . The row of lights is inclined in the direction of the turnout. For a MEDIUM TURNOUT aspect, the turnout signal will flash. Otherwise the appearance is the same as above. As for double light signalling, a theatre route indicator may be used in conjunction with the main signal aspect, where several routes are possible from one signal. Again the route indicator is not illuminated when the signal is at stop. When the signal is required to clear, the route indicator will illuminate for the appropriate route. The normal construction of signals is to provide a separate lamp unit for each light to be displayed . Some signals, however, are of the "searchlight" type. In this type of signal, the lamp is continuously illuminated and coloured lenses are moved in front of the lamp according to the aspect to be displayed. The lenses are moved by a relay mechanism inside the signal head. The lights visible to the driver are the same for either type of signal. 2.2 Subsidiary Signals Associated with Main Signals As well as normal running movements, signals may be required for some of the following movements:- Entering an occupied section Shunting into a siding Running on to a line used for traffic in the opposite direction. Attaching or detaching vehicles or locomotives. The main types are described below. As for the running signals, only current practice is covered in detail. Although the SRA (TfNSW) practice will allow subsidiary signals to clear immediately the route is set, many railway administrations employ approach control to delay clearance of the subsidiary signal until the train has come at or almost to a stand at the signal. This is usually achieved by timed track circuit occupation. The driver will receive a caution at the previous signal and will be preparing to stop. Subsidiary signals are short range signals which are only visible within a short distance of the signal. 2.2.1 Subsidiary Shunt and Calling-on Signals These authorise a driver to pass a main signal at stop for shunting purposes or to enter an occupied section. The driver must be prepared to stop short of any train or other obstruction on the line ahead. He must therefore control the speed of his train so that he can stop within the distance he can see. The appearance of these signals is a small yellow light below the main aspect. On some older double light signals the letters "CO" in a round lens illuminated in white may be used. 2.2.2 Shunt Ahead Signal This signal is generally found on single and double lines worked under absolute block conditions. It permits the movement of a train past the starting signal for shunting purposes. It does not require a block release from the signal box ahead and the movement will eventually come back behind the starting signal when shunting is complete. As this method of working is generally only found outside the suburban area, a shunt ahead signal will normally be provided on single light signals only. It consists of a small flashing yellow signal below the main running signal and marker light. 2.2.3 Close-up Signal This is similar in appearance and application to the low speed signal. 2.2.4 Dead-end Signal This is for entering short dead end sidings directly from a running line. The only difference between this and a subsidiary shunting or calling-on signal is that the dead end signal is offset from the post on the same side as the siding leads off the main line. Subsidiary signals display no aspect when not in use. The associated main signal will remain at stop when the subsidiary signal is in use. Route indicators may be used in conjunction with subsidiary aspects to give an indication to the driver where multiple routes are available. In the case of a movement on to another running line in the wrong direction (i.e. opposite to normal direction of traffic) a route indication is always provided. 2.4 Shunting Signals Signals may also be required for shunting movements in positions where no main signal is necessary. The most common locations are:- Entrance to and exit from sidings. At crossovers to allow a wrong road movement to regain the right line. In yards and depots where main signals are not required. As they have no associated main signal, they must display a stop as well as a proceed aspect. 2.5 Dwarf and Position-light Signals Two main types are in use, the dwarf signal, with all lights arranged vertically and the position light signal. The diagrams show the various signal profiles. Both types display two red lights for stop. The proceed aspect is normally only a yellow indicating caution. Shunting signals do not always prove track circuits clear and the driver must be ready to stop if there is another train occupying the section ahead. The proceed aspect may be accompanied by a route indication. Shunting signals are normally mounted at ground level, although they may be elevated if required for sighting. 2.3.2 Stop Boards If a wrong road movement is authorised from a shunting or subsidiary signal there must be another signal ahead to limit the wrong road movement. If no such signal was provided, the movement could continue past the protecting signal for the normal direction of traffic and cause a collision. The usual signal is an illuminated notice board carrying the words "SHUNTING LIMIT". It can be considered as a shunting signal permanently at danger. 2.3.3 Facing Shunt Signals A shunting signal is sometimes needed in a position where it is passed in the normal direction by running movements. To avoid confusing the driver by displaying a yellow light for a movement which may well be running under the authority of clear signals, these facing shunt signals are provided with an additional green light (clear aspect) for use only in this situation. 2.3.4 Point Indicators Although not signals in the same way as those just described, point indicators are important in sidings to avoid derailments and damage to equipment. They provide a visible indication of the position of hand operated points. Operation may be either mechanical or electrical. Located alongside the point switches, they display an illuminated arrow in the direction of the line for which the points are set. 2.5 British Signalling Aspects For the benefit of those who may at some time have to read signalling plans drawn to British standards (e.g. for the IRSE examination), this is a brief summary of the aspects in use, their meanings and how they are drawn. 2.5.1 Main Signals As with SRA (Currently TfNSW) practice, three colours are used, red, yellow and green. On the plan, a red light is denoted by a circle with a horizontal line across it. A yellow light has the line at 45° and a green light has a vertical line. The "normaJ" aspect of the signaJ (i.e. with no routes set and all track circuits clear) is shown by a double line in the appropriate light(s). There are four available aspects; STOP is a red light, CAUTION is a single yellow light, PRELIMINARY CAUTION is two yellow lights and CLEAR is green.The stop, caution and clear signals have the same meanings as the corresponding SRA aspects. The preliminary caution is similar to the MEDIUM indication of SRA signaJs. The double yellow aspect is only used in situations where signals must be positioned closer together than braking distance. Many lines use red, single yellow and green only. Marker lights are not used. There is no equivalent of the LOW SPEED signal. An equivalent control (to allow trains to close up provided they are running at very low speed) is provided by delayed clearance of the yellow aspect. The train must be almost stationary at the signal before the aspect will change from red to yellow. This is achieved by applying approach control with timed track circuit occupation. 2.5.2 Junction Signalling Where a signal has more than one route, a distinct route indication must be given for each route, except that the highest speed or straight route need not have a route indication. This may take one of two forms, a junction indicator (a row of five white lights) normally above the main signal and pointing in the direction of the divergence or a multi lamp or fibre optic route indicator displaying one or two characters. There are six available junction indicator positions. Positions 1, 2 and 3 (at 45°, 90" and 135° respectively) indicate diverging routes to the left. Positions 4, 5 and 6 provide equivalent indications to the right. Multi-lamp or fibre optic route indicators are restricted to routes with a speed of 40 mph (64 km/h) or less. Where necessary, clearance of the junction signal is delayed by occupation of the approach track circuit (timed if necessary) to enforce a speed reduction. This is because the driver may receive no warning at previous signals of the route set from the junction signal 2.5.3 Subsidiary Signals The standard subsidiary signal is a position light with two white lights at 45°. The proceed aspect is both lights illuminated. There is no stop aspect - the associated main signal remains at red. Route indicators are provided where necessary but are not obligatory - if they are provided, route indications must be displayed for all routes. The subsidiary signal is used for all shunting and calling on moves. This is a short range signal. An approaching train must be brought to a stand before clearance of the subsidiary aspect. 2.5.4 Shunting Signals The position light shunting signal has two white lights and one red light. The proceed aspect is identical to the subsidiary signal. The stop aspect is one red and one white light, horizontally placed. The white light at the lower right {the "pivot" light) therefore remains continuously lit. A shunting signal with two red lights only is used as a "limit of shunt" indicator. 2.6 Summary Whatever the system of signalling, the signal engineer must have a detailed knowledge of the aspects displayed to the driver and the instructions conveyed. He must then design the layout of the signalling and the associated controls so that the driver can safely obey all signal aspects. This must apply for all types of train likely to use a line. When required to reduce speed or stop, trains must have adequate braking distance under all conditions. The driver of a train must never be given an instruction by a signal that he is unable to comply with. TO BE CONTINUED - SIGNALLING BOOK | CHAPTER 2 | PART 2...........
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CH3 | SIGNALLING A LAYOUT | PART 1
SIGNALLING BOOK | CHAPTER 3 | PART 1 CONTENTS 1. Introduction - In Part 1 2. Headway - In Part 1 3. Positioning of Running Signals - In Part 2 4. Types of Signal - In Part 2 5. Points and Crossings - In Part 3 6. Track Circuits - In Part 3 7. Identification of Signals, Points & Track Circuits - In Part 3 8. Examples - In Part 3 1. INTRODUCTION One of the first steps in any signalling project is to determine the method of train working. Having decided this, it is then necessary to decide the position and spacing of signals. This section will assume throughout that colour light signalling to track circuit block principles will be provided on all main lines. Although other methods of working may well be more appropriate, particularly for lightly used single lines, these will be covered later in the course. It is useful at an early stage to determine whether 2, 3 or 4 aspect signalling will be required. This will be governed by the required line capacity, which in turn will be determined by the timetable to be operated. Having this information and an approximate signal spacing, we can then proceed to position the signals on a scale plan of the track layout. Their position relative to stations, junctions etc. will be decided largely by operating requirements. The most economical arrangement that meets all operating requirements is the one that should be adopted. In order to produce a safe and economical signalling scheme, the designer must use his knowledge of signalling principles and be provided with all necessary details of the train service pattern required, the track layout, gradient profiles, line speeds and train characteristics. If this information is not immediately available, it must be requested from the appropriate authority. Sometimes operating requirements conflict with each other and with safety standards — the engineer must then use his experience to reach a satisfactory compromise whilst maintaining the safety standard. 2. HEADWAY The headway of a line is the closest spacing between two following trains, so that the second train can safely maintain the same speeds as the first. This usually means that the second train is sufficiently far behind the first that its driver does not see an unduly restrictive signal aspect. Headways can be expressed in terms of distance but more usefully as a time (e.g. 2 1/2 minutes between following non-stop trains). It can also be converted to a line capacity (trains per hour). Care must be taken when using a "trains per hour" figure if the trains are not evenly spaced in the timetable. The signalling must be able to handle the minimum headway, not the average. Headway will depend on a number of factors:- D = Service Braking Distance d = Distance between STOP signals S = Sighting Distance (usually 200 yds/metres or distance travelled in 10 seconds) O = Overlap Length L = Train Length (less than 100 yds/metres for a short suburban train but possibly over 1km for a heavy freight train) V = Line Speed (or actual train speed if lower) a = Braking rate Where any of these factors are not given to you, you should always state your assumptions. In practical situations, it is vital to obtain accurate information regarding the braking performance of trains. It is also vital to standardise your units of distance and time. If you work in imperial, yards and seconds are most useful; in metric, metres and seconds would be most appropriate. Whichever you decide, you must use the same set of units consistently throughout to avoid confusion and error. 2.1. Service Braking Distance This is the distance in which a train can stop without causing undue passenger discomfort. It will depend on the line speed, gradient, and type of train. It is usually significantly greater than the emergency braking distance. Theoretically, the Service Braking Distance can be calculated using the line speed and braking rate This is derived from the 3rd Law of Motion. This calculation will depend upon the braking characteristics of the type(s) of train using the line and must take into account the worst case combination of train speed and braking rate. If this calculation is to be performed frequently, it is useful to show the service braking distances for different combinations of speed and gradient in tabular or graphical form. Gradient should always be taken into account. A falling gradient will increase braking distance, a rising gradient will reduce it. As gradients are rarely uniform between signals, we need to calculate an average gradient using the formula: where G is the average gradient D is the total distance g and d are the individual gradients & distances. For a gradient of 1 in 100, G = 100. If the gradient is expressed as a percentage, G is the reciprocal of the percentage gradient. Falling gradients taken as negative, rising gradients as positive. 2.2. 2 Aspect Signalling 2 aspect signalling will generally be adequate on lines where traffic density is low. The required length of block section is much greater than braking distance. Only two types of signal are used, a stop signal showing stop and clear only and a distant signal showing caution or clear. Each stop signal will have its associated distant signal. As 2 aspect signalling will mainly be found outside the suburban area, the example shows single light signals. The distance (d) between stop signals is variable according to the geography of the line, positions of stations, loops etc. The headway distance can be calculated as: H = D + d + S + O + L giving a headway time: Note that the headway time for the line is that of the longest section and cannot be averaged. To obtain the greatest signal spacing to achieve a specified headway, we transpose the equation to give: d = (V x T) - ( D + S + O + L) 2.3. 3 Aspect Signalling With 2 aspect signalling, as the required headway reduces, each stop signal will become closer to the distant signal ahead. it is therefore more economic to put both signals on the same post. This then becomes 3 aspect signalling. Each signal can display either stop, caution or clear. The distance (d) between signals must never be less than braking distance (D), but to ensure that the driver does not forget that he has passed a distant at caution, (d) should not be excessively greater than the service braking distance. The current SRA recommendation is for signal spacing to be no greater than three times braking distance. BR has adopted a maximum of 50% (i.e. 1.5D) although this is often exceeded at low speeds. The headway distance is given by:- H = 2d + S + O + L So the best possible headway, when the signals are as close as possible (exactly braking distance), is: H = 2D + S + O + L The headway with signals spaced 50% over service braking distance is: H = 3D + S + O + L The headway with signals spaced at three times braking distance is: H = 6D + S + O + L 2.4. 4 Aspect Signalling Where signals are closer together than braking distance, a preliminary caution or medium aspect is needed to give trains sufficient warning of a signal at danger. This medium aspect must not be less than braking distance (D) from the stop aspect, so the distance (d) between successive signals must on average be no less than 0.5D. The headway distance is given by:- H = 3d + S + O + L where d > 0.5 D So the best possible headway with 4 aspect signalling is given by:- H = 1.5 D + S + O + L In practice, the geographical constraints of the track layout will probably prevent regular spacing of signals at 0.5D. If the total length of two consecutive signal sections is less than braking distance, an additional medium aspect will be required at the previous signal. In other words, the first warning of a signal at stop must be greater than braking distance away. If more than two warnings are required, the medium aspect is repeated, not the caution. Signals should however be positioned so that this situation is as far as possible avoided. 2.5. Application of Low Speed Signals and Conditional Caution Aspects In normal use, the addition of a low speed signal provides the driver with a fifth aspect. It is important to realise that this does not have any effect on the headway of through or non-stopping trains running at their normal speed. In this situation, the engineer will arrange the signals so that each driver should, under normal conditions, see only clear aspects. The preceding headway calculations apply regardless of whether low speed signals are provided or not. A low speed signal tells the driver that he has little or no margin for error beyond the next signal and should control the speed of his train accordingly. The benefit of low speed signals is in allowing a second train to close up behind a stationary or slow moving train by reducing the length of the overlap, provided the speed of the second train has been sufficiently reduced. The same effect can be achieved by delaying the clearance of the caution aspect. This is now preferred, provided an overlap of the order of 100 metres can be achieved. The clearance of the signal should be delayed to give a passing speed of approximately 35km/h. Low speed signals should only be used where the reduced overlap is very short (less than 50 metres) and/or there are fouling moves within 100 metres of the stop signal. 2.5.1. Station Stops With an overlap of 500 metres, a train stopped at a station will have at least 500 metres of clear track behind it. A following train will stop at the first signal outside this distance. By the addition of a low speed signal or a conditionally cleared caution, the overlap distance can be reduced and the second train can approach closer to the station. When the first train leaves the station, the second train can enter the platform earlier, thus giving a better headway for stopping trains. A conditionally cleared caution aspect will normally be used unless the overlap is less than 50 metres. 2.5.2. Approaching Junctions Trains awaiting the clearance of another movement across a junction can approach closer to the junction while keeping the overlap clear of other routes across the junction. A low speed aspect will normally be used in this situation. 2.5.3. Recovery from Delays A line which is operating at or near its maximum capacity will be susceptible to disruption from even minor train delays (e.g. extended station stops at busy times). Low speed signals and or conditionally cleared caution aspects can allow trains to keep moving, even if only slowly, to improve recovery from the delay. The total length of a queue of trains will be less and the area over which the delay has an impact will be reduced. 2.6. Summary For 2 aspect signalling, the headway distance is:- H = D + d + [S + O + L] For 3 aspect signalling, the headway distance is:- H = 2D + [S + O + L] (minimum) where signals are spaced at braking distance H = 2d + [S + O+ L] (general case) for an actual signal spacing of d For 4 aspect signalling, the headway distance is:- H = 1.5 D + [S + O + L] (minimum) where signals are spaced at braking distance H = 3d + [S + O + L] (general case) for an actual signal spacing of d Note the factor [S + O + L] is common to all equations. Headway time is then calculated as: 2.7. Determining Signal Type and Spacing Because cost is generally proportional to the number of signals, the arrangement of signalling which needs the smallest number of signals is usually the most economic. It must, however, meet the headway requirements of the operators. For non-stop headways it is likely that the same type of signalling should be provided throughout. Otherwise there will be large variations in the headway. Remember that the headway of the line is limited by the signal section which individually has the greatest headway. This section will briefly describe a technique for determining the optimum signalling for a line. There may need to be localised variations (e.g. a 2 aspect signalled line may need 3 aspect signals in the vicinity of a station or a 3-aspect line may need to change to 4 aspect through a complex junction area). These variations will depend on the requirements for positioning individual signals and can be dealt with after the general rules have been determined. Firstly, determine braking distance, train length and overlap length required. Each must be the worst case. Knowing the required minimum headway, use the H = 2D + S + O + L equation to determine the best possible headway for 3 aspect. Compare the results with the required headway to check whether "best case" 3 aspect signalling is adequate. There should be a margin of 25–30% between the theoretical headway and that required by the timetable to allow for some flexibility to cope with delays. 2.7.1. If the Headway is Worse than Required 3 aspect will not be adequate and 4 aspect must be used. Recalculate for 4 aspect to confirm that this does meet the headway requirement. T = (1.5D + S + O + L) / V If the non-stop headway requires 4 aspect signalling, it is likely that station stops will cause further problems. Signal spacing near stations should be kept to a minimum and low speed signals or conditionally cleared cautions with reduced overlaps may also be required. 2.7.2. If the Headway is Much Better Much better generally means a headway time of 30% or less than that required by the timetable. If this is the case 2 aspect will generally be adequate. Calculate the greatest signal spacing that will achieve the headway with 2 aspect signalling. d=(V x T) - (D + S + O + L) Remember that in this distance d there will be two signals, a stop signal and a distant signal. Then compare this with the maximum permissible signal spacing for 3 aspect. In the absence of any firm rules, a judgement must be made on the amount of excess braking which is acceptable. SRA recommends that signal spacing is no more than three times braking distance while BR signalling principles specify no more than 1.5 times braking distance. If the two calculations produce a similar total number of signals (i.e. d for 2 aspect is approximately twice the value of d for 3 aspect) a 3 aspect system will be the better choice. The cost of the signals will be similar and the operator may as well benefit from the improved headway provided by 3 aspect. 2.7.3. If the Headway is Slightly Better It is probable that 3 aspect is the correct choice. Check that there is sufficient margin between the required and theoretical headway. 2.7.4. Signal Spacing Having evaluated that the chosen arrangement of signalling will provide the required headway, the relevant equation should be transposed to calculate the greatest possible signal spacing that can be allowed with the specified headway: eg. for 3 aspect signalling: V x T = H = 2d + S + O + L therefore 2d = (V x T) - (S + O + L) from which the post to post spacing (d) can be calculated Remember, there may be a constraint on the maximum signal spacing. The value of d should not exceed this. Geographical constraints may also require signals to be closer together than braking distance, in which case the 4th (medium) aspect is used where required. It does not need to be used throughout unless for headway [puposes]. 2.8. Example Information given:- Max. Line Speed...... 90 km/h Gradients .......... Level Train Length.............. 250 metres Headway Required..... 2 1/2 mins. (non-stop) Before we start, we need the Service Braking Distance, either by calculation or from tables/curves (where available). We will assume that D = 625 metres. Note : S assumed to be 200 metres. O assumed to be 500 metres (although overlaps may need to be more accurately calculated if trainstops used) V = 90 km/h = 25m/s First, check 3 aspect signalling:- H = (2D + S + O + L) = (1250 + 200 + 500 + 250) = 2200 metres so T = H/V = 88 seconds. This is much less than the 150 seconds (21/2 mins) specified. We will therefore consider the alternative of 2 aspect signalling. We cannot calculate a theoretical headway for 2 aspect signalling as the signal spacing is not fixed. Instead, we calculate the greatest 2 aspect signal spacing to give us the 150 second headway specified : d = (V x T) - (D + S + O + L) = (25 x 150) - (625 + 200 + 500 + 250) = 3750 - 1575 metres = 2175 metres Hence 2 aspect signalling, with the stop signals no more than 2175 metres apart, would give the 2 1/2 min. headway required. However, each stop signal also requires a distant signal. Two signals are therefore required every 2175 metres. 3 aspect signalling with signals every 1088 metres would require no more signals but would give a better headway of: H = 2d + (S + O = L) = 2175 + (200 + 500 + 250) = 3125 metres So T = H/V = 3125/25 = 125 seconds In fact, the signal spacing could be extended further within the headway requirement of 150 seconds. This would give a better headway with fewer signals than 2 aspect. This demonstrates that 2 aspect is generally worth considering only for very long headways. We could now calculate the maximum possible 3 aspect signal spacing allowed by the headway : V x T = H = 2d + S + O + L therefore 2d = (V x T) - (S + O + L) = (25 x 150) - (200 + 500 + 250) = 3750 - 950 = 2800 metres d = 1400m As this is over twice braking distance, it should be confirmed that this signal spacing is operationally acceptable TO BE CONTINUED - SIGNALLING BOOK | CHAPTER 3 | PART 2...........
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SRA (TfNSW) SIGNALLING & ELECTRICAL SYMBOLS
CONTENTS The following pages show the signalling and schematic symbols used by the State Rail Authority of New South Wales on their plans and drawings. They are provided for reference use throughout the course. They are reproduced from SRA (Currently TfNSW) training material. TRACK PLAN AND WORKING SKETCH SYMBOLS MECHANICAL SIGNALS POWER WORKED SIGNALS SINGLE LIGHT INDICATION WAYSIDE BUILDING AND STRUCTURES TRACK CIRCUIT DEVICES INTERLOCKING SYMBOLS APPARATUS HOUSINGS RELAYS AND CONTACTS CONTACTS OPERATED BY SIGNALS LOWER QUADRANT SEMAPHORE SIGNALS, INCLUDING BANNER SIGNALS CONTACTS OPERATED BY POINTS LEVER CONTACTS CATCH ROD CONTACTS MISCELLANEOUS APPARATUS
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CH4 | HEADWAYS
SIGNALLING BOOK | CHAPTER 4 CONTENTS Introduction Theoretical Headways Practical Headways Headway Charts Producing a Headway Chart Station Stops Speed Restrictions & Junctions Gradients Varying Train Speeds Single Lines Terminal Stations 1. INTRODUCTION The prime function of a signalling system ,irrespective of fixed block or moving block is to protect trains so that they run safely, including maintenance of a safe distance between following trains. Design headway can be defined as the theoretical time separation between two Trains travelling in the same direction on the same track. It is calculated from the time the head-end of the leading Train passes a given reference point to the time the head-end of the following Train passes the same reference point.The run profile for both trains shall be the minimum run time that the rollingstock and track conditions permit. For a fixed block signalling once the signals have been positioned, this minimum distance has effectively been fixed. This in tum governs the capacity of the line, or how many trains per hour can use it,whereas for a moving block headway is the minimum distance can be maintained between two moving block (Rollingstock) with a safety margin. Refer figure 1 below Figure 1 Head way of Moving block system It would be ideal to learn the headway with a fixed block signalling.Line capacity is derived from the minimum headway time between trains. Although this can be deduced mathematically from a knowledge of Signalling Principles and Equations of Motion, it is very laborious and time-consuming to go through these calculations for every signal, although it is often useful to do so for a rough assessment and to examine any critical sections. In practice, the headway time is often found graphically, by producing a HEADWAY CHART, or time-distance-speed curve. These are often drawn for lines where headways are important, such as those with an intensive service or major junctions or termini. On the majority of lines fitted with 4 aspect signalling, the headway provided is usually much better than actually required, so a headway chart for the whole line is unnecessary. However, it is still important to have an appreciation of those factors which affect line capacity when signalling a layout. Additionally, headways are often adversely affected if the trains do not actually behave according to the simplified theoretical performance assumed when positioning the signals. This section will also examine some of the problems which arise and suggest possible solutions. Although the problems of optimising headway have been known to the signal engineer for many years, after introduction of computers that most engineers have had readily available computer aided design facilities and problem solving. The solution of headway problems is well suited to the application of computers. The details of signals, gradients, speed restrictions and the other geographical features of the railway can be held on a database. Train performance generally follows fairly simple mathematical equations. Given the correct data, computer simulations of the passage of trains along the line may be performed very quickly, providing the signal engineer with an accurate output of the headway at each signal. However, it is important that the engineer understands how these results are derived. These notes will therefore concentrate on the graphical calculation of headways. 2. THEORETICAL HEADWAYS The headway time of a given line can be calculated theoretically by the equations: From the 3 basic equations of motion: we can derive the relationship: where B is the braking rate, and D the braking distance. Substituting this in the 3 aspect headway formula, for example, gives To understand how the headway varies with train speed, we will use the second of the above expressions. It consists of two parts. Both have units of time (normally seconds). The first part (V/B) represents the time taken to cover two signal sections. As braking distance (which determines signal spacing) increases in proportion to the square of the speed and time taken to cover a given distance is inversely proportional to speed, the resultant time increases directly in proportion to speed. If we could ignore factors such as train length, overlaps and sighting distance, a headway could always be improved to the required value by reducing the speed. This is however offset by the second part of the expression, (S+O+L)/V, which represents the time for the train to traverse a distance equal to the sum of the overlap, the sighting distance and the length of the train. This distance is usually fairly constant (although sighting and overlaps can be reduced to a limited extent at low speed). The time taken therefore reduces as the speed of the train increases. Figure 2 Headway-Line speed Graph As V/B increases, (S+O+L)/V decreases. There is therefore an optimum speed at which the headway is at a minimum. Increasing or decreasing this speed will reduce line capacity. Figure 2 shows this effect. It can be shown mathematically that the best possible headway is defined by the equation:- At low speeds, train length, overlap and sighting distances dominate the value of the headway. At higher speeds, the time to traverse the required number of signal sections will be the most significant. However, in practice, the signal engineer is usually presented with a situation where these factors, particularly the line speed and the braking performance of the trains, have all been previously decided. This theoretical treatment should nevertheless assist in the understanding of specific situations. 3. PRACTICAL HEADWAYS The equations and formulae we have dealt with so far are for the very simple case of two identical trains running at the same constant speed as each other, with signals placed at ideal positions for headway purposes. In practice these are unrealistic assumptions, and the headway of a line is affected by:- station stops speed restrictions signal positioning constraints different types/speeds of trains trains travelling at less than line To calculate the headway mathematically, taking all these factors into account would be very time consuming. It would be necessary to calculate the headway time individually for each signal to find the "worst case". For the level of accuracy required, the use of graphical techniques (the headway chart) will usually produce a solution more quickly. It is also possible to perform the same calculations with the aid of a computer. To ensure a thorough understanding, this section will concentrate on graphical solutions. Some of the diagrams refer to British signal aspects. The Australian equivalents are:- 4. HEADWAY CHARTS The headway chart, or time-distance-speed curve, is a plot of the train's position against time, from which headways can be measured directly. Where the headway times achieved do not meet the Operators' requirements, then the positions of signals may be adjusted by eye, or extra intermediate signals added, provided that braking distances are not infringed. Ideally, signals should be spaced to give equal headways, even where the "worst case" is better than that required, to avoid creating a "bottleneck". The chart is drawn with distance along the horizontal (x) axis, and time along the vertical (y) axis. It is once again important to use consistent units throughout, time in seconds and distance in meters. A train travelling at constant speed is therefore represented by a straight line, the higher the speed the closer the line becomes to the horizontal, while a stationary train is represented as a vertical line. A plot of a train running at constant speed, stopping at a station, and then accelerating away again to reach the same speed as before would look like in figure 3 & 4 Figure 3 Time- Distance Graph Figure 4 Time -Distance Graph 5. PRODUCING A HEADWAY CHART 5.1 The horizontal and vertical scales are drawn, with the position of signals shown on the horizontal scale. Ends of overlaps, stations and any significant speed restrictions are also drawn. 5.2 The path of a train is then plotted on the chart. This may be done using point-to-point times (where known), or by assuming that the train will travel at the maximum permitted or attainable Suitable time should be allowed for a station stop. If the timetable shows this information, it should be used, otherwise 30 seconds is probably a reasonable assumption. It is very important to obtain accurate information. Remember that at the busiest times of day, when the shortest headway will be needed , trains often take longer to load and unload when much larger numbers of passengers are travelling. Accelerations and decelerations are curved plots, calculated from S=at²/2, often drawn on a template or stencil, while constant speed is a straight line calculated from S =vt.The time/distance curve is constructed by joining the acceleration/deceleration curves together with the constant speed lines, ensuring that the transition from one curve/line to another occurs at points of equal speed. 5.3 Having drawn the curve, you should check that the line speed and other speed restrictions have not been 5.4 For headway purposes, it is necessary to show both the front and rear of the train, so an identical curve is then drawn for the rear of the train, displaced horizontally a scale train length in rear of the first 5.5 The headway distance for a 3 aspect signal is from its sighting point to when the rear of the previous train clears the overlap beyond the appropriate signal in advance. Rather than calculate how long it would take for a train to cover this distance, we can read the headway time directly from the chart. From the sighting point of each signal project a horizontal line forward to the end of the overlap beyond the headway signal, and then measure the running time vertically from this line to the REAR of the train. It is usual on a headway chart to show the sighting allowance as a time (10 seconds) rather than a distance, although it is a simple matter to measure off a distance if preferred for a particular situation. 5.6 This process is repeated for each signal, and the headway time for each aspect noted on the horizontal line from that signal. Figure 5 Headway Plotted Curve 5.7 The headway of the line is then given by the worst-case signal For non-stop services the headway on clear aspects is always quoted, but if lower speed stopping services can in practice run at their normal speed on double yellow (medium) aspects, then that headway may often be quoted. In addition, there is generally no objection to stopping trains entering a platform on a caution aspect (i.e. platform starter at red/stop), but the platform "starting" signal should permit a train to make an unrestricted departure after the station stop ,ideally showing green/clear before the train is due to leave. Separate curves are often plotted for stopping and non-stopping services. 5.8 When quoting headway times from a chart, you should make allowance for errors in plotting the curves and intersections, for the reaction time of signalmen, and for the fact that in practice drivers do not make a uniform brake application but reduce speed in stages. An allowance of typically 25% is often added to the derived values to take these and other factors into 6. STATION STOPS Considering the headway chart for stopping trains, we see that:- 6.1 Providing a platform "starting" signal allows signals in rear of the station to clear as quickly as possible when a train · 6.2 The headways of signals in rear of the station include the braking period, the stopping time, and at least a portion of the acceleration Any signal which includes any of these factors in its headway should be located as close to the station as possible to keep the headway figure down. 6.3 Similarly, any signals in advance should be located as close to the station as possible, if they control the headways of signals approaching the station. In severe cases, if the stopping headway is most critical, it may be necessary to impose speed restrictions on the approach to a station in order to close up the signals sufficiently. Where conditional caution or low speed aspects are provided (with reduced overlaps), the clearance of the signal should be measured from the point at which the rear of the train clears the reduced overlap. It must be remembered that a separate curve may need to be drawn to take account of the speed reduction due to the approach control. 6.4 It is very often the case that 4 aspect signalling is necessary to provide adequate headway for station stops even though 3 aspect would have been satisfactory for non-stopping 7. SPEED RESTRICTIONS & JUNCTIONS S peed restrictions at junctions and on plain line have a similar but less marked effect than station stops. When a train slows down for a speed restriction, any following train will still be travelling at full line speed and so will tend to catch up with the train ahead. If the speed restriction is severe, this can have a serious affect on headways. As with station stops, it is desirable to have the signals either side of the restriction spaced closer together. Signals can be closer together after the restriction because the maximum attainable speed is less, but on the approach side braking for full line speed must be maintained. Temporary speed restrictions can have a serious effect on headways and line capacity, particularly as the speeds imposed are often low. Unfortunately, the signal engineer has little control over temporary speed restrictions. In severe cases, the imposition of a temporary speed restriction can make a timetable based on normal line speeds unworkable. This is another good reason why the headway provided should always be better than that required by the operators. 8. GRADIENTS Signals will generally need to be spaced further apart on falling gradients because of the greater braking distanc.es. This will increase the headway and thus reduce line capacity. On rising gradients, the braking distances (and therefore signal spacing) will be reduced, which may improve the headway, but train speeds may also be reduced by the gradient, making line speed unattainable. The effect on headways may be particularly noticeable if the speeds of some trains are more reduced than others (e.g. heavy freight trains on rising gradients). 9. VARYING TRAIN SPEEDS So far we have only considered the headways between similar trains. However, if a line is used by both fast and slow, or stopping and non-stop services, this can have a marked effect on line capacity. Fast trains will "catch up" slower trains ahead, while a slow train starting out closely behind a faster train will follow a progressively larger distance behind as it travels down the line. This effectively makes some of the line capacity unusable. Either trains must be timetabled so that the "catching up" does not occur or faster trains will have their speed reduced and journey time extended by a slower train ahead. There is thus a compromise between line capacity and the attainable speed of the faster trains. Consider the case of two passenger trains running at 90 km/h over a 30 km section of line, with an intervening 60 km/h freight train. Figure 6 Time -Distance Graph The freight train will take 10 mins. longer than the faster passenger trains, so if the headway of the line is 2 mins, then the "fast to slow" headway will be 12 mins. Mixing dissimilar services in this way can lead to a very low line capacity. The situation can be improved by either running similar trains together in groups ("flights"), or by providing loops or sections with additional running lines to allow slower trains to be overtaken. The best line capacities are always obtained on those lines where all trains perform identically. 10. SINGLE LINES 10.1. Calculation of maximum capacity The maximum number of trains that can use a single line is set by the number of crossing(passing) loops: Figure 7 Single Line with Passing Loops T he capacity of a single line is set by the spacing between loops. If the loops are not equidistant, then trains will be delayed awaiting entry to the longest single-line section, which will determine the minimum headway for the whole line. Figure 8 Passing loop case In the example in Figure 8 , once train A has cleared the section, train B has to travel a distance (d + D + S + L) before train C can follow A, which sets the minimum crossing time between trains in alternate directions. The headway between following trains on an alternating service will be twice this time. 10.2 Additional Factors If trains have to stop at the crossing loops, this can increase the crossing times due to the delay in accelerating and decelerating. For trains to run through a loop without significantly reducing speed, we must ensure: The Loop is long enough Running times between loops are as near as possible the same for each Signals are suitably positioned with free overlaps. The method of signalling does not require trains to stop (eg to exchange tokens). Unfortunately, it is often the case that loops and stations occur together, so trains are required to stop anyway. 10.3 Single Line Section on a Double Line Railway Where a stretch of single line is necessary in an otherwise double line, this can seriously affect the capacity of the line. However, this effect can be minimised by providing extra signals at line headway throughout the single line section, and "flighting" groups of trains through in each direction. For example, if the crossing time is 15 mins, then 4 trains per hour can be run on .an alternating service; however, if it is possible to run trains in flights with a 5 minute headway between trains, then 6 trains per hour can be achieved with 2 trains per flight, or 8 trains per hour with 4 trains per flight. 10.4 Effect of Gradients Many single lines occur in rural and/or mountainous areas where steep gradients prevail. Gradients can affect both line capacity and the speed of trains. Let us examine a line with ideally spaced (equidistant) passing loops operating at or near full capacity, with a train passing one in the opposite direction every time it reaches a passing loop. The line is on a severe gradient and it takes 10 minutes for a train to clear a single line section downhill but 15 minutes for a train going uphill. Although the downhill train may reach the next passing loop in a shorter time it cannot proceed further as there is a train in the single line section coming the other way. The effective capacity of the line is therefore only 2 trains per hour. In addition, the effective speed of the downhill train is no better than that of the uphill train. It simply spends more time waiting at passing loops. The only solution to this problem, where it is necessary for the capacity of the line and the speed of the trains and can be financially justified, is to provide additional lengths of double track over the most critical sections. These will generally be those with long continuous gradients in one direction. 11. TERMINAL STATIONS The capacity of lines entering and leaving a terminal station should ideally be twice that of the main lines served by the station. This is because some arriving movements will completely block the station approach to departing movements and vice-versa (Refer Figure 9). In practice, it may not be possible to provide this capacity. Careful planning of the timetable to maximise the number of parallel arriving and departing movements may give some improvement. However, if a terminal station is operating near to its maximum capacity, late running of a small number of trains can quickly disrupt the entire service. Figure 9 11.1 Signals Approaching the Station At most busy stations the signalman will only be able to set an incoming route shortly before the arrival of the train. To keep trains moving, the driver should see the most favorable aspects possible on the approach to the station. The final signal will of course always be a single yellow. If possible, speed restrictions which reflect the actual attainable speeds should be applied on the approach to the station to permit the closest possible signal spacing. Remember that all trains will be braking in order to stop .at the terminus. On the layout shown in figure 9 , assuming 3 aspect signals approaching the terminus the headway from the signal in rear of signal 2 will be determined by the time taken for the rear of the train to clear the relevant points (103 for the move shown if the following train is destined for platforms 1,2 or 3, 100 for platform 4). However, this time may vary significantly for each platform. The worst case should be used. 11.2 Signals Leaving the Station Platform starting signals are invariably provided. For braking purposes, the first signal after leaving the station can be as close as necessary to the platform starting signals as all trains will be starting from a stand. For operating convenience, the next signal should be at least a train length beyond the platform ends (so that trains will always completely clear the platform on departure). If possible there should be standing room for the longest · train clear of all points and crossings. Another platform starting signal cannot be cleared until the rear of a departing train has cleared the overlap of this signal. Headway requirements may have to override the desire to provide standing room. Platform starting signals are usually 3 aspect (making the first TWO signal sections 3 aspect). This increases the likelihood of a train departing with an unrestricted green (clear) signal and clearing the station as quickly as possible. A more restrictive aspect might encourage the driver to make a slower departure and is not in any case necessary for braking. The spacing of the first few signals leaving the station should be as close as possible. Speed restrictions based upon the attainable speed of the trains rather than the design speed of the track will enable the signals to be more closely spaced.
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CH6 | ASPECT SEQUENCES
SIGNALLING BOOK | CHAPTER 6 CONTENTS Introduction Plain Line Sequence Selection of Aspects Diverging Junctions Converging Junctions Transition between Aspect Sequences Isolated 4-aspect Signals INTRODUCTION Control tables contain details of the aspects displayed by each signal and the conditions under which they are displayed. To simplify testing, it is common to show the conditions for which each signal displays each of its aspects on a single document. As the final testing of signal aspects requires several persons to observe the aspects of a succession of signals the aspect sequence chart or table is of great assistance to the person coordinating the testing of aspect sequences. The aspects of shunting and subsidiary signals are not shown as they convey no information regarding the aspect of the signal ahead or the distance for which the line is clear. Similarly BR practice has Call On Route ( To send another train to share platform ,already occupied with another train),Proceed On sight Class (When Track equipment status is unknown ,but route reserved in interlocking and point are in desired position ) are not covered. The text of these notes will refer to both British and NSW- New South Wales of Australia (Former State Railway Authority /Railcorp) aspects. New South Wales of Australia has two operators Transport for New South Wales (here by will be referred as TfNSW ) and ARTC (Australian Rail Track Corporation ) exclusively for freight operation. PLAIN LINE SEQUENCE For any signal showing a stop or danger aspect, there will be one or more aspects preceding it, warning of the need to stop. The aspect sequence information should already have been shown on the control tables. It can, if necessary be derived independently according to the following simple rules. Measure back full-service braking distance from each signal at stop. The next signal outside this distance (still measuring back) will display the first caution to be seen by the driver. If there are no other signals between, this will be a caution (British single yellow) aspect. If there is one signal between, this will show caution and the signal outside braking distance will show preliminary caution/medium. British practice does not permit repeated cautionary aspects of any type but, for NSW signalling, if there are two or more signals between the first signal outside braking distance and the signal displaying stop, the medium aspect will be repeated as necessary, and a caution aspect will precede the stop. Move forward to the next signal and repeat the process. When the aspect sequence is complete, a full set of aspects should be written down for each signal. To establish the conditions for a particular aspect to be displayed (e.g., when testing) the line from that aspect should be followed forward to its conclusion to show the aspects of signals in advance. Where low speed and/or conditional caution (warning) aspects are required, the aspect sequence should distinguish between the two by reference to the different overlaps and/or the clearance conditions required. Refer Below diagrams for normal two, three and four aspect sequences. Examples of conditional caution, low speed and repeated medium aspects are also shown. Figure 1: Normal Two Aspect Sequence -New South Wales, Australia Figure 2: Normal Two Aspect Sequence -British Figure 3: Normal 3 Aspect Sequence (Single light)-New South Wales -Australia Figure 4: Normal 3 Aspect -British Figure 5: Normal 4 Aspect Sequence (Double Light) -New South Wales -Australia Figure 6: Normal 4 Aspect Sequence -British Figure 7: Conditional Caution and Low Speed Aspects -New South Wales -Australia Figure 8 Repeated Medium Aspects (Closely Spaced Signals) SELECTION OF ASPECT It will be helpful to know how an Engineer decide 2 aspect or 3 aspect or 4 Aspects are suitable for the line in a brief. However this has been detailed in previous chapter headways. Please refer article HEADWAYS in RailFactor.First caution aspect for a stop signal is installed in advance at braking distance from the stop aspect. This is a trade off between Safety & Service. Safety is the minimum required separation between two signals which must be minimum at breaking distance (S) and must not be greater than 1.5S for a 2 aspect and 3Aspect signal . DIVERGING JUNCTIONS Depending on the type of signalling indication of a route diverging from the main line must be given at the junction signal and may also need to be given at the previous signal or signals. A separate line of the aspect sequence will be required from the signal at which the aspect sequence for the diverging (turnout) route differs from the plain line sequence. British practice requires approach control where a significant speed reduction is required. For Example, a junction turn out where approach locking starts when cleared, train is brought to a signal on low speed displaying red aspect and clears junction indicator as train approach the signal). This should be noted on the aspect sequence chart. NSW, Australia does not use approach control for junction signalling so the aspect sequence is correspondingly simplified. If a junction signal is showing a proceed aspect for the turnout. the previous signal will display medium. It is recommended that the braking distances are checked to ensure that the required speed reduction can be achieved before the turnout. If not, two options are available; the medium aspect could be repeated or the aspect sequence leading up to the junction signal should be the same as that for the signal at stop. It should be noted that normal NSW, Australia practice normally requires the use of the medium aspect for junction signalling even where it is not used for the through route. The following diagrams show the aspect sequences for a junction signal on lines signalled with three and four aspect signals for the main line. The aspect displayed by the junction signal should refer where necessary to any turnout route or junction indicator displayed as this will need to be checked when testing the aspect sequence. Figure 9: Junction Signalling (4 Aspect -Double Light) -NSW Australia Figure 10: Junction Signalling (4 Aspect) British Figure 11: Junction Signalling (3 Aspect -Single Light) -NSW Australia CONVERGING JUNCTIONS Because all trains at converging junctions must make the same speed reduction, and the driver is expected to know this, no special provisions are necessary for converging junctions. The sequences will combine at the first signal beyond the junction. Figure 12: NSW-Australia 3Aspect Sequence at Converging Junction (Single Light ) TRANSITION BETWEEN ASPECT SEQUENCES The transition between different aspect sequences sometimes causes difficulties, for example when running from a 3-aspect line to a 4-aspect line. The engineer must decide exactly where the transition occurs. This should of course have been considered at the time the signalling plan was produced to be able to decide the possible aspects displayed by each signal. The simplest rule to ensure getting the aspect sequence correct is to start from the stop aspect and work back. There should be no caution aspects further back than the first signal at or beyond braking distance. Where a 3-aspect line leads on to a 4-aspect line at a junction, the only reason for carrying the 4-aspect sequence back on to the 3-aspect line is if the signal spacing is inadequate for 3 aspect over the junction or the first section past (taking into account any speed restriction over the junction). Only use a medium aspect if it is necessary to obtain adequate braking (or to warn if a turnout ahead) not simply because the signal can display a medium. With NSW, Australia practice, the aspect sequences through diverging junctions will be similar, regardless of whether plain line aspect sequences are 3 or 4 aspects. In British practice, the aspect sequence should be maintained through the junction (together with any route or junction indicators) according to the aspects used on the diverging route. A junction signal reading on to a 3-aspect line will usually only display yellow (caution) or green (clear) for the turnout. Figure 13: NSW-Australia 3 Aspect to 4 Aspect Transition - Figure 14: NSW-Australia 4 Aspect to 3 Aspect Transition ISOLATED 4 ASPECT SIGNALS At certain locations (approaching stations for example) it may be necessary to position two signals closer than braking distance in what is otherwise a 3-aspect sequence. The remaining signals are all at least braking distance apart. In the example shown on Figure 15, there is insufficient braking distance from 105 to 107. Note that 103, the signal before the section shorter than braking distance is the 4-aspect signal. Note also that when 107 changes from stop to caution, 105 and 103 will both change to a clear aspect together. Figure 15: NSW-Australia Isolated 4 Aspect Signal (Single Light)
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CH8 | LEVEL CROSSINGS
CONTENTS 1. INTRODUCTION 2. LEVEL CROSSING MODERNISATION 3. AVAILABLE TTYPES OF LEVEL CROSSING 4. MANUALLY CONTROLLED CROSSINGS 5. AUTOMATIC CROSSINGS 6. OPEN CROSSINGS 7. OTHER VARIATIONS 8. WESTERN AUSTRALIAN LEVEL CROSSINGS 9. SINGAPORE LEVEL CROSSING FOR FIRE VEHICLE/MAINTAINER ACCESS 10. LEVELCROSSING PREDICTORS 1. INTRODUCTION One of the early problems encountered by railway engineers was that of crossing existing roads. The operators and the engineers are very fortunate if all crossings can be achieved by the construction of bridges. The level crossing was however a cheap and effective means of dealing with the problem: With the increases in speed and volume of both road and rail traffic, level crossings may cause greater operational problems. However, geographical and cost factors may require many level crossings to be retained. Level crossings have the following disadvantages:- a) They often require additional staff to operate. b) They can reduce line capacity and increase the risk of delays to rail traffic. c) They are an additional safety risk. d) They may be unpopular with road users. When railways were first built. the type of level crossing protection provided varied according to the terrain, the type of train service and the density of population. Political considerations were also significant. In countries like the United Kingdom where centres of population were already established and most land was privately owned, there was generally an obligation on the railway companies to fence off the railway. In Australia, no such obligation exists. Many lines in sparsely populated areas will not be fenced. Early level crossings in the UK therefore consisted of gates which could be placed across the road or the railway to protect one from the other. Many early level crossings in Australia were totally unprotected. There was not the need or the available finance to provide anything more. Looking at the UK example, therefore, most level crossings required an operator or attendant to operate gates across the full width of the roads.Nowa days , most gates have been replaced by lifting barriers. This section will deal with the basic requirements of level crossing protection and ways in which level crossings can be made more economic and efficient in operation. Because most countries have extensive regulations to deal with the control of road traffic, details of crossing layout and construction and the operation of specific types of equipment are not covered in these notes. The general principles of operation of the main types of modem level crossing from the railway operating viewpoint will be dealt with. 2. LEVEL CROSSING MODERNISATION On most railways, the signal engineer is responsible for providing any level crossing protection other than the provision of basic warning signs. There will normally be pressures on the signal engineer to improve the level of protection and/or reduce operating costs. Any equipment provided must, of course, be safe and reliable. Operation of level crossings can be very expensive. In recent years British Rail bas been engaged in an extensive programme of level crossing modernisation. The main factors to justify such a programme are given below. 2.1 Staff Savings This is probably the main reason for modernisation. Where local conditions require the road to be closed across its whole width, some form of human supervision is essential to check that the crossing is completely clear before permitting trains to pass. Instead of providing a local attendant, closed circuit television will permit a signalman or crossing attendant to supervise one or more remote level crossings, often in addition to one adjacent to the crossing/signal box. Alternatively, it may be possible to automate the operation of the level crossing. Some form of local or remote monitoring for correct operation is still required. The level of monitoring for correct operation will depend on local circumstances. In the UK, levels of road and rail traffic require continuous monitoring. Any failures could have a serious impact on the safety and flow of traffic. The Australian approach is to perform a daily inspection or test. In remote areas, the person performing this test may not necessarily be a railway employee. The availability and cost of available persons could lead to some form of remote monitoring being considered in the future. 2.2 Improvements in Line Capacity Manually controlled crossings, whether gates or barriers, are interlocked with the signals. If the driver of an approaching train is not to see a restrictive aspect, the crossing must be closed and the signals cleared some time before the arrival of the train. It may not be possible to open the road to traffic between closely following trains. This may cause severe delays to road traffic. Conversely, leaving the road open for sufficient time to clear a backlog of road traffic may delay an approaching train. Assuming that the options to close the road or to build a bridge have been discounted, the only solution is to reduce the road closure time. This can be done by removing the interlocking with signals and operating crossings automatically. The crossing is then only closed for a short period before the arrival of each train until it bas completely cleared the crossing. To ensure safety, road traffic must not be obstructed on the exit side of the crossing. 2.3 Improvements in Safety The use of barriers is inherently much safer than gates, partiatlarly if used in conjunction with road signals. Opinions differ on the effect of automatic crossings on safety. The reduction in road closure time obviously reduces traffic congestion and gives road users less cause to disobey the road signals (regular users will know that the road will only be closed for a short period). As there are never barriers on the exit side of the crossing, road vehicles and pedestrians cannot get trapped on the crossing. However, the removal of interlocking with signals may also remove the opportunity to stop a train in sufficient time if the crossing becomes obstructed. In all cases road users must be disciplined to obey the signs and signals on the approach to the crossing. Pedestrians may be very diffiatlt to control where there are no barriers or half barriers. The problems will often vary according to the culture of the country. In the UK, level crossing automation has often been perceived by the public as a reduction in protection because the local attendant' is no longer visibly in charge of all traffic. In addition road users do not appear to pay the same regard to road traffic signals as railway personnel do to their signals. In countries having a large number of unprotected open crossings, any form of protection is seen as an improvement. In the UK a large quantity of statistical information has now been built up which appears to indicate that automatic half barrier crossings are in fact very safe (as compared with other types) regardless of the volume of road traffic. Automatic open crossings, to achieve a similar level of safety, must be restricted to situations with lower road traffic density and/or speed. These findings may not always be applicable to other countries. As an example, one of the problems of open and automatic crossings in the United States is that of trying to beat the train to the crossing, regardless of any road signals which may be displayed. This is probably because a long, slow moving train may block the crossing for several minutes (a train 2km long running at 15km/h would block a crossing for over 8 minutes!). The provision of barriers may also vary. Australian practice is to provide half barriers where the road crosses two or more tracks, as a physical reminder to the road user when two trains approach the crossing at the same time. On a single track railway, where this problem does not arise, barriers are not normally provided. In most cases the public perception of level crossings will influence the amount of government regulation. As a minimum, there are usually certain standard road traffic signs which need to be erected. In the UK, government regulation extends to the determination of the type and layout of each level crossing on an individual basis. Any alterations to operation or appearance also have to be approved. 3. AVAILABLE TYPES OF LEVEL CROSSING Modem level crossings can be broadly divided into the following categories:- a) Manually worked, normally with full or half barriers according to local requirements and/or legislation. With local attendant Remotely supervised (closed circuit television - CCTV) User worked Operated by train crew b) Automatic (half barriers or open - no barriers). Remotely monitored (from adjacent signal box) continuously Locally monitored (by driver) with the passage of each train No continuous monitoring but regularly tested and inspected. In the UK, this type of crossing would not be permitted. The period before a failure would become apparent is considered unacceptable. c) Open - no road or rail signals - suitable warning notices only. While some types may be used regardless of traffic density or speed, others have slight or severe practical restrictions on their use. The following descriptions are based on UK practice. 4. MANUALLY CONTROLLED CROSSINGS The most common type is the Manually Controlled Barrier (MCB) which may be either locally controlled or remotely supervised using CCTV. The crossing is directly interlocked with all signal routes over the crossing. The signalling layout for a typical MCB installation is shown on Figure 1 The main features of the MCB crossing are as follows:- Barriers across the full width of the road.2 or 4 barriers may be provided dependent on the width of the road. Lowering of the barriers will be preceded by operation of road traffic signals. On lines with overhead electrification, the barrier arms will normally be earthed. An audible warning will be provided for pedestrians from the start of the operating sequence until the barriers are fully lowered. Figure 1 MANUALLY CONTROLLED BARRIER (MCB) LEVEL CROSSING Although not desirable, overlaps may extend over the crossing without requiring the barriers lowered provided the signal is at least 50m (25m if a platform starter) from the edge of the crossing. Routes may be set while the barriers are raised. Signals will not clear until the barriers are fully lowered and the crossing is clear. The signalman/attendant must operate a special "crossing clear" button for this purpose. The signalman/attendant will have an indication of road signals operating and barriers lowered on his control panel (or equivalent). Signals will clear for one movement only. Under appropriate conditions, facility may be provided to lower the barriers automatically. In most cases, an automatic raise facility is provided which operates as soon as the train bas cleared the crossing and the signal approach locking is released (provided no other routes have been set). Crossings supervised by CCTV are provided with a Local Control Unit (LCU) which permits local operation in the event of CCTV failure or maintenance or for other engineering work. When the LCU is in use the signals are maintained at danger. In general, a signal passed at danger will immediately operate the road traffic signals. Safety of MCB crossings is ensured by: Interlocking with signals. Detecting the barriers down and the road signals operating. Provision of a separate "crossing clear" button. Maintaining the barriers down until the approach locking on all protecting signals is released and the crossing is clear of trains. The MCB is generally the most expensive type of crossing to provide. In the UK there were no restrictions on its use. 5. AUTOMATIC CROSSINGS Automatic crossings will generally have no barriers or half barriers. This is to ensure that vehicles and pedestrians do not become trapped on the crossing. They will always be provided with road traffic signals. In general, the operating sequence will be timed so that at least 27 seconds (UK practice, determined by government regulation) elapses from the start until the arrival of the fastest train. This timing is calculated from the operating sequence of the particular type of road traffic signals in use, the lowering time of the barriers (if any) and a suitable margin of time before the train reaches the crossing. It could therefore vary for other types of road signal and/or barrier equipment. The crossing will reopen to road traffic provided:- a) The train is clear of the crossing. b) The crossing can remain fully open to road traffic for at least 10 seconds after the passage of the train. Therefore, if another train is approaching the crossing within this period, the crossing will remain closed to road traffic until both trains have passed. Automatic crossings may be monitored by an adjacent signal box (remotely monitored) of by the driver (locally monitored). At locally monitored crossings a flashing white light indicates to the driver that the road signals are operating. Provision is generally made for local control, to cover periods of maintenance, failures or track maintenance in the vicinity of the controlling track circuits. Local control may also be necessary in the event of planned or unplanned single line working on a double track railway. It may, however be cost effective to equip crossings for bi-directional working on all lines. The provision of the additional circuitry could well be more economic than the cost of providing crossing attendants for single line working. 5.1 Automatic Half Barrier Level Crossing (AHB) The automatic half barrier crossing is the earliest and most widespread automatic crossing in the UK. Each barrier is pivoted on the left hand side (for left hand road traffic) and covers slightly less than half the width of the road. It is monitored from an adjacent signal box. A dedicated telephone circuit and indications for barriers and power supply are provided. Operation can be initiated by track circuits, treadles (or a combination of both). The running on or "strike-in" end of the track circuit may be provided with a welded stainless steel strip on the rail surface to protect against bad contact due to rust The simplest arrangement for the AHB crossing is on a single line. On most single lines there is no possibility of a second train striking in before the crossing has been open for 10 seconds. Therefore, the operation of the crossing is initiated by a track circuit approaching the crossing from either side becoming occupied. The crossing will remain closed to road traffic until the train has cleared the crossing. A treadle may be provided at the crossing to safeguard against false operation of the track circuit by proving that the front of the train has reached the crossing. The controls are more complicated in the case of a double line. In the example on Figure 2, the crossing operation is initiated by timed occupation of the approach track circuit. Occupation of the other approach track circuit while the crossing is closed to the road will maintain the crossing closed until both trains have passed. The example also shows the provision of emergency replacement on the automatic signal approaching the crossing. There must be a minimum of 50 metres and a maximum of 10 minutes between a stop signal and the crossing. The signal must either be a controlled signal or an automatic signal with an emergency replacement facility. In the opposite direction, a station is located next to the crossing. Special arrangements are necessary if the road is not to be closed for an excessive length of time by a stopping train. The signalman may select between an operating sequence for a stopping or a non stopping train. For a non-stopping train the platform starting signal clears immediately and the normal sequence of operation will apply. For a stopping train, the signal will be maintained at danger for a suitable time (to allow for the station stop). Crossing operation will commence before the signal clears. The signal will clear so as to permit the minimum road closure time before arrival of a train starting from the platform. There is no restriction on the volume of road or rail traffic. The speed of rail traffic must be below 100 mph (160km/h). To limit the time for a road vehicle to cross, a maximum of 2 running lines and 2 other lines are permitted. If any of these conditions cannot be fulfilled or the crossing and approaching road layouts are unsuitable, the MCB type of crossing must be used. Figure 2 AUTOMATIC HALF BARRIER (AHB) LEVEL CROSSING If signals are located within the "strike-in" distance of the level crossing, the controls can become very complicated. The crossing operation must not commence unless a route has been set (or an automatic signal can show a proceed aspect) over the crossing. The clearance of such a signal may need to be delayed to ensure adequate crossing closure time. If a train passes a signal at danger, crossing operation should commence immediately. If a signal is replaced to danger·after showing a proceed aspect and the train is successfully brought to a stand at the signal, the crossing may be opened after the approach locking bas been released. 5.2 Automatic Open Crossing Remotely Monitored (AOCR) This type of crossing is effectively an AHB without barriers. Due to the absence of barriers, its use is restricted to situations where the road traffic is very light. The crossing is also restricted to a maximum of 2 lines and a line speed of 75 mph (120km/b). Additional safeguards due to the absence of barriers are:- a) An illuminated "Another Train Coming" sign which operates as the first train reaches the crossing when two trains are to pass over the crossing before it reopens to road traffic. This is to ensure that road users do not assume it is safe to proceed (or fail to check the main road signals) after the passage of the first train. b) In conjunction with the operation of the "Another Train Coming" sign, the audible warning will change in pitch. 5.3 Automatic Open Crossing Locally Monitored (AOCL) Instead of providing indication and telephone circuits to a remote monitoring point, many crossings can more economically and effectively be monitored by the driver as be approaches the crossing. One vital provision is that be must be able to stop the train short of the crossing in the event of any failure of the crossing equipment or obstruction of the crossing. A flashing white light facing in each direction of rail traffic is provided at the crossing which operates when the road signals are operating correctly. The speed of approaching trains must be restricted so that the driver can stop short of the crossing if the white light fails to operate (or if the crossing is obstructed). Warning boards are provided on the approach to the crossing. An overall maximum speed limit of 55 mph (88km/b) or lower is applied to ensure adequate sighting. If there is a station on the approach side of the level crossing where trains normally stop then ALL trains must stop to ensure correct operation of the crossing. This is normally enforced by a stop board although a signal could be employed instead. It is thought that the provision of a Main signal and a flashing white light signal in the same place could cause confusion. Some crossings therefore exist where the proceed aspect of the signal performs the function of the white light. All trains will initially be brought to a stand by the signal at danger. There is some restriction on the volume of road traffic for which an AOCL is suitable. This is not as severe as for an AOCR. Figure 3 AUTOMATIC OPEN CROSSING ,LOCALLY MONITORED (AOCL) If the train does not reach the crossing within a reasonable time the crossing will reset to open the road. This is quite safe because the driver's white light will already have been extinguished and the driver will therefore be prepared to stop (if a train is actually on the track circuit it will obviously be travelling very slowly). This is a useful safeguard against track circuit failure causing serious road traffic delays. 5.4 Automatic Barrier Crossing Locally Monitored (ABC-L) This is a new addition to the available types of level crossing. It has the operational advantages of the AOCL but is also provided with half barriers. It is therefore suitable for situations with heavier road traffic. Operation is the same as for the AHB. The flashing white light operates when the road signals are operating and the barriers have commenced to lower. Proving the barriers down would effectively reduce the train speed over the crossing (due to the longer operating time and the effective upper limit on sighting) or increase crossing closure time. Automatic reset facilities are provided similar to the AOCL 6. OPEN CROSSINGS On some lines it may be acceptable for all trains to severely reduce speed at a level crossing. If both road and rail traffic are low, the provision of an Open Crossing (without road signals) may be adequate. Suitable road signs are provided on the road approaches and a warning board at braking distance on the rail approaches. A speed restriction of 10 mph (16 km/h) applies to all trains. Road traffic is instructed to give priority to rail traffic. Train drivers must ensure the crossing is clear of obstruction before proceeding. This type of crossing is suitable for single lines only. There are no signals therefore no warning can be given of the approach of a second train. 7. OTHER VARIATIONS On crossings where either the road or rail traffic is very infrequent, other alternatives may be used. If the railway crosses a private road with generally a small number of regular users and protection is considered necessary due to the frequency of rail traffic or the approach view of rail traffic, the crossing may be a barrier or gate crossing operated by the user. Telephone communication and/or warning signals to indicate an approaching train would normally be provided. The gates or barriers would normally be left closed across the road. If it is acceptable for the trains to stop, the crossing may be operated by the train crew. At least one other person in addition to the driver is desirable - the train will have to stop, set down the crossing operator, who then closes the crossing to the road, proceed over the crossing and stop again to pick up the crossing operator after he has reopened the crossing. This method of working will generally not be acceptable for a regular passenger service. 8. WESTERN AUSTRALIAN LEVEL CROSSINGS TransPerth network is mainly electrified hence predictors arenot type approved and level crossing is controlled with Track circuits .There are controlled level crossing for Road Traffic and separate pedestrian crossing Roads are equipped with half boom barriers ,warning flashig light and audible alarm and pedestrian crossings are equipped with electronically controlled swing gate . There are active level crossing with out half boom as well but protected with audible alarm and visual warning lights.All the requirements are in compliance with the Australian Stanadard AS 1742 8.1 Level Crossing Protected with Flashing Light Signals In its quiescent state if no train is detected approaching or passing over the level crossing,flashing light warning signals will be extinguished and audible warnings will be silent. If a train is detected as approaching the level crossing within the approach area the flashing light warning signals will commence and continue to flash alternately and the audible warning will commence and continue to operate. When the rear of the train passes clear of the road area of the level crossing, the flashing light warning signals will become extinguished and the audible warning will be silenced. 8.2 Level Crossings Controlled by Flashing Light Signals, Half-Boom Barriers and Audible Warning In its quiescent state where no train is approaching or passing over the level crossing, all flashing light warning signals will be extinguished, the half-boom barriers will be in the fully raised position and audible warnings will be silent. If a train is detected as approaching the level crossing within the approach area, then the flashing light warning signals will automatically commence and continue to flash alternately and the audible warnings will commence and continue to operate. After a predetermined period (normally a minimum of 6 seconds) the half-boom barriers will commence descent.After a predetermined period (normally 10-12 seconds) the half-boom barriers will reach the fully horizontal position and all of the audible warnings will be silenced unless there is a designated pedestrian crossing. After the minimum design warning period, the front of the approaching train will reach the level crossing. The minimum warning time for all new boom barrier installations will be 25 seconds. When the rear of the approaching train passes clear of the level crossing then both the halfboom barriers willl commence to rise and any audible warning will be silenced. When both half-boom barriers reach the fully vertical position, the flashing light warning signals will become extinguished. In multiple track level crossings, if a second train is approaching the level crossing on another track, as the rear of the first train passes clear of the level crossing, and if there is insufficient time for the half-boom barriers to rise and remain in the fully raised position for the predetermined minimum road opening time (normally 15 seconds) then they remain lowered until the rear of the second train has also passed clear of the level crossing. 8.3 Pedestrian and Cycleway Level Crossings Controlled by Lights and Audible Warnings Only If no train is detected as approaching or passing over the pedestrian level crossing then the warning lights will be extinguished and audible warning devices will be silent. If a train is detected as approaching or passing over the pedestrian level crossing then the warning lights will display and flash red warning lights and audible warning devices will commence and continue to sound. When the rear of the train passes clear of the pedestrian level crossing then the warning lights will become extinguished and the audible warning devices will be silenced. 8.4 Pedestrian Level Crossings Controlled by Lights and AutoLocking Gates If no train is detected as approaching or passing over the pedestrian level crossing then the warning lights will be extinguished, the gates will be fully open and the audible warning devices shall be silent. If a train is detected as approaching or passing over the pedestrian level crossing, then the warning lights will display and flash red lights and the audible warning devices will commence and continue to sound. a. After a predetermined period the gates commence to close. b. After a predetermined period the gates will be fully closed. One or all of the audible warning devices may be reduced in level. c. After the predetermined minimum period the front of the approaching train will reach the level crossing. When the rear of the approaching train passes clear of the level crossing then the gates shall commence to open, the warning lights will become extinguished and the audible warning devices will be silenced. If a second train is approaching the level crossing as the rear of the first train passes clear of the level crossing and there is insufficient time for the gates to open and remain in the fully open position for a predetermined period before commencing to close for the second train then they remain closed until the rear of the second train has also passed clear of the level crossing. 9. SINGAPORE LEVEL CROSSING FOR FIRE VEHICLE/MAINTAINER ACCESS Singapore SMRT operate moving block Grade of Automation 4(GoA4) with a designed headway of 88 seconds maninly on elevated track or tunnled Track with a bit of at grade track .It is practically impossible to maintain a normally open level crossing .They do have test track with design speed 80kmph and unmanned depot operation with operational speed of 18km/hr.Recently built Thomson East Coast Line Depot has an at Grade depot (Mandai Depot) with a test track where access is given through normally closed level crossing .There are four types of such crossing used for safe passage of fire engine ,maintainer and drivers on emergency. 9.1 Type 1 Low speed levelcrossing with gates ,normally closed to road traffic ( Fire Engine & Train Delivery Road ) These are slow speed levelcrossing with gates ,normally closed to road traffic.This type of crossing is used for those level crossings that are occasionally used by road traffic and in depot only. They are suitable for Train Consists travelling up to 18 kph.The gates are electrically detected as closed and locked by double pole SIL 4 detection switches.When gates are detected not closed and locked, the signalling system will safely stop Train Consists which are routed to the level crossing and When gates are detected not closed and locked, the signalling system will safely stop Train Consists which are routed to the level crossing.Train Consists are not allowed to stop on the level crossing. When the gates are detected as closed and locked after the gates are detected not closed and locked, the signalling system prompt the operator at the Depot Control Centre to confirm that train operation at the level crossing can be resumed and operator can remotely request train operation at the level crossing to resume. When the resume train operation request is received, the signalling system will safely check that the gates are detected closed and locked before allowing train operation at the level crossing to resume.Indications are provided for the depot controller as below (a) Gates not closed and locked indication (b) Gates closed and locked indication (c) Prompt to confirm train operation to resume 9.2 Type 2 Slow speed Level Crossing with gates, normally closed to human traffic This type of crossing is used for those level crossings that are occasionally used by human traffic and in depot only. They are suitable for Train Consists travelling up to 18 kph.It is equipped with three-position spring loaded local switches at each side of the level crossing. The three positions of the switches are (a) Request To Use Crossing (b) Normal position (c) Cancel Request to Use Crossing When the switch is set to Request to Use Crossing to cross, the signalling system will safely stop Train Consists which are routed to the level crossing. Train Consists are not allowed stop on the level crossing. When trains have been stopped from approaching the level crossing a safe to proceed lamp at each gate will be lit.t has facilities at the Depot Control Centre to allow the operator to remotely request train operation at the level crossing to resume. When the switch is set to Cancel Request to Use Crossing ,system will prompt the Depot Control Operator to resume train operations and the safe to proceed lamps are e extinguished. Indications at the DCC are : (a) Request to Use Crossing (b) Cancel Request to use Crossing (c) Prompt to confirm train operation to resume 9.3 Type 3 Level Crossing with no gates, normally open to road traffic This type of level crossing shall be used where road traffic across the level crossing is moderately frequent. Its use are restricted to cases where rail traffic across the level crossing is restricted to a maximum speed of 18 km/h. Track circuits as required are utilised for the operation ,along with warning light and audible alarms at either side of the level crossing .Operating Principles are as below (a) When a signalled route is set across the road level crossing, and the berth track circuit to the signal is occupied, the warning lights will flash red along with audible alarm. (b) The railway signal of the route that has been set across the level crossing will not clear until the flashing road crossing signals have been proved illuminated for a pre-determined time. (c) The failure of one lamp of each road crossing signal shall still allow the relevant railway signal(s) that read over the level crossing to clear and failure is alarmed to the Depot Control Centre (d) The failure of both lamps of one road crossing signal will prevent clearance of the relevant railway signals that read overthe crossing. This failure is alarmed to the Depot Contrl Centre. (e) If a route is set across the level crossing, when the berth track circuit to the signal is clear, the level crossing warning lights and audible alarm will not be initiated and the signal will remain at red. When the berth track circuit to the signal becomes occupied, warning lights and audible alarms are initiated as described in (a). (f) In case the Train Consist passes a red railway signal before travelling over the road crossing, the road level crossing warning lights and audible warning will be initiated when any track circuit between the signal and level crossing are occupied. (Unless Train Consist is routed away from level crossing). There are road warning light indication provided to the Depot Controller 9.4 Type 4 Level Crossing with gates, normally closed to road traffic integrated with Fire Alarm Signal This type of crossing is used for those level crossings that are occasionally used by road traffic and in depot only. They are suitable for Train Consists travelling up to 90 kph, e.g. level crossing of test track in the depot track in depot.This gates are equipped with electrically released locks ,which can be electrically detected in closed and locked position with double pole detction switches .Locks are controlled by signalling system "Gates Locked" (RED) and "Gates released" (Green) lamps are provided on each side of the gate .Depot Controller can remotely release the switch to unlock the gate at same time each side of level crossing has three position spring loaded locake switches and the positions are (a) Gates release: to request the gates to be unlocked (b) Normal position (c) Gates lock: to request the gates to be locked Appropriate Fire Alarm signal is received by the interlocking which command to release the lock automatically 10. LEVELCROSSING PREDICTORS These are the relatively new trends in level crossing .Signal engineers releaized that if the train driver dont maintain the allowed speed limit and its possible train can reach the level crossing island much later that required also driver is suppose not to excced his alllwed speed limit for the train to reach the island earlier . Thease are potential threats with the track circuit based controlls.Engineers thought of detecting the speed of the train when it strikes the warning point and activate the crossing accordingly to avoid such threats .Not forgetting the fact that driver cannotexceed the speed after his train strikes the point of level crossing activation Hence a btetter equipped predictors come into existence .It used Narrow band shunts ,wide band shunts to make it accurate .GCP of Siemens(Former Westinghouse) and XP4 of Alstom (Former GE) are well know level crossing predictors .We will discuss level crossing tedictors in a separate chapter with logic ,circuits and settings.
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CH9 | TRACK CIRCUITS
TRACK CIRCUITS CONTENTS Introduction Principles of Operation Practical Considerations Insulated Block Joints Track Circuit Types D.C. Track Circuits AC. Track Circuits Jointless Track Circuits Impulsing Track Circuits Rail Circuits and Overlay Track Circuits Shunt Assisters Alternatives to Track Circuits INTRODUCTION The track circuit is an important component of modern signalling systems. Its main purpose is to continuously detect the presence and position of traffic within the signalled area. Track circuits are electrical circuits formed in part by the rails of the track. The detection of vehicles is possible because their wheels and axles reduce the rail to rail resistance significantly below the value relating to a clear track condition. Eventhough modern communication based train control system make use of track installed transponders otherwise know as balises or beacon (French) as a primary means to identify the position,over laid secondary train systems are implemented (Can be axle counter as well) to recover the train ,if communication is lost and primary train detection is impossible. The first track circuits were used only as indication devices, to remind the signalman of the presence of a train, for example standing at the home signal or on a section of track which might be obscured from his view. Later they were used to lock facing points, as a replacement for fouling bars. In this role, the track circuit directly controlled the operation of the point lever through an electric lock. Track circuits are now used to control the signalling directly in legacy fixed block signalling system,by preventing a signal being cleared if the track ahead of the signal is occupied by a train, by holding the route after a train has passed the signal, and to lock a set of points if a train is standing on them. It is important to remember that a track circuit is designed primarily to detect the absence of a train and not its presence. There are however many signalling controls which require track circuits to be occupied (e.g. release of overlap after a train has come to a stand) and the track circuit must therefore be reliable enough to be used for this purpose also. A rail circuit is the reverse of a track-circuit, and is used to prove the presence (not absence) of a train at a particular place. Rail-circuits are not as commonly used as track circuits. To understand the basic principles of track circuit operation, the easiest type of track circuit to study is the simple, battery-fed D.C. track circuit. Many other types have been developed to cover particular situations such as electrified areas, jointless track etc. These will be discussed later. Most of the principles of the simple D.C. track apply equally to other types, although the configuration of the equipment may differ. PRINCIPLES OF OPERATION A track circuit employs the rails as a transmission line. The electrical signal, in this case a d.c. voltage, is applied to one end. At the other end a relay is connected across the rails as a receiver of the voltage applied at the opposite end. The physical limits of the track circuit must be defined by insulated rail joints to prevent interference between adjacent track circuits or isolate the track circuit from sections of line where track circuits are not required. When there is no train on the track circuit, the relay will be energised via the full length of each rail. When a train stands on the track circuit the wheels and axles create a short circuit between the rails. The current takes the lower resistance path via the train and the relay becomes de-energised. All track circuits depend for their operation on the wheels of a train forming a low resistance path between the rails. Current from a battery (or rectified power source) at one end of the section, energises a relay at the other end via an adjustable resistor and the rails. Each rail forms one leg of the circuit. Although the rail joints are metallically joined by fishplates, they are electrically unreliable and it is therefore necessary for the rails in each leg to be bonded together to form a good path for conduction of the electrical current. The end rails of the track circuit are insulated from the adjoining rails by means of insulated rail joints. The track circuit is an independent circuit with its own power supply at one end, and its own relay at the other end. Normally, when the track is clear of trains, current flows through the rails and through the coil of the tractive armature electromagnetic relay, thereby energising the relay which closes its front contacts. When a train occupies the track circuit, the two rails of the track circuit are short-circuited by the axles of the train. The relay is shunted by an electrical path of extremely low resistance, provided by the wheels and axles of the train, the result being that virtually no current flows through the relay coil because all but an insignificant amount is diverted through the axles. The relay becomes de-energised and the armature drops to close the back contacts. Thus the condition of the track relay determines whether the track circuit is clear or occupied. The most important feature of the track circuit is the fact that it fails to safety, i. e. is "fail-safe". The moment a rail or a wire breaks, or a metallic obstruction straddles the track, or the power supply fails, the relay will become de-energised indicating the track occupied condition Figure 1 Track Circuit Clear (relay Energized) -Simple DC Track Circuit Figure 2 Track Circuit Occupied (relay De-Energized) -Simple DC Track Circuit The relay is therefore energised to prove that there is no train on the track, and de-energised when there is a train on the track. The relay will also de-energise if: a) A rail connection becomes detached b) The power supply fails c) A rail to rail bond is broken or becomes detached d) A rail breaks The circuit is therefore designed to be "fail-safe", as under most fault conditions it indicates that the track is occupied. This may not be totally true in practice, as will be seen later. 3. PRACTICAL CONSIDERATION Figure 3 Practical Track Circuits A practical track circuit differs significantly from the theoretical example shownin Figure 1 & Figure 2 The two major difficulties are:- a) The rails are not perfectly insulated from each other. Current "leaks" from one rail to the other, through the rail fixings, the sleepers and the ballast. This rail-to-rail resistance is known as "ballast resistance". It can vary widely 300 metres of standard gauge (1435mm) track may have a ballast resistance as high as 50 ohms for well ballasted, dry track, or as low as 0.5 ohm with waterlogged or dirty ballast. b) The presence of the wheels on the rails may not provide a perfect short circuit. The axles may have a small resistance, but the main problem is the contact between wheel and rail. This may be due to rust or other causes such as fallen leaves ,or ice on track during winter . Although the rails also have a resistance, this is normally insignificant compared to the above two problems. Rails will also have an inductance. This will be significant when dealing with some types of a.c. track circuit. 3.1 Ballast Resistance This is a factor over which the signal engineer usually has very little control. It will vary according to the type of ballast, its condition, the type of sleepers, the method of fixing of rails to sleepers, track drainage and, most importantly, the weather conditions. These will all vary in time during the life of the track circuit. Some factors will change slowly, such as the condition of the ballast. Others, like the amount of moisture in the ground, can vary rapidly. The design of the track circuit equipment must be able to compensate for the long term changes by some means of manual or automatic adjustment. The short term changes due to weather must be contained within the normal operation range of the track circuit. If not, failures (either right side or wrong side) will result. The inclusion of a series resistance at the feed end is an essential feature of most track circuits to avoid excessive power consumption when the track is occupied. It also permits components of a lower current rating to be used. This will also make the track circuit more difficult to set up for reliable operation. As ballast resistance decreases, the current drawn will increase, the voltage drop across the feed resistor will increase and the relay voltage will be correspondingly reduced. Too large a reduction in ballast resistance could cause a right side failure of the track circuit. Under the most adverse conditions, the minimum ballast resistance should not be less than 4 ohms per 1000 ft. Short track circuits will of course have a higher ballast resistance. The value of ballast resistance may be calculated from the measurements taken for rail resistance: - Vf = Volts across rails at feed end. Vr = Volts across rails relay end. It = Current to track. Ir = Current to relay. Mean voltage across rails = (Vf + Vr) / 2 Current in ballast = It - Ir Ballast Resistance = (Mean voltage across rails) / (Current in ballast) For a typical DC track circuit calculation, the following figures for a track circuit will be used: - EMF of battery 1.2 volts. Resistance of relay 9 ohms Pick-up value of relay 0.36 volts Drop-away value of relay 0.27 volts Minimum ballast resistance 2 ohms Working voltage 25 per cent above pick-up 0.45 volts Current to relay = Voltage across relay/Resistance of Relay = 0.45/9 = 0.05 ampere. Current to ballast = Voltage across rails/Ballast resistance = 0.45/ 2 = 0.225 amp. Current to track = 0.05 + 0.225= 0.275 ampere. Voltage across feed resistance = EMF of battery - EMF across rails = 1.2 - 0.45 = 0.75 volts Value of feed resistance = Voltage across feed resistance/Current to track = 0.75 / 0.275 = 2.73 ohms The value of the resistance which, when placed across the rails of the track circuit with a ballast resistance of 2 ohms, would reduce the voltage across the relay to the drop-away value, i. e. 0.27 volts, can be calculated as follows Voltage across feed resistance = 1.2V - 0.27V = 0.93 volts. Current to track = 0.93V/2.73= 0.341 ampere. Current to relay = 0.27V/9 = 0.03 ampere. Current to ballast = 0.27V/2 = 0.135 ampere. Current to relay and ballast = 0.03 + 0.13 5 = 0.165 ampere. Current to shunt resistance = 0.341 - 0.165 = 0.176 amp. Shunt resistance = Voltage across track/Current through shunt= 0.27/ 0.176 = 1.53 ohms. The above figures are given as an example to show how the various values can be obtained. 3.2 Rail Resistance This is the total resistance of rails, fishplates and bonds. Its value remains appreciably constant. The rails of a track circuit form the conductors of a simple circuit in that current is supplied at the feed end and less current appears at the relay end. The loss in current is due to leakage and the loss in voltage to rail and cable resistance. The fall in current and voltage is not uniform along the length of the track circuit, but falls faster near the supply end than the relay end. In most cases, however, the rail resistance is small compared with the ballast resistance and it may be assumed that the voltage and current fall off uniformly along the length of the track circuit. The value of the rail resistance in a track circuit may be obtained from the following measurements Vf = Rail voltage at the feed end of track circuit. Vr = Rail voltage at the relay end of track circuit. It = Current to circuit. Ir = Current to relay. From Ohms law it is known that: Rail resistance = (Voltage drop in rails) / (mean current in rails) 3.3 Train Shunt Resistance This is the joint resistance of all the parallel paths from rail to rail of a train or vehicle. The train shunt of a vehicle is greater than that of a train and that of a pair of wheels greater than that of a vehicle. It should be noted that the ordinary resistance of a pair of wheels and axle is not the same as the train shunt of a pair of wheels resting on the track, due to films of oil, scale and rust frequently found between the wheels and rail. 3.4 Train Shunt The train shunt resistance varies with the voltage between the rails, in that as the voltage increases the train shunt resistance decreases. The lowest allowable drop shunt is 0.5 ohm irrespective of the track voltage. The shunt and pick-up of a track relay can be taken with the aid of a variable resistance with 0.1 ohm tappings. When taking the pick-up value the resistance should be set at a low value and gradually increased until the relay armature just picks up. This should be repeated two or three times until consistent results are obtained. When taking the train shunt value the resistance should be set at a high value so that the relay remains in the energised position, and then the resistance is decreased until the armature of the relay just falls away and breaks its energised, i.e. front, contacts. The reading of the adjustable resistance is the drop train shunt of the track circuit. This should also be repeated two or three times to obtain the true value of the drop train shunt. 3.5 Relay Characteristics There are two different values for train shunt. The drop shunt is the largest value of train resistance that will cause the track relay to release. The pick-up shunt is the smallest value of train shunt which will allow the relay to operate. Due to the method of operation of all relays, the drop shunt will always be the lower of the two values. As the energisation of the relay closes the air gap in the relay's magnetic circuit, the reluctance of the magnetic circuit reduces. The relay therefore requires less current to hold it in the energised position than to pick it up initially. Conversely, it will require a significant reduction in this current to release again. Whenever track circuits are tested the drop shunt should always be observed and recorded. Track relays are designed specially to minimise the difference between the pick-up and hold-up currents. Nevertheless, a hold-up current of only 0.5 to 0.7 times the pick-up current is typical. Another problem is the time of release of the track relay. With both relay and rail being inductive, a loop will be formed by the rails, the train and the relay. The back-emf caused by the reduction in current as the train shunts the track will tend to cause a circulating current in this loop which will delay the release of the relay. Short, fast moving trains could momentarily be undetected as they pass from one track circuit to the next. This has serious implications for the release of route locking and the holding of points. If the type of track relay used is not inherently slow to pick up, a repeater relay which is slow to pick must be used in all vital signalling controls. 3.6 Maximum Length The combination of the above factors will generally limit the maximum length of any type of track circuit. The longer the track-circuit, the lower will be its ballast resistance, and so the attainable voltage at the relay will be lower, as more current leaks between the rails. Eventually, a length is reached at which it is no longer possible to operate the track relay. This maximum length will be lower for lower values of ballast resistance. The maximum length must therefore be based on worst case ballast conditions. A track circuit adjusted for minimum ballast resistance (e.g. during wet weather) will have a relay voltage higher than the minimum pick-up voltage when ballast conditions improve. Adjustment must be such that the track relay does not remain energised when a train of maximum value of drop shunt occupies the track circuit. This requirement also effectively imposes a maximum length on the track circuit. Maximum length will depend on the type of track circuit, the components used and the conditions under which it operates. Most track circuits have maximum lengths in the 600 to 2000 metres range. 3.7 Minimum Length For short track circuits, adjustment for reliable operation does not usually present any problem. But if a track circuit is too short, a vehicle with a long wheel-base may stand completely over the track circuit without it being detected. The minimum length of a track circuit is set by the vehicle with the)opgest wheel-base. For bogie vehicles, this distance is measured between the inner axles of each bogie. In most cases, this minimum distance is more likely to be determined by bogie vehicles than by two axle vehicles. 3.8 Staggered Block Joints Ideally, insulated joints are installed in pairs opposite each other. In practice, it is frequently difficult to ensure that they are exactly opposite, such as within pointwork. Joints may be provided to separate adjacent track circuits or to allow a polarity change within a track circuit. Where joints are not exactly opposite, they are said to be staggered. There is a short length of track where both rails are of the same polarity. In this area, an axle will not be detected. If the stagger is too great, a bogie or even a complete vehicle may remain undetected. The maximum stagger is determined by the vehicle with the shortest wheelbase. Although it is still possible to "lose" one axle, the . other axle will be detected, Care must be taken to correctly position joints for clearance purposes to take account of this problem. Staggered joints must not be used to define critical clearance points. If two sets of staggered joints are too close together, both axles of a short, four wheel vehicle could be undetected at the same time. A set of joints which is not staggered but defines the end of the track circuit could also fail to detect a vehicle if too close to staggered joints. Therefore, unless joints are exactly opposite, they must always be separated by the length of the longest vehicle. Figure 4 Track Circuit Minimum Length Figure 5 Staggered Joints -Maximum Stagger Figure 6 Joints in Close Proximity -Loss of Train Detection 3.9 Equipment Details To ensure effective operation equipment types and values must be carefully chosen. Whether a.c. or d.c. power supplies are normally of low voltage. This is desirable as the presence of a train presents a short circuit to the feed set or transmitter. Similarly, there is usually a series impedance in the transmitter/feed set to limit output current, avoid damaging circuit components and prevent excessive power consumption. Where power supplies are derived from an ac mains supply, each track feed will normally have its own supply to prevent track circuit signals feeding through between adjacent track circuits via the power supply. Where a feed resistor is provided it is often adjustable to deal with different track circuit characteristics. When a track circuit is set up, account must be taken of the weather & ground conditions at the time. An incorrectly adjusted track circuit may either fail to detect all trains under conditions of high ballast resistance (dry weather) or fail to energise the relay with no train present if the ballast resistance is extremely low (wet weather). Many audio frequency track circuits are designed to be adjusted by varying the gain of the receiver. This makes setting up easier and also permits track circuits to be centre fed where required. Frequencies for a.c. track circuits are normally within the audio frequency range. Higher frequencies are increasingly attenuated by the inductive impedance of the rail. This would make the maximum length impractically low. Mains frequency may be used in non-electrified or d.c. traction areas (with suitable precautions). Due to the low power used, relays are normally of low coil resistance (typically 4 to 20 ohms). For the same reason, relays may only be provided with two front contacts. Electrical continuity of both rails is essential. Any rails not welded or bonded for traction return must be bonded by the signal engineer. Bolted fishplates do not in practice provide a reliable electrical connection. For rails which carry track circuit currents only, an adequate connection can be made by using two galvanised steel bonds pinned at each end.Welded or brazed connections may also be used. Where electric traction is used, the return traction current will be many times greater than the track circuit currents. The bonds must be much heavier and will often be provided by the electrification engineer. Connections from the rail to lineside equipment should be firmly secured to the rails and employ flexible cable to counteract the effects of vibration. INSULATED BLOCK JOINTS The insulated rail joint is installed in the track at the extremities of a track circuit and also within a track circuit at points and crossings in order to achieve reversal of polarity. When 40-ft. rails were used, the tensile stresses were negligible and to provide an insulating medium the steel fishplate was planed down by 1/4 inch and a specially formed sheet of bone fibre was inserted between the fishplate and the fishing surfaces of the rail. Fibre ferrules and washers were used to insulate the bolts and provided the joint was correctly packed in the ballast bed to prevent excessive flexing of the joint and crushing of the fibre insulation, the result was satisfactory. To insulate the gap between the rails, a fibre sheet shaped to the contour of the rail was inserted between the rail ends. This latter insulation, which is known as an end-post or "T" piece, was amply strong to resist the compressive load when the 40-ft. rails expanded. This form of insulated rail joint gave reasonably good service even when 120-ft. rails came into use. However, the crushing of the fibre fishplate resulting in electrical failure of the insulated joint could not be observed and therefore the failure could not be anticipated. This fact necessitated the use of a joint where signs of impending mechanical failure could be seen long before replacement became necessary. For this joint, the steel fishplate was discarded and replaced by one made of an insulating material, the most common one being resin impregnated, compressed laminated wood. Such joints will take a tensile load of 2000 psi but the bending strength is limited to a smaller loading. The fishplate will be very quickly smashed unless the joint is well supported. From a track point of view a rigidly supported joint is not favoured since track resilience is the ultimate aim. Modern railway practice is to provide a running top without any joints whatsoever as distinct from the use of 40 ft. or 120 ft. rail lengths joined together by means of steel fishplates. Rails of 45 ft. are depot-welded to 360-ft. lengths, transported to site and then thermit-welded into 1440-ft. lengths in the track. Thus the civil engineer has a track which is smooth, resilient and silent, requiring comparatively little maintenance and one which increases the life of rolling stock. The signal engineer, however, is obliged to request that the long-welded rail be cut at selected points in order that insulated rail joints may be installed to suit the track circuiting arrangements. Insulated rail joints, for which a design to resist the stresses set up in long rails has yet to be perfected, are inserted in the track. In order to protect the insulated joint against the high tensile forces, it is normal practice to install a splice joint next to the insulated joint. The splice joint allows the long rail to expand and contract, but apart from its prohibitive cost, it breaks the pattern and, together with the insulated joint, tends to nullify the advantages of long-welded rails. A possible solution to the problem lies in the application of an insulated joint which reverts to the use of steel fishplates which, after all, give maximum bending strength. The joint is known as a "frozen" joint or "glued" joint because over and above the use of high-tensile steel bolts, the entire assembly is rigidly held together by means of an adhesive glue, 1/16 inch thick, which acts both as a strengthening bond and as insulation. The entire assembly must be made to close tolerances under workshop supervision. Two short lengths of rail are used, the completed joint then being taken to site and thermit-welded into the track. Except for jointless track circuits on plain line, insulated block joints are used to define the limits of a track-circuit. It is important that the failure of an insulated joint does not cause a wrong side failure. Joints may be provided in one rail only or in both rails. This will usually be decided by requirements for electric traction return. With joints in both rails, if a single block joint fails, both track circuits should continue to work satisfactorily. If both block joints of a pair fail, there is a danger of a relay being falsely energised from the adjacent track-feed. To avoid this problem, the polarities of the rails on either side of every insulated joint must opposite. If both block joints of a pair fail, the two feeds will short-circuit each other, and both relays will de-energise. In the vicinity of points and crossings,it may be necessary for sections of one rail to be common to two track circuits. Failure of any common blockjoint should cause both track circuits to show occupied. In electrified territory, joints may be in one rail only. Maintenance of opposite polarities at every blockjoint is essential as only one joint separates the track circuits. Figure 7 Track Circuit polarities -Double Rail (Joints in Both rails) Figure 8 Single Rail (Joints in One Rail Only -Other Rail Continuous) TRACK-CIRCUIT TYPES Many different types of track circuit are available, designed to be used in particular applications such as electrified lines, continuous welded rail, etc. A list of the main types of track circuit is given below. As track circuit development continues, further types will no doubt become available. It is not necessarily an exhaustive list. 5.1 D.C. Track Circuits D.C. track circuits have the advantage that they can be simply fed from a battery, either as the main or a standby supply. Installation, setting up and subsequent maintenance is straightforward. They have the disadvantage that they are unsuitable where d.c. traction systems are in use. Battery - fed (primary cell or trickle charged) Non AC - immune AC - immune Coded "Swept" 5.2 Fixed Frequency A.C. Track Circuits All track circuits in this category work from a mains power supply at the same frequency. Mains frequency (e.g. 50Hz) vane relay (single-rail or double-rail) Traction immune, vane relay (operates at a different frequency to the traction supply for immunity from a.c. and d.c. traction currents) A.C./D.C (a.c. track circuit employing a d.c. relay) various types 5.3 Multiple Frequency A.C. Track Circuits Track circuits in this category are normally designed and installed so that adjacent track circuits operate at different frequencies. In most cases they may be used without insulated blockjoints. Reed Aster Hitachi UM71 ML TI-21 Siemens FS2500/FS3000 Alstom CVCM 5.4 Impulse Track Circuits These track circuits employ short pulses at a relatively high voltage to overcome contact resistance at the rail while keeping power consumption at a low level. Jeumont Lucas Although in some cases proprietary or manufacturer's names have been shown above, similar equipment may be available from other manufacturers. 5.5 Functions Other Than Train Detection As the rails are a continuous transmission line from one end of the track circuit to the other, it is possible to use the track circuit for more than simple train detection. To economise on lineside circuits, additional controls can be added into the feed end of the track circuit, typically in automatic sections. For example, the track feed can include proof that the signal ahead of it bas returned to normal after the passage of the previous train and/or proof that the signal is alight. The disadvantage of this type of circuit is that it complicates fault finding and may give the signalman misleading track circuit indications. A more recent application is the coded track circuit. The track circuit current is interrupted at different intervals or modulated at different frequencies to convey information to the train, usually for automatic train control (ATC). The current passing from one rail, through the wheels and returning to the other rail is detected by one or two receivers on the front of the train. Different signals may correspond to different commands to the on-board ATC equipment. To operate correctly, the track feed must always be at the end of the track at which the train leaves. Otherwise the train will shunt its own track circuit signal and the receiver on the train will see no signal· current in the rails. This is not a problem where trains run in one direction only. On bi-directional lines, this means that the track feed and relay circuits must be capable of being switched from one end to the other or two sets of track circuit equipment must be provided for each bi-directional track. Coded track circuits also create complications in train detection at the relay or receiver end. The simplest approach is to build in some form of delay longer than the code pulses to maintain the relay energised. The relay/receiver could be set up to recognise only the available set of codes, any invalid code causing the track circuit to show occupied. The third option is to compare the signals at the feed and relay ends. Figure 9 Coded Track Circuits Operating Principle D.C. TRACK CIRCUITS In its simplest form the d.c. track circuit is as described in the earlier sections. Power was originally derived from one·or more primary cells which would be replaced· as necessary during routine maintenance. Where a lineside signalling power supply is available (normally a.c.) this will be used to operate the track circuit, either via a transformer & rectifier or by trickle charging a battery of secondary cells. In more recent years, solar power has become a viable power source in areas where the climate is suitable. The choice of feed arrangements will depend upon the reliability of the main power supply. If there are standby arrangements for the supply itself, a battery is not normally used. In ac. electrified areas, d.c. track circuits may be used but with special ac. immune feed sets and relays. Immunity to the a.c. traction currents must ensure that the feed circuit cannot be damaged by traction currents and the relay will not pick or be damaged by a.c. A combination of fusing, series chokes and a.c. immune relays will normally provide acceptable protection. Purpose built a.c. immune feed sets are available. Obviously, d.c. track circuits cannot be employed where d.c. traction is in use as there is no satisfactory means of preventing traction currents from passing through the relay. A.C. TRACK CIRCUITS A.C track circuits, normally at mains frequency, can be used in areas where the presence of high levels of d.c. would prevent the use of d.c. track circuits. This is most commonly due to the use of d.c. traction, but may also be due to the presence of certain chemicals in the ground, causing a voltage to appear across the rails due to electrolytic effects. It is also possible to use a.c. track circuits in non-electrified areas. In place of a d.c. feed, an a.c. source is used. A two element a.c. vane relay is usually used in places of the d.c. relay. AC. vane relays cannot be energised by a d.c. voltage. The vane relay is operated by two windings, both of which must be energised simultaneously. One winding is fed locally, the other through the rails. The two relay feeds are then approximately 90" out of phase. If either feed is absent, or the phase angle between them is wrong, the relay will not energise. To isolate the track circuit equipment from the effects of very large traction currents, it is normal to connect the track circuit equipment to the rails via either a transformer or a series capacitor. If a capacitor is used in the feed end, this can be variable to perform the function of the feed resistor in a d.c. track circuit. Fuse protection is also desirable. One point to be borne in mind when designing locations for a.c. track circuits is that the supplies for the two windings must come from the same source. Signalling power supplies are single phase. This is normally derived from a 3-phase supply and adjacent power feeders may not be fed off the same phase. As the relay is phase sensitive, failure to observe this rule could lead to incorrect operation and/or adjustment of the track circuit. If d.c. traction is used, voltages are usually low and currents are therefore extremely high. To provide an acceptable low resistance return current path to the substation, both rails may be needed for traction return. Two basic configurations of A.C. track circuit are required, Double Rail and Single Rail. This refers to the traction current, not the track circuit current. Phase must be alternated between adjacent tracks for the same reason as polarity is staggered with D.C. track circuits (i.e. failure of blockjoints, connecting two supplies in antiphase will produce an effective voltage which is insufficient to energise either of the relays). 7.1 Double Rail Track circuits With low voltage d.c. traction systems, currents of 3,000A or more are commonplace. If current is to pass along both rails, special arrangements are made at track circuit joints to pass d.c. but not a.c. The arrangements include the use of an "impedance bond", which presents a low impedance to d.c., but a high impedance to a.c. by virtue of its two coils being wound in opposite directions. Often the impedance bond is "resonated". The impedance bond is provided with a secondary winding of a larger number of turns than the primary windings. This is connected in circuit with a capacitor, giving a band-stop filter effect at the track circuit frequency to further increase the a.c. impedance. The impedance bond is effectively a centre tapped inductor. The d.c. traction currents in each half of the winding set up opposing fluxes in the magnetic core which cancel each other out. To the a.c. track circuit current, the impedance bond presents a rail to rail impedance which is high compared with the ballast resistance. Failure of a blockjoint or the disconnection of a traction return bond between rails may unbalance the d.c. currents in the impedance bond sufficiently to saturate the magnetic core of the impedance bond and cause the track circuit to fail. In addition to the extremities of a track circuit, impedance bonds will also be found:- a) Where it is necessary to bond the traction return between adjacent tracks (to provide a lower resistance path). Depending on the power supply arrangements, this is usually necessary at approximately 0.9 km (BR third rail 750v) to 1.5 km (TfNSW 1500v overhead) intervals. b) Immediately adjacent to substations for connection to the traction supply. 7.2 Single Rail Track Circuits If traction currents can be kept low enough it may not be necessary to use double rail track circuits throughout an electrified line. Through points and crossings the use of double rail track circuits may not always be possible or desirable. Single Rail track circuits will then be used. The length of single rail track circuits through pointwork should be kept as low as possible where D.C. traction is in use as the impedance of the traction return path will be increased. Figure 10 A.C Double Rail Track Circuits (Typical TfNSW , Former SRA) Figure 11 BR(British) Capacitor Fed Figure 12 Typical Single rail Track Circuits Figure 13 Double Rail Track circuits with Additional Impedance Bond 7.3 Dual Electrified Lines In a few cases, more than one electrification system is in use on the same line (e.g. 750v d.c. third rail and 25kv a.c. overhead) or lines on different systems run in close proximity. If mains frequency a.c. track circuits are used, they will not be immune to the a.c. traction current. Similarly, d.c track circuits cannot be used either. It is however possible to operate at a supply frequency different to the traction supply (and any harmonics), Immunity will then be provided against both traction systems. For a 50Hz traction supply, frequencies of 83.3Hz and 125 Hz have been used. A separate signalling supply at the required frequency can be provided and distributed along the lineside. Alternatively, the supply for each track feed can be produced at each location by an inverter. Filters must be incorporated in both feed and relay ends of the track circuit. Impedance bonds on double rail track circuits must be resonated at the track circuit frequency (not 50Hz). This will require a different (usually lower) value for the resonating c3pacitor. The impedance bond will then appear as a much lower impedance at 50Hz. This arrangement is particularly useful where the traction system is to be converted from d.c. to a.c. where much of the existing track circuit equipment can be retained and track circuit conversion can be undertaken in advance of the traction changeover. The use of such track circuits is now falling in popularity as a number of dual trnction immune audio frequency track circuits are available. 7.4 A.C/D.C. Track Circuits The simple d.c. track circuit has an inherent slow-to-release characteristic, because the track relay has a high inductance, and a low resistance. The d.c. track relay is however much simpler and cheaper to manufacture than the a.c. vane relay. The high inductance delays the release of the relay. This is an unwanted characteristic for a track circuit, because it delays detection of fast moving vehicles and could leave a short train or vehicle undetected as it passes from one track circuit to the next. This obviously has serious implications on the safe operation of route locking. If a relay with higher resistance, and lower inductance is used, the relay releases quicker. However, a relay with higher resistance requires a higher operating voltage. An equivalent effect can be obtained by feeding a d.c. track relay through a step up transformer rectifier arrangement. One example of this is the B.R. "Quick Release" track circuit. The voltage on the rails is ac. The resistance in series with the track relay increases the value of resistance in the shunted circuit, and reduces the release time. The fact that the relay end has a high resistance also makes it possible to locate the relay some distance from the end of the track circuit (useful in restricted access situations such as tunnels). This type of track circuit is not traction immune. It is also not frequency selective, and will operate at frequencies other than 50Hz, which causes problems when used adjacent to audio-frequency jointless track circuits. JOINTLESS TRACK CIRCUITS Insulated block joints are an additional cost in modem welded rail. Although the strength and reliability of insulated joints has generally reached a high level, most permanent way engineers prefer to avoid insulated joints wherever possible. Jointless track circuits have been developed to operate without insulated block joints. Various arrangements of electrical components are used to terminate the track circuits, and define their limits, electrically. All jointless track circuits employ an a.c. signal. A transmitter at a specified frequency is connected to one end of the track circuit. At the opposite end, a receiver is connected which will only respond to the signal produced by its own transmitter. The receiver will normally be used to operate a relay which interfaces with the signalling system in the normal manner. The insulated joint normally takes the form of a filter which will prevent signals of the track circuit's operating frequency from penetrating more than a short distance beyond the transmitter or receiver and ensure that the track circuit is not operated by signals from adjacent track circuits. Adjacent track circuits must therefore operate at different frequencies. The frequencies used must also be immune from mains interference from the signalling power supply and, where necessary, the traction power supply. Early jointless track circuits were generally not traction immune. Several types of jointless track circuit are now available, many of which are immune to both a.c. and d.c. traction. The GEC "Reed" track circuit was also developed to be used as both jointless and conventionally jointed track circuits, immune to both a.c. and d.c. traction. However, problems were encountered with the operation of the track circuit in its jointless form in certain situations and it is at present only used on BR as a conventional jointed track, often in areas with both ac. and d.c. traction together. It will however be described in this section. Due to the absence of insulated joints, the limits of jointless track circuits cannot be as precisely defined as for conventional track circuits. This generally presents no problem on plain line but, as the extremities of track circuits through pointwork need to be precisely defined to ensure safe clearances, block joints must be used. Track circuits which define precise clearances will thereftill need insulated joints although the same jointless track circuits transmitters and receivers may often be used. Joints are also required at the extremities of jointless sections where they adjoin conventional track circuits. To permit the filters to operate correctly, each type of track circuit will often have a minimum length. This may be of the order of 50 metres. Shorter track circuits may therefore be unsuitable for use with jointless track circuit equipment. To permit adequate flexibility in the positioning of track circuits to meet all requirements, at least four distinct frequencies must be used, two on each line of a double line railway. This prevents interference between track circuits on adjacent lines. An early type of jointless track circuit, the Aster 1 watt, employed six frequencies, three for each line. On railways with more than two parallel lines, each set of frequencies could be used on alternate lines. Aster track circuits were not traction immune. A further development was the Aster U type. This employed only four track frequencies (two on each line) and was also not traction immune. To permit alterations (addition or removal of a track circuit) with only two frequencies per line, the track circuit may be centre fed if required. It is still a simple example of a jointless track circuit to study and will be described first. 8.1 Aster Track Circuits The connection of the transmitter and the receiver to the rails is by secondary coils of the track transformer. This defines the approximate position of the boundary between the two track circuits. The tuning units operate in conjunction with the inductance of the rails to form filters. In the diagram below, L1-4 are the inductances of the rails between the tuning units and the track transformer. At frequency F1, C2 and L6 are series resonant and are therefore an effective short circuit to F1. The remaining components present a parallel resonant circuit of high impedance to the F1 signal. F1 will thus reduce in amplitude progressing from Tuning Unit 1 to Tuning Unit 2. A similar situation arises for F2 with L5 and C1 acting as a series resonant short circuit. Four frequencies are used. 1.7KHz and 2.3KHz would alternate on one line and 2.0KHz and 2.6KHz would alternate on the other line. Track circuit length may be from 50m to 1000m long. By using a centre feeding Transmitter the track circuit may be extended up to 2000m either as two individual track circuits or a single long track circuit (the TPR circuit would of course include both receivers in this case). Figure 14 Aster U Type Track Circuit -Tuned Area Between Adjoining Tracks 8.2 ABB /ML Style TI-21 Track Circuits The TI21 track circuit is similar in operation to the Aster track circuit already described, but has been designed to provide traction immunity. This track circuit requires a tuned length of 20m with the tuning unit F1. acting as a short circuit to F2 and Tuning Unit F2 short circuiting F1. Transmitter and Receiver connections are made via the respective tuning units. The security of the system is enhanced and traction immunity provided by operating each track circuit at two frequencies 34 Hz apart and modulating between them at a rate of 4.8 Hz. The receiver must detect both frequencies and the correct rate of modulation to energise the follower relay. It is considered that any signal produced by the electric traction supply and its harmonics would be unable to simulate the track circuit signal for long enough to energise the relay. For traction return purposes, parallel tracks are normally bonded together at regular intervals. It is therefore not possible to re-use the same frequencies on third and fourth parallel tracks. Eight frequencies are available, four of which are preferred and thus used on two track lines. The remaining four are used on third and fourth lines when required. A minimum length of 200m is required (300m if centre fed) and a maximum of 1100m is permitted (1000m per end if centre fed). Length can be reduced to 50m in a special low power mode of operation but the adjoining track circuits either side must then be connected with the transmitter nearest to the low power track to prevent the signal feeding through and falsely energising the receiver of the next track of the same frequency. On d.c. electrified lines, double rail traction return is usually required and impedance bonds will be required at feeder points and wherever adjacent tracks are bonded together. They will not be required at the extremities of the tracks. Special resonating units are provided to resonate the bonds to the track circuit frequencies. Figure 15 TI21 Transmitter & Receiver Connections 8.3 GEC Type RT Reed Track Circuits The GEC Type RT Jointless Track Circuit operates by detecting the current in the track circuit loop, as opposed to the voltage at the end of the track circuit like the jointless track circuits already described. It is always centre fed in the jointless mode, although the last track before block joints may be end fed. The Type RT Track circuit may also be used as a jointed track circuit, in which case it may be centre or end fed. Each track circuit operates at a frequency defined by a mechanical reed filter in the transmitter. The filter is very stable in frequency and has a bandwidth of less than 1 Hz. This allows track frequencies to be only 3 Hz apart, using 366, 369, 372, 375, 378, 381 and 408 Hz. On each line a four frequency cycle should be maintained. The transmitter is connected directly to the rails. The receiver only responds to its own frequency due also to a reed filter at its input. When used in its jointless configuration, each track circuit is centre fed. The receiver is energised by a_n aerial positioned between the rails. The aerial comprises 36 cores of a 37 core cable wired to give a 36 turn inductive loop. The 37th core is connected to a series resonant shunt which terminates the track circuit. At any frequency other than the track circuit frequency, insufficient current will flow in this loop to energise the receiver. As the limits of the track circuit may not extend as far as the rail connection to the loop, the ends of adjacent tracks must overlap as shown. The inefficiency of the shunt has led to problems in practice on B.R. where track circuit lengths are often short and/or more than two parallel tracks occur on electrified lines. The result is that signals from nearby track circuits of the same frequency feed through sufficiently to be detected by other receivers. This type of track circuit is only used on B.R. in its conventional jointed form, where it provides dual (a.c. & d.c.) traction immunity. Receiver connection is then directly to the rails. Tracks may be either centre or end fed. Figure 16 GEC Reed Track Circuit Arrangement for Jointless Operation 8.4 Siemens FS2500 Track Circuit This is another recent addition to the range of jointless track circuits available. The location equipment can be mounted on a standard BR930 relay rack. The track circuit is traction immune and its circuitry includes the use of microprocessors to process the track circuit signal digitally. It employs a similar signal to the 1121 track circuit, a modulated ac. signal where the level of modulation and the modulation frequency must be correct to indicate a clear track circuit. An interface has been designed to connect direct to a Solid State Interlocking (SSI) trackside module without the use of a track relay. The receiver has a built in "slow to operate" feature. It employs four sets of frequencies, two on each line. It may be centre or end fed. In addition, the receiver may be positioned remote from the end of the track circuit, useful in tunnel situations. Different termination arrangements are used in each situation. For a series of end fed track circuits, each transmitter and receiver is connected to the rails via a tuning unit. The tuning unit performs three functions:- a) A high impedance to its own frequency b) A low impedance to the frequency of the adjacent track circuit. This prevents transmission of the signal through to the next track circuit of the same frequency and shunting of the track by a train outside its nominal limits. c) A transformer for transmission of the signal from transmitter to rails and from rails to receiver. Adjustment of the signal level can be performed at both transmitter and receiver. It is usual for the transmitter to be set up on installation according to the length of the track circuit. Finer adjustment according to individual track characteristics is at the receiver. At the end of a jointless section, an end tuning unit, which is slightly different to the normal tuning units for track circuit separation, is connected across the rails. The same tuning unit is used at any centre feed to provide a high impedance connection between transmitter and rails. It can also be used in conjunction with an impedance bond where connection between tracks, to traction feeders or to conventional jointed track circuits. Air cored inductors (often referred to as SI units) are used where it is necessary to make earth connections to the track without direct connection to the rails. These are installed in the centre of the tuned area between tuning units. Intermediate receivers can be installed within a track circuit to indicate the occupation of part of a track circuit, for example approaching level crossings. Maximum track circuit length is 1.3km end fed (although TfNSW adopts a maximum of 900m). Minimum track circuit length is 50 metres. 8.4.1 Siemens FS3000 Track Circuit FS3000 is the latest addition to the family .This Audio frequency track circuit has tranceiver unit and different types of tuning units for various configurations .This type dosent have impedenace bond for the application .Singapore Thomson East Cost Line use this type 8.5 CSEE UM71 (Hitachi) Track Circuit The operation and configuration is similar to other types of traction inunune jointless track circuits. It is not intended to repeat the details contained in earlier sections. Only the differences will be identified. Instead of the tuning unit, separate tuning and matching units are provided at the trackside for connection and track circuit separation. Resonating units for each frequency are available for use with impedance bonds. Maximum length is 600m and minimum length is 50m. Length may be extended up to 2km by the use of compensating capacitors connected at intervals between the rails. Two types of equipment are in use, the "T1" for most normal applications and the "T2" for extended connections in tunnel situations. IMPULSING TRACK CIRCUITS Rusty rails, as may occur on lightly used lines, will tend to cause wrong side failures whereby vehicles will remain undetected. To overcome this problem two basic methods are used. One is to weld a stainless steel strip onto the surface of such rails in critical areas; the strip will not go rusty. The other is to use impulsing track circuits. The principle of an impulse track circuit is to use high voltage "spikes" to be fed to the rails. If a train or single vehicle occupies the track the voltage of these spikes is enough to break down the resistance caused by the rust on the rails. The voltage spikes applied to the track have insufficient energy to electrocute personnel on the line, although their voltage is of the region of 150V. Frequency is of the order of 3 spikes per second. Although a higher ac. voltage on the rails would produce a similar effect, the power c:onsumption would be excessive. An impulse type track circuit generally consumes no more power than a conventional track circuit. Two types, the Jeumont and the Lucas track circuit, are in use on B.R. although they are not very common. TfNSW uses the Jeumont track circuit extensively. On the Jeumont track circuit, / a typical voltage waveform on the rails is shown on the following page. The receiver detects the correct polarity of the pulse (important in the event of a failed insulated joint) and the correct ratio of voltage and duration between the positive and negative parts of the waveform. It must also provide an output to the track relay in the interval between pulses. The arrangement of connections to the track is also shown. Figure 17 Jeumont Track Circuit -Typical Rail Voltage Waveform Figure 18 Jeumont Track Circuit -Typical Rail Voltage Waveform It is not intended to describe the construction and operation of Jeumont tracks in detail. They are generally considered to be traction immune due to the low frequency of operation (3Hz). Due to the high voltage on the rails, they can be used for long track circuits, up to 1500 metres on electrified lines and 3km on non-electrified lines. Double rail track circuits on d.c. electrified lines will be connected at each end via an impedance bond. The relay is peculiar to the Jeumont track circuit and is designed to operate with the receiver. RAIL CIRCUITS AND OVERLAY TRACK CIRCUITS Unlike a track circuit, which detects the absence of a train, a rail circuit detects the presence of a train. It may be used at automatic level crossings, for approach control of signals, or in place of treadles on high speed lines. It has also been used in situations such as bump marshalling yards to control the operation of points, due to its higher speed of operation when compared with the track circuit. To detect the absence of trains, a rail circuit is not fail safe. The simplest form of rail circuit consists of a standard track relay connected in series with its feed across the rails. A train or vehicle on the track will then complete the circuit, energising the relay. An alternative to the rail circuit is the overlay track circuit. It uses a much higher :frequency signal than conventional track circuits. The inherent impedance of the rail limits its extent of operation, so insulated joints are not required and it may be overlaid on most conventional track circuits. SHUNT ASSISTERS As mentioned earlier, increasing problems are being experienced with track circuits due to the introduction of lightweight vehicles with improved riding characteristics. Obviously, unreliability of track circuits puts the integrity of the whole signalling system at risk. Large scale replacement of existing track circuits with those that will operate satisfactorily with the new vehicles may not be practical or economic. If it can be shown that the problem is confined to certain types of train, it may be possible to deal with it by modifying the vehicles instead of the track circuits. On B.R. certain classes of one and two car diesel multiple unit vehicles are being fitted with shunt assisters. These operate by applying a high voltage (compared to most track circuits) signal to the rails. This is outside the frequency range of normal track circuit operation (50khz) and does not affect the operation of relays and/or receivers. It does however break down the resistance between rail and wheel and allow the track circuits to operate more reliably. Such a solution is not without problems. The on-board equipment must be monitored for correct operation and its failure could prevent further operation of the vehicle until the problem has been rectified. It is not therefore a substitute for ensuring reliable track circuit operation for normal traffic. ALTERNATIVES TO TRACK CIRCUITS In locations where track circuits cannot be installed or where a long track circuit is required, axle counters may be a possible solution. A set of axle counter equipment is generally more expensive than a single track circuit. If one set of axle counters can substitute for several track circuits, the overall cost may be cheaper. If required, axle counter sections can be designed to overlap, reducing the overall quantity of equipment. The track equipment may also be located remotely from the electronic logic equipment. Axle counters will be covered in more detail in a later section. Although not yet used in any working system, the use of Automatic Vehicle Identification transponders on vehicles has the potential for use as a train detection system. An esential requirement is, of course that all vehicles are fitted with transponders and that they can be proved to be functional.
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CH10 | TRACK CIRCUIT BONDING
TRACK CIRCUIT BONDING CONTENTS Introduction Fouling & Clearance Points Positioning of Insulated Joints Jointless Track Circuits Bonding of Rails Track Circuit Interupters Other Information on Bonding/Insulation Plan NOTE: While these notes are based on the authors' understanding of railway signalling practice in New South Wales of Australia, they must not be taken to modify or replace any existing rules, instructions, or procedures of any railway administration. Where any apparent conflict exists, reference should be made to the appropriate documents produced by the administration of your Railway. This article will give fair bit of knowledge on bonding, there are numerous types of track circuits (FS2550, FS3000, CVCM, SDTC, TI21, MicroTrax). Bonding rules vary for these, and the respective manual shall be referred to. Bonding requirement for Axle counters is very limited (for Traction purpose only) and are out of scope for this article. 1. Introduction A modem signalling installation will use many track circuits. The limits of each track circuit must be precisely defined, and the track circuit must be connected to operate safely and reliably, even through the most complex points and crossings. In many cases this will involve the installation of insulated joints, although jointless track circuits are available to suit many applications. An insulated joint is relatively simple to provide in jointed track. The fishplate and its bolts are insulated from the rail and an "end post", a piece of insulating material of similar shape to the rail cross section, is inserted between the rail ends. In welded rail, this operation is much more complex. Either the rail must be cut and an insulated joint inserted or a length of rail is removed and a pre-assembled joint in a section of rail is substituted. The rail is then welded. Both operations involve the adjustment of the rails to allow for internal stresses. After installation an insulated joint will generally be weaker than the rail on either side. Therefore, although insulated joints are essential, their position must be chosen correctly, and the number of joints minimized. In addition, it may be desirable to avoid joints in positions of greatest wear, vibration, or stress on the rails. The positioning of insulated joints may often be a compromise between the requirements for an ideal track circuit and the practicalities of permanent way construction and maintenance. For correct operation, the two rails of each track circuit must both be electrically continuous between all extremities of the track circuit. The rails must also be insulated from each other. This requirement is just as important through points and crossings as on plain line. There should be no position within a track circuit where a vehicle can be totally undetected. Each track circuit must operate reliably and must as far as is practical fail safe. Any normal failure mode should result in the track indicating occupation by a train. In areas where electric traction is employed, one or both rails will be used for the return traction current. The track circuit arrangement must permit an adequate traction current return path while maintaining safe operation of track circuits. In addition to the signalling plan, it is customary to prepare a plan showing the arrangement of the track circuits. Instead of a single line for each track, it will show each rail individually. Its main purpose is to show the bonding and insulation arrangements for all track circuits. In addition, it may show other useful information such as position of cable routes and locations, overhead structures, traction power supply connections, earth bonding and further details of the track circuit equipment. 2. Fouling & Clearance Points Many insulated joints are positioned to prove clearance in the vicinity of points and crossings. The following terms will be used in these notes to describe the positioning of insulated joints. The fouling point is the position at which the extremity of a vehicle on one track is clear (by an adequate margin) of a movement on a converging line. The placing of an insulated joint at this position will not, however, ensure sufficient clearance. The clearance point is the position at which an insulated joint must be placed to ensure a vehicle stands beyond the fouling point. The distance of this from the fouling point will be determined by the longest overhang of all vehicles operating on the line. It follows that a joint which is intended to prove clearance must be positioned beyond the clearance point. although the signalling plan is not of adequate scale to show these joints accurately the track circuit bonding plan, or insulation plan, must be of a large enough scale to do so. If necessary, critical measurements must be taken from permanent way construction drawings. The limits of jointless track circuits cannot be precisely defined and cannot accurately determine clearance points. Even if jointless track circuit equipment is used through points and crossings, insulated joints will be needed to define clearance points and to electrically separate opposite running rails. 3. Positioning of Insulated Joints The positioning of insulated joints must fulfil all of the following requirements:- a) Within any track circuit, the two rails must always be of opposite. b) Unless adjacent track circuit signals employ different frequencies, the polarity (d.c. tracks) or phase (a.c. tracks) must be opposite on each side of all insulated joints. This is equally important whether the joint separates two track circuits or the two rails of the same track circuit. c) Where it is unavoidable to stagger block joints (i.e. they are not exactly opposite), the separation must be limited so that complete vehicles cannot remain undetected. d) Separation of a staggered pair of joints from an adjacent pair of joints (whether staggered or not) must not result in a vehicle being undetected. Critical clearance points cannot be defined by staggered. e) Minimum track circuit length must be greater than maximum vehicle wheel base. f) Maximum and minimum track circuit lengths must be within the specified range of operation of the type of track circuit. Most railways now employ a high degree of standardization of permanent way components. This will often restrict the position of insulated joints within points and crossings. Preferred positions must be used wherever possible to avoid additional cutting of rails and subsequent track maintenance. The use of these preferred positions will often result in joints being staggered. At a turnout, if there is a choice between joints in a high speed or low speed line, the low speed line is usually preferred. In cases where electric traction employs a single rail return, joints in plain line will usually be provided in one rail only. Pairs of joints will occur however in points and crossings and where it is desired to change the traction return rail from one side to the other. The above rules for staggering will still apply. In areas with double rail traction return, jointless track circuits must be used or joints must be provided in both rails, together with impedance bonds for continuity of traction current past the joint. 4. Jointless Track Circuits For adjacent jointless track circuits of the same type, no joints are necessary unless clearance points must be accurately defined. For replacement of signals and defining the end of an overlap, tolerances of 5-10 metres are usually acceptable. Adjacent jointless track circuits of the same type must be of different frequencies. Where a jointless track circuit adjoins a jointless track circuit of a different type, block joints will usually be needed due to different operating characteristics. A filter designed to operate with one type of track circuit is unlikely to discriminate correctly between signals of a different type of track circuit. Where any extremity of a jointless track circuit must be accurately positioned for clearance purposes, insulated joints must be provided. 5. Bonding of Rails Each rail must be bonded to give electrical continuity throughout the track circuit. The arrangement of bonding may be dependent on traction power supply arrangements. For fail safe operation feed/transmitter connections must be at one extremity of the track circuit and relay/receiver at the other. Between the feed and relay connections, the bonding must as far as possible be fail safe. If the rails are not welded together or otherwise bonded (e.g. for traction current) the signal engineer must provide adequate bonding throughout the length of the track circuit. The safest way of bonding the rails together is in series. On plain line this is the only practical method of bonding so no problems will arise. Within points and crossings, all track circuits will have additional branches which must also be bonded. All sections of a track circuit should still be bonded in series, but this may not be possible in all cases due to traction requirements. A certain amount of parallel bonding may be necessary. Figures 2 & 3 below demonstrate the difference between series and parallel bonding. In Figure 2, the rails in the turnout are parallel bonded. A break in any bond or rail in this section of track could leave a vehicle undetected - the track circuit will indicate clear when a train is standing on part of it. If the connections are rearranged as in the Figure 3, only very short sections of rail are now parallel bonded. Other than in these short sections, a break in a bond or a rail will cause the relay to drop, indicating an occupied track. Even in non-electrified areas, there will always be short branches of a track circuit which. cannot be connected in series (e.g. where the switch and stock rail adjoin). When the arrangement of track circuit bonding and insulation is being designed, the length of these branches should be kept as short as possible. 5.1 Non-electrified Areas It is usual, although not essential , to install insulated joints in both rails where electric traction is not used. In this case, series bonding should be employed throughout. To maximise the amount of series bonding, certain portions of rail through points and crossings may be common to two adjacent track circuits. In general, this should not cause a problem but in complex track layouts, the bonding and insulation must be checked very carefully to ensure that the use of a common rail between several nearby pairs of track circuits does not provide an electrical path for false operation of a track circuit and does not cause any track circuit to be shunted by trains outside its limits. Refer FIGURE 4 for a Track Circuit Sharing a common rail. 5.2. Single Rail Traction Return On many electrified lines, particularly those with a high voltage a.c. supply; traction return is via one rail only. The signal engineer still has exclusive use of the other rail. The two rails are normally designated the signalling rail and the traction rail. Of course, the signalling equipment must always use both rails. Track circuits connected in this way are described as single rail track circuits. The signalling rail will be series bonded. The traction rail must be connected to give the lowest impedance path back to the feeder station. Track circuits should be connected so that as much as possible of the traction rail bonding is in series. Often, however, parallel bonding must be accepted in the traction rail. A typical example of bonding for a single rail track circuit is shown on Figure 5. The signalling rail is connected to provide the maximum amount of series bonding. The traction rail shows a significant amount of parallel bonding. 5.3 Double Rail Traction Return On Some lines, particularly those with d.c. traction, a lower supply voltage increases the traction current. This will often require both rails to be used for traction return. Track circuits must therefore operate safely and reliably while sharing both rails with the much larger traction currents. To allow traction currents to pass conventional insulated joints, impedance bonds are used. Where two double rail tracks adjoin, the centre connections of the two impedance bonds are joined together. The ends of each coil are connected to the rails on either side. Where a double rail track circuit is of a jointless type, impedance bonds are not normally needed where tracks of the same type adjoin. At the end of a section containing a number of jointless track circuits, insulated joints are usually required and an impedance bond (resonated to the track circuit frequency) will be needed to pass the current around the insulated joint. Impedance bonds are also needed wherever the traction return must be connected to the supply at a feeder point and where adjacent roads are bonded together to reduce the impedance of the return path. Plain line track circuits are inherently series bonded. Through points and crossings, however, if the double rail traction return is to be continuous, a proportion of parallel bonding is required. Although it prevents proof of continuity of the track circuit and its bonding, parallel bonding must be accepted as the only means of providing a low impedance traction return. Additional security is usually given by duplicating some or all of the rail-to-rail bonds (which of course will all be traction bonds and must consist of a suitably sized conductor). Even in series bonded sections of rail, bonds or jumpers are often duplicated for reliability reasons. It must be remembered that maintenance routines must include regular checking of the integrity of these bonds because the disconnection of a single bond will not become evident as a track failure. Refer Figure 6 for Double Rail Track Circuit with Parallel bonding. 5.4. Transition Between Single & Double Rail At the end of double rail sections, where they adjoin single rail track circuits, insulated joints will be required in both rails. An impedance bond will generally be needed for the double rail track. Its centre connection in this case will be bonded to the traction rail of the single rail track circuit. Refer Figure 7 for Typical Connections Between Single and Double Rail Track circuits. Where double rail tracks are in general use, even a small section of single rail return will increase the impedance of the traction return path. Single rail tracks in this situation are kept as short as possible. In some cases, two separate track circuits may be provided where a long track includes both points and plain line, even if one single rail track circuit could perform adequately over the total distance. 6. Track Circuit Interrupters Track circuit interrupters are used to detect that a train has been derailed by a set of catch (trap) points by maintaining the track in the occupied state. The reason for this is that a derailed vehicle may be completely clear of the rails while still in a position which would be foul of other movements. If track circuit interrupters are provided, the following rules will generally apply to the track circuit bonding: - a) The track circuit interrupter will be insulated from the rail upon which it is mounted. b) It will be bonded in series with the opposite rail to the one upon which it is mounted. c) Traction current should not pass through the track circuit interrupter. If mounted on a double rail track circuit, the interrupter must be connected in a separate circuit. A contact of the interrupter repeat relay must be included to cut the TPR circuit of its associated track. 7. Other Information on Bonding/Insulation Plan The bonding or insulation plan will often show other additional information such as:- a) Position of overhead electrification structures. b) Bonding between overhead structures and traction return rails. c) Positions of locations, cable routes and signal structures. Note : Please comment in the query section ,if you wish to discuss about Bonding for Track Circuits like FS2500/FS3000, CVCM ,SDTC,TI21 etc on real layout .
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CH12A | CABLES
CONTENTS Introduction Signalling Cables Telecommunication Cables Power Cables Selection of Cable Type Methods of Termination Cable Routes Cable Construction Electrical Properties Cable Testing Data Cable Fiber Cables INTRODUCTION Railway signalling now involves a wide range of equipment and techniques to transmit information, ranging from simple d.c. to carrier and data transmission. Associated with these is an equally wide range of interconnecting cables. Some are peculiar to railway signalling. Others are generally produced for telecommunications or general electrical purposes and have found applications within railway signalling. Refer Article Signalling Cable Standards on Rail Factor for a comprehensive list of cable standards, user of this article shall make effort to cross refer their current local guideline/standard in this regard. 2. SIGNALLING CABLES Most lineside signalling circuits are d.c. or mains frequency a.c. Voltages are low, typically 24 - 120 volts. In some cases point machine can be 3 phase ,415/400 Volts ,3 wire system. Cables designed for conveying d.c. or low frequencies are generally far simpler than those for a.c. use. This is because the transmission of d.c. is far less demanding on the type and dimensions of the insulating materials and also on the construction of the cable. Under d.c. conditions the voltage drop along the cable conductor is the product of the current flowing and the resistance of the conductor. As long as the insulation material and its thickness is chosen such that its resistance is very high then it will have little or no effect on the voltage drop along the conductor. Although the electrical characteristics of most signalling cables will be similar, cable construction will vary according to the environment in which the cable is to be installed. The most common variants are:- 2.1 Internal wiring This is usually flexible (stranded conductor) for easy installation along relay racks ,cable frame ,interlocking cabinets and cable ducts. The cable will generally be installed in a controlled environment so the sheath will not have to withstand such great changes in temperature, humidity and mechanical stresses as those installed outside. Many countries now have quite stringent requirements for the sheath material to satisfy fire regulations, the objective being to avoid emission of harmful gases in the event of a fire. Many existing internal cables have a PVC sheath. EVA (Ethylene Vinyl Acetate, otherwise known as Cross Linked Polyethylene) is often preferred now as it generally satisfies fire regulations. Some railways insist to use double sheathed insulation. Requirements for emission of smoke /and hazardous gas can be referred to IEC 61034-1/-2 and IEC60754-1 respectively. Similarly, IEC 60332-1 & IEC 60332-3 for flame test on Single wire and bunched wires respectively. UL1581 is an American Standard for electrical wires. Standard Size of wires are defined by AWG (American Wire Gauge)/European Or British Standards. Similar requirements exist for cables in tunnels and EVA sheathed cables are also used on underground railways. Internal cables are generally required as multicore cables (e.g. wiring between racks) and single core (individual circuits in interlockings and locations). Annealed Copper wires are used for electrical conductor compliant to IEC 60228(Australian AS/NZS 1574 & section 1&2 of 1125 ) 2.2 Lineside Cables Although these may carry similar circuits to the internal cables, they must withstand a more hostile environment. Typically, they will be installed in lineside troughs or buried and will be subject to changes in temperature and humidity, often lying-in waterlogged ground. Often UV Rays, oil, rodent and vermin will also be a problem. Individual conductors are insulated by an ethylene propylene rubber (EPR) compound/ while the outer sheath is polychloroprene (PCP) to give an oil resistant cable which will also withstand abrasion. If the cable requirement ask for UV Protection, Low Smoke Zero Halogen, Flame/Fire Retardant, carbon/iron/ wash plant liquid protection, outer sheath shall be selected accordingly . HDPE (High Density Polyethylene) cross linked polythene cover majority of such requirements. Most cables are multicore, to carry many separate signalling circuits. A single conductor will normally be adequate as the cable will not be subject to significant vibration once installed. Power cables are of similar construction but generally consist of two cores (for d.c. or single phase a.c. distribution and will generally have a stranded conductor due to the cross-section required. Some railways favour some form of armoured sheath (e.g. steel wire) for added mechanical protection. 2.3 On-track Cables Trackside electrical equipment is generally connected by cables across or under the ballast. Such cables must be strong and capable of withstanding considerable vibration. British Railway uses a flexible, multi-strand cable. Materials and construction are as for the lineside cables above, but the sheath is thicker, and the conductor is composed of a larger number of smaller strands (50/0.50mm). Again, many railways prefer an armoured cable but this can also present problems with earth faults where the risk of damage to the cable is high. 3.TELECOMMUNICATION CABLES Under a.c. conditions the inductance and capacitance of a cable can have a considerable bearing on the voltage drop and these factors become of major importance as the frequency increases. The capacitance of a cable pair or conductor will depend on the type and thickness of the insulation material. For a given material the thickness required for ac. transmission will generally be greater than for d.c. transmission. Suitable insulation materials for a.c. cables are dry air, paper and polythene. P.V.C. and rubber are not considered to be satisfactory, except in the case of low frequency cables such as 50Hz power. With multicore cables there is the additional problem of interference in one circuit due to the current in a neighboring circuit, commonly called "Crosstalk". The degree of crosstalk which may be encountered can depend on a number of different factors e.g. a) The frequency of the disturbing signal e. crosstalk increases with frequency (square waveforms, because of their high harmonic content are particularly troublesome). b) The magnitude of the current flowing in the disturbing i.e. crosstalk increases with the current. c) The position of the conductors in the cable relative to each To satisfy the latter problem, cables are manufactured with cores twisted together to form pairs, and adjacent pairs may be twisted at different rates. A circuit should always utilise the two conductors of a pair for out and return. Common returns should not be used in such cables. The effect of twisting the conductors together in such a fashion is that cancellation of any induced voltage occurs across any load terminated at the ends: of the cable pair. Often cables are made up in multiples of quads instead of pairs. In such cases a pair is formed by utilising the diametrically opposite cores. The resultant cable is smaller in diameter than the equivalent multi-pair cable, since there is less wasted space within the cable. With the use of very high frequencies on carrier circuits and for broad bandwidth applications such as closed-circuit television, the problems of crosstalk and attenuation increase with frequency. Eventually, twisted pair cables become unsatisfactory. Coaxial cable, effectively a cable pair consisting of a central conductor surrounded by and insulated from an outer metallic sheath, is employed. The fields created by the high frequency signals are contained in the cylindrical space between the inner and outer conductors, thus alleviating the crosstalk problem. A larger spacing between conductors also reduces the high frequency attenuation. 4. POWER CABLES For both signalling and telecommunications applications it is necessary to distribute power to the lineside. For most purposes this is best distributed as a.c. and transformed and/or rectified locally to the equipment served. A wide range of power cables is manufactured to supply the electrical industry, and these are generally employed within signalling and telecommunications systems. The sheath and/or insulation material may have to be modified to suit the specifications for signalling cable. As most power distribution is single phase, 2-core power cables are generally used. The conductors may be stranded copper or solid aluminium. 5. SELECTION OF CABLE TYPE 5.1 Lineside Signalling Circuits Signalling circuits will be taken to mean d.c. or mains frequency a.c. circuits between an interlocking and a lineside location (or between locations) to convey signalling information. They are normally laid in main cable routes and may be very numerous in areas of complex signalling. Cables for signalling circuits normally utilise conductors with only one strand, as mechanical strength is not of major importance. Typical cross sectional area is 0.5 to 2.5mm 2. To economize on installation costs, a smaller number of cables with a higher number of cores is preferred. 5.2 Tail Cables On-track cables (tail cables) to lineside equipment (e.g. signal heads, point machines etc.) from relay rooms or location cupboards must withstand physical damage and vibration. The B.R. standard cable has 50 strands to give it flexibility and a heavy-duty sheath for protection. The following sizes are commonly used: 5.3 Power Distribution Standard industrial sizes of 2-core power cable are used. The size of the conductors will depend on the power loading and the consequent voltage drop along the line. For most types of signalling equipment, a voltage drop of 10% is usually the maximum acceptable. It is therefore important to perform an estimate of power consumption of all equipment before deciding upon the size of cable to install. Allowance should be made for possible future additions to the electrical load. Renewal of power cables at a later stage can be expensive. 5.4 Internal Relay Room/Location Wiring Due to the diverse termination points of internal circuits, the only means of installing internal circuits is to use individual wires. Cables may be useful for wiring between racks or between different rooms/floors of a building to simplify installation by allowing factory pre- wiring. Alternatively, armoured (e.g. steel wire) cable could be used. This has fallen in popularity in recent years due to cost, difficulty of handling and difficulty in performing a satisfactory repair in the event of damage. 6 . METHODS OF TERMINATION All cables and wires must be terminated to connect on to other equipment. Each individual conductor in the cable will normally be terminated, even though some cores may be spare. In addition to terminating the individual cores the cable sheath is often clamped to a fixed bar in the location or equipment room. This is to avoid the weight of the cable placing a strain on the individual terminations which could lead to breakage of the conductors. The following are the most widely used forms of termination: - 6.1 Crimped Termination This method is mainly used for stranded cables. It is unsuitable for small cross-section single conductors as it weakens the conductor mechanically. It is a mechanical method which involves compressing a metal termination on to the wire. Various types of crimped connector are available:- a) A "ring" type connector to fit over a screw terminal. This is secured by nuts and washers. b) "Spade" type connectors which can be inserted into equipment terminals without the need to completely remove nuts and washers. c) A special type of spade connector for use in BR930 relay bases. These are simply inserted into the relay base. The crimps are constructed so as to spring apart slightly and lock the wire in position. A special tool is required to extract this type of connector d) "Shoelace " type connectors which can be inserted into equipment terminals without the need to completely remove nuts and washers. There are terminals available capable of inserting wires without the need of ferrules. Its recommended to use ferrules for multi stranded wires, however single wire cores can be inserted without a ferrule. For all types of crimped connection, it is important to choose the correct size to suit the terminal, the size of conductor and the size of insulation. Its also important to ensure the terminal can withstand the current carrying capacity of the wire. 6.2 Screw Terminals These are used in conjunction with crimped terminations or to directly terminate the conductors of solid cored cables. The cable is held tightly by a screw and/or nuts and washers to provide mechanical strength and electrical continuity. Where screw terminals are used, it is common to provide disconnection links to enable portions of the circuit to be isolated. 6.3 Plug Couplers Where modular equipment requires to be unplugged it is common to connect all wiring to a plug coupler. Wires may be soldered or wire-wrapped for security. This allows very quick disconnection and reconnection and avoids the need to check and/or test the wiring when a module is replaced. Plug couplers are often electrically or mechanically coded to ensure that they are only connected to the correct equipment. 7. CABLE ROUTES A safe route must be determined for the cables, both within buildings and externally. Within buildings, suitable ducts may be provided as part of the building or the equipment racking. If the cable route is shared with other than signalling cables, it must be ascertained that there is no hazard from electrical interference. Outside buildings there are several methods of cable installation used. These are described below. 7.1 Troughing Troughing is laid on the surface. It is usually inset into the ground for stability and the safety of staff walking along the track. Cable installation is simple. The lids are removed, the cables placed in the trough and the lids replaced. Two types are now in common use:- a) Ground Level Sectional Concrete Troughing. b) Ground Level Plastic Troughing. Plastic troughing is easier to handle but requires more accuracy in installation. Lids must be clipped on rather than being held in place by their own weight. 7.2 Ducts The following types of cable duct are in general use:- a) Earthenware duct b) Thick wall Rigid V.C. duct c) Thin wall V.C. duct. This duct must be laid in concrete d) Asbestos Cement duct - not generally available but has been used in the past in large quantities. e) Steel duct f) Flexible plastic pipe - this is corrugated along its length to allow the pipe to bend A common problem with all ducts is that over a long period of time they tend to accumulate debris which is either washed or blown into them. After a long period of time has elapsed, this may make further cable installation more difficult. 7.3 Cable Racks or Trays Slotted steel or plastic trays may be fixed to lineside structures, retaining walls etc. Cables may be fixed to this using plastic cable ties. The cable route is easily accessible for installation purposes but is also exposed to the environment and may be unsatisfactory in areas where vandalism is a problem. A similar method is to use cable hangers - hooks on to which the cable is placed and held in position by its own weight. This method is particularly useful in tunnels where there is little risk of vandalism and clearances are limited. 7.4 Buried Cables Provided the cable sheath is suitable cables may be buried direct in the ground. This avoids the cost of providing an expensive cable route. Cables are normally buried in one of two ways:- a) Laid in a trench with protective tapes or tiles and backfilled. b) Mole ploughed using a mechanical mole ploughing machine. In both cases warning markers should be provided on the surface at regular intervals. Buried cables suffer from several problems:- a) It is difficult to install further cables at a later stage without risk of damaging existing cables. b) Cables are vulnerable to excavations by others as they are not visible. Cable markers may eventually become obscured. c) Fault location and rectification is more difficult. d) Over a long period, ground movements and the growth of tree roots etc. may stretch and damage the cables. When specifying the type of cable installation, the engineer should take all costs, risks and benefits into account. 7.5 Aerial Cable Where an existing pole route is in good condition, it may be economic to install aerial cable. Aerial cable is however vulnerable to damage (gunshot, falling branches etc.) and the effects of lightning. Ordinary cable is unsuitable for aerial installation. Aerial cable has a steel strainer wire installed to provide the necessary tensile strength for hanging on poles. 7.6 Application In general the application of the above methods are as follows:- 7.6.1 Track-side cable routes Ground Level Concrete Troughing and Plastic Troughing is usually preferred where a large number of cables is required. Burying may be employed for small cable routes (i.e. single lines). Aerial cable could be considered if an existing pole route is still in good condition and the risk of damage or interference is low. 7.6.2 Under Track Crossings Practices vary but in general steel or asbestos cement ducts are preferable because of their ability to withstand vertical impact. In some cases thick wall P.V.C. duct has been used. Ducts should be installed sufficiently far below the track to be clear of track maintenance equipment (tamping machines, ballast cleaners etc.). 7.6.3 Platform Routes In general, platform routes should be provided with cable ducts and associated manholes for access to joints and pulling through of cables. Usually, it is more convenient to use earthenware ducts for platform routes. Surface troughing or hangers along the platform edge may interfere with track maintenance. 7.6.4 Tunnels The method used will depend upon the construction and cross section of the tunnel. It is often difficult to install a ground level route which will be clear of track maintenance machinery. Cable trays or hangers on the tunnel wall are therefore the most suitable. It is of course possible to design new tunnels to accommodate a cable route. Whether this is best located on the tunnel wall or floor will be determined by the type of track and tunnel drainage requirements. 7.6.5 Tail Cables Opinions vary on the best method. It is unlikely that there is a best method which suits both the signal engineer and the permanent way engineer. Cables may be simply laid across the top of the ballast. These are visible and easily removed and replaced. They are also vulnerable to damage by track maintenance and trailing objects on trains. Use of surface level ducts provides added protection but interferes with track maintenance. Placing the ducts below ballast level increases installation costs. Ducts can also become blocked with ballast and other debris. Surface cables secured to the top surface of the sleepers have greater protection than those laid loose across the ballast. This method may not always be acceptable to the track engineer. Tracks laid on concrete sleeper can use hard hats to cover the cable where hard hats are fastened on the concrete. 8. CABLE CONSTRUCTION A vast range of cables is now available. it is only possible to cover some of the main features of modem signaling cables in this section. There are many other cables widely in use for different application including control voltage to a Point Machine. Refer Figure 2 for the construction of a modern quad cable with Water Resistant property. Even though below section is based on Quad Cable Construction, we use this opportunity to discuss other alternative options available for armour, screen, internal & external sheath and the conductor insulation. Selection of PVC materials (XLPE -Cross Linked Polyethylene, LDPE -Low Density Polyethylene HDPE -High Density Polyethylene) and the addictive included for UV, Pest, Carbon /Iron /Copper/Mineral Dust, Acid /Salt, Industrial Cleaning solution shall be selected according to the operators need and the type of environment for the intended application. It is signal engineers’ responsibility to ensure that cable will not fail and select the property in case operator doesn’t specify detailed requirement. 8.1 Conductor Materials Copper is the most widely used conductor material due to its very low resistance and excellent mechanical properties. It is also available in sufficient quantities at an acceptable price. The copper is manufactured as wire of a consistent cross-section by repeatedly drawing the copper through holes in dies of reducing cross-section until the desired size is reached. As this process tends to harden the copper wire, it is usually annealed to restore its ductile properties. Annealed copper wires are complied to EN60811-203 /IEC 60228 .Refer RailFactor Article “Standard for Signalling cables “ as well for more details. Where rubber insulations are employed, the copper is generally coated with tin to prevent a chemical reaction between them causing corrosion of the copper and a change in the mechanical properties of the rubber. Where a large cross-section is required, aluminium may be used in place of copper (e.g. for power cables). Although Aluminium is more resistive, it is lighter and cheaper and has greater mechanical strength. Aluminium cables generally employ a solid conductor and are therefore more rigid than the copper equivalent. 8.2 Insulating Materials Each core of a cable must be insulated from all others and must therefore be surrounded by an insulating material throughout its length. Signalling cables use thermoplastic (P.V.C. or Polyethylene) or elastomeric (natural rubber or polychloroprene (P.C.P.)) insulations. The addition of P.C.P. to rubber improves its resistance to weathering. In very wet conditions, however, it has a tendency to absorb water over a long period of time. This will adversely affect its electrical characteristics. With telecommunications cables, the capacitance between cores becomes significant with a.c. signals. Dry air is the best insulator but impractical for cable construction. In older cables paper was widely_ used. This effectively consisted of a mixture of organic fibres and a large proportion of air spaces. As paper insulation is no longer used, polyethylene ie: HDPE (High Density Polyethylene-Solid) insulation is common these days with Low Smoke, Zero halogen property especially in the tunneled application. 8.3 Formation of Conductors Quad formation of conductors are the most recent trend for multicore cables .They are constructed in 1 Quad (4 Core ) ,3Quad (12 Core) ,4 Quad ( 16 Core ) , 5Quad (20Core ), 7Quad( 28Core) and 10Quad ( 40 core) .Each quad is arranged in the form of a Star (Diamond) and twisted together to get the best electrical property in terms of mutual Capacitance ( <45nF/Km) and Electromagnetic Compatibility compared to other formation which makes higher mutual capacitance .Lightly twisted pair formation is also available for better Electro Magnetic Compatibility. German DB(DB416.0115-Standard) defined Quad cable is popular for CBTC /ETCS application. Quads are helically stranded in concentric layers and cables more than 7Quad include two extra conductors with perforated insulation for surveillance. Signal Engineer shall select the compatible cable with respect to their signalling solution. 8.4 Core Identification Cores are identified in a number of ways, for a DB defined Quad Cable as shown in Figure 2 on light brown conductors black bracelets are printed in different combination .Eg: Two black circles printed together and repeated on fixed length apart ,One circle printed and repeated on certain length etc . As a Designer I am not a big fan of such identification as its confusing for the technicians for the first time. Methods of identification can be classified into 5. a) Coloured Tape wrapped around each core b) Coloured Insulation (Especially for Signalling Application Power Cables) c) Numbered Tape d) Numbering Printed on the insulation. e) Identification concentric rings (as mentioned above on DB defined Quad Cable) When numbering is used care must be taken to avoid confusion between 6 and 9. The easiest method is to write the number (six or nine) as well as or instead of the numerals. Various systems of colour coding are used depending on the size, type and manufacturer of the cable. In a paired cable only one conductor may be colour coded. The numbering of cable cores/pairs always starts from one at the centre and increases towards the outside of the cable. Each end of the cable is identified - the A end is the end at which the numbering of each layer runs in a clockwise direction, the opposite end is the B (or Z) end. 8.5 Core Wrapping, Screen, Inner Jacket, Swellable Tapes & Armour Plastic tapes are overlapped around cores which collectively hold all the cores together, on top of it, a copolymer coated aluminium tapes are wrapped. Tinned copper wire of 0.5 mm. run along the cable making in contact with the aluminium tape. This arrangement provides the functional earthing (EMC) option for the cable. This continuity wire needs to be earthed along with the armour for earthing the induced current generated due to parallelism (double sided for AC Traction line and singe sided for DC Traction). This arrangement is protected with Black coloured Zero Halogen Low Smoke compound PVC Inner jacket. Steel Tape armour is taped around the inner jacket. Swellable water blocking tapes are wrapped around the Inner jacket and Steel armour to avoid longitudinal water penetration. This arrangement along with inner and outer sheath is making the cable compliant to moisture barrier requirement per BS-EN 50288-1, EN 50288-7 or equivalent, water immersion test complying to IEC 61156-1. This construction is ensured to pass the Transversal water tightness and armour long water tightness test according to EN50289-4-2/A and water absorption for conductor insulation and outer jacket according to EN60811-402. 8.5.1 Types of Armour Armour provides additional mechanical heavy-duty protection, such as crushing, and resistance from pest such as rodent penetrating into inner conductor. Some cables will have nylon wrapping beneath outer sheath to protect from Termites and steel tape armour for rodent protection. Metallic armour not only provide mechanical protection it can also offer EMC protection but dose not replace the need for screen but lines less than 25kV can consider avoid screen under specific conditions. 1) Steel Tape Armour Steel Tape armour is sandwiched between water blocking tapes for DB cable. These are used for buried cables. According to American Railway Engineering and Maintenance of way Association (AREMA) states that tape armouring provide high degree of shielding protection than shield wires. 2) Steel Wire Armour Steel wire surround lead sheath for some cable design and are used for buried cable. This will surround the braided sheath and such sheath are used for high frequency emc protection 3) Corrugated armour Corrugated steel /copper surround the cable lengthwise beneath the outer sheath which is used for lines less than 25kv .This is mainly used in Optical Fiber Cable for optimum flexibility and recommended to replace with Fibers Reinforced Plastic (FRP) for electrified territory more than 25kV due to chance for high induced voltage. 8.5.2 Types of Shield/Screens Screening /shielding is used for reducing the effect of electromagnetic interference (EMI) or electrical noise which can disrupt the transmission performance in some environments. This noise may be because of external interference from other electrical equipment or because of interference generated within the cable from adjacent pairs (cross talk). 1) Aluminium /polyester tape with a tinned copper drain wire DB 416.0115-Standard Quad Cable referenced in Figure 2 have aluminium foil with attached tin plated copper wire . 2) Copper /polyester tape with a tinned copper drain wire This solution can provide better screening effect compared to aluminium foil. 3) Bare copper braid This is good for electromagnetic interference when the cable is subject to movements 4) Tinned copper braid Good for electromagnetic interference in presence of corrosive atmosphere or high temperature 8.6 Outer Jacket In addition to insulating individual cores, the entire cable must be contained within a sheath for both mechanical strength and environmental protection. Cross Linked Polyethylene (High Density Polyethylene Otherwise known as HDPE) are the most widely used. Low Density Polyethylene-LDPE) are also used depends on area of usage. Some older cables used lead, but its expense and associated health risks have led to its disuse. The choice of sheath material should consider the environment in which the cable will be used. Factors such as moisture, exposure to light and heat, the presence of oils and solvents, presence of carbon/iron dust, Train washing plant solutions, temperature, water immersion (IPX7) /submersion (IPX8) and the required level of resistance. In nutshell outer jacket is one of the most important elements for mechanical protection from external damages such as chemical (oxidation acid, oil), Mechanical (Abrasion), Environmental (Heat, Sun exposure, moisture, water), Fire exposure etc. Thermoplastic or Thermoset polymers are widely used where Thermoset have excellent properties against threats. 8.6.1 Ingress Level- Mechanical Properties. Mechanical shock severity shall be shared with the cable supplier whether its Low (Energy shock of 0.2 J, mainly for household installation hence not applicable) or Medium (Energy shock of 2J-standard industrial application) or high severity (Energy shock -5J) 8.6.2 Ingress Level- UV Resistance. Designer shall share the UV intensity requirement to the cable supplier based on the regional severity and exposure levels whether its Low (AN1 – Intensity ≤500 W/m²) or Medium (<500 W/m² intensity ≤700 W/m²) or High (<700 W/m² intensity ≤1120 W/m²). Refer Rail Factor Article “Standards for Signalling Cables” for more details. 8.6.3 Ingress Level- Water 8.6.3.1 Water Environment 8.6.3.2 Water Penetration The factor defines the water penetration in cables and to prevent the entry and migration of moisture or water throughout the cable. Water ingress can happen through Radial due to sheath damage and in this case, water enters in the cable by permeation through protective layers or due to any mechanical damage. Once water enter the cable, it travels longitudinally through out of the cables core. Where as longitudinal penetration moisture or water enters inside cables core due to ineffective capping or poor cable joint /termination .Please note water proofing and water absorption tests are different .There are no specific test for longitudinal water penetration for power cables .Radial water penetration test shall only be applied .Separate water penetration barrier are applied below the armour (or metallic screen layer ) and along conductor .Refer cable construction above in Figure 2 8.6.3.3 Moisture Protection Resistance offered by the jacket and the additional chemical used are ensuring the protection. However, the material with highest degree of water resistance is often not flame resistant, hence a tradeoff must be made between these two contradictory requirements. Choice of sheath material, make use of chemical moisture barrier and water blocking tapes can protect the cables from moisture. 8.6.4 Flame & Fire 8.6.4.1 Low Smoke Zero Halogen (LSOH) This is the property of cable to emit very low smoke and zero halogen and ensuring low corrosivity and Toxicity. Even though normal PVC cable ensure better mechanical and electrical properties, its poor in fire retardancy, corrosivity and low smoke capability. 8.6.4.2 Smoke Density Smoke can prevent fire fighters’ visibility and evacuation, especially in tunnel, work areas, control room and public areas. 8.6.4.3 Flame Retardant Flame Retardant property is vital, during a fire flame spread shall be retarded to limit to a confined area thus eliminating fire propagation. 8.6.4.4 Fire Retardant Property which when ignited do not produce flammable volatile products in required amount to give rise to a secondary outbreak of fire. 8.6.4.5 Fire Resistant Fire resistant cables are designed to maintain circuit integrity of emergency services during fire. Please note that Fire resistant cables are super expensive and normally considered for very vital cables (Eg; Fire cabinet ,depend on contractual requirement .Please note that Fire Resistant and fire retardant are different property and fire resistant is more stringent requirement. The individual conductors are wrapped with a layer of fire resisting mica/glass tape which prevents phase to phase and phase to earth contact even after the insulation has been burnt away. The fire-resistant cables exhibit same performance even under fire with water spray or mechanical shock situation. 8.6.5 Pest Resistant Depends on the intended area of the project ,there could be various threats such as ants, termite ,rodents , squirrels ,wood peckers ,other birds ,beetle and larva where cable contact with any plants to mention some .Various chemical compounds added on to the sheath ,depends on pest chemical and armours are protecting the cable .It may not be practical to have armoured cable for indoor application due to flexibility issue and outdoor environment have more threats . 9. ELECTRICAL PROPERTIES SIGNALLING CABLES 9.1 Voltage Rating of Cable Signalling control cables are normally rated for 600v/1000V. Voltage range classification for LV, HV, AC & DC according to IEC 60038 are as shown in Table 3 Maximum for High Voltage for IEEE is 35kV and in some countries its 45kV, which is country specific. Refer Table 4 for maximum permitted voltage Vs Rated voltage. 9 .2 Resistance of Cable This is the resistance of wire which increases with distance and normally included in the cable data sheet from the supplier. Its measured Ω/kM @ 50Hz (or 60Hz) and 20°C. This is the main parameter to calculate the voltage drop of cable. Voltage received at the end gear shall not be less than 10% of the source voltage. This means when you feed 130V from the SER, signal at the track side should at least get 117V. Cable conductor size shall be selected based on voltage drop calculation and shall cross check with field gear data sheet that the 10% voltage drop allowed will still fall in the minimum required voltage Important Note: Signal Engineer shall ensure that the resistance value (Ω/kM) provided by the supplier in the data sheet is loop resistance or wire resistance. Loop resistance means it’s the value for two conductors. While calculating voltage drop, number of loops used for the respective circuit and its distance shall be used. 9.3 Reactance of Cable This is defined in Ω/kM @50Hz (or 60Hz). This is important parameter for MV cables which need to be asked from the supplier but for LV, designer can define the allowed limit for LV 9.4 Capacitance of Cable Measured in µF /KM which is mandatory parameter for MV cable and shall be requested from supplier. As mentioned above Quad formation have less than 40µF /KM. Lower the capacitance better the cable property. 9.5 Maximum Short Circuit Current (Conductor and Screen) Maxum short circuit current in kA for conductor and screens for 1.0 seconds and 0.5 seconds respectively shall be requested and obtained from supplier. 10 CABLE TESTING It is not the purpose of this article to give detailed instructions on the procedures for testing and maintenance of different types of cable. However, the general principles of cable testing are described here. In general, whenever a cable is installed, repaired, re-terminated or jointed and at regular intervals during the life of the cable, tests must be made to ensure that: - a) Each core is continuous and of the correct resistance. A rise in the resistance of a core could indicate a potential fault. b) Each core is insulated from all other cores. It is normal for the insulation resistance to fall slightly during the life of a cable. Serious deterioration must, however, be detected before it causes any safety hazard. c) Each core must be adequately insulated from earth. Unwanted connections to earth are a potential danger to all signalling circuits and must be avoided. Where the cable has a metallic sheath, the insulation tests must include the sheath. Where the sheath is earthed and/or bonded for reasons of safety or noise immunity, the continuity of the sheath is also important. The continuity tests may be made using a suitable digital or analogue multi-meter set up to measure resistance. All tests will require the cooperation of persons at each end of the cable. A telephone circuit between the ends (using the cable to be tested if convenient) is essential to carry out an efficient test. The simplest method is to put a loop between one conductor and each other conductor in turn at one end of the cable. The loop resistance is measured at the other end using the meter. Any variation between individual readings (and changes since the previous test) should be investigated and resolved before the cable enters (or re-enters) service. Insulation and earth tests should use a suitably rated insulation tester (1000-volt Megger or similar for signalling cables). Tests should be performed between each core and each other core in turn. The acceptable value of resistance for a cable will depend on the circuits connected through it. However, as a general guide, a new signalling cable should give readings better than 10MΩ (when terminated). Readings less than 1MΩ could potentially be dangerous and require urgent investigation. The earth test may initially be carried out between earth and all cores connected in parallel. Only if this test is unsatisfactory need individual cores be tested to earth. Although a new cable is always completely tested before being brought into use, a complete test of a working cable is not always practical without serious disruption. In this case, routine tests are often carried out on a sample of cable cores (spare cores if available). Previous readings should be retained for comparison. 11. DATA CABLES Ethernet cables falls under this category. They are classified into different category Cat 1 to Cat 9, whereas Cat 1-4 are not suitable for modern day rail application and above Cat7 is not yet came into application while preparing the article. Refer Table 4 for category classification. Data cables with twisted pairs have different construction depends on the purpose, cable shall be selected. Refer Table 6 for various construction. 12. Fiber Cables Although many signalling applications must use metallic cables, the availability and cost of fiber optic cables is rapidly improving. Instead of electrical signals, they transmit information by passing light signals along the length of a glass fiber. Internal reflection contains the light signal within the fiber. Although not specifically employed in conventional signalling systems, fiber optic technology has the following advantages and necessary for modern communication-based train control system: - a) An extremely high capacity and bandwidth. b) Immunity from all types of electrical It is therefore of great use for communications purposes on electrified lines. Conventional jointing techniques are not applicable to glass fiber cables. Instead, the two ends must be cut squarely, butted up to each other and fused together by the application of heat. This is a very precise operation as any irregularities in the fiber will cause attenuation of the signal. Much of the work of jointing fiber cables can now be done automatically by sophisticated (and usually very expensive) fusion splicers. The action of cutting the two ends squarely, aligning them for a parallel joint and fusing for the correct period of time is largely automatic. Even with the high degree of automation, fusion splicing is not always 100% satisfactory each time. It is therefore usual to provide additional spare fiber at the joint. This must be accommodated within the joint closure. Category cables have limitation to transfer data more than 100meter, Fiber has significance in this case. There are two types of fibers: Single Mode: Long Distance Application Multi-Mode: Short Distance Application Single mode Fiber must be complied to G652-D type as per ITU-T standard and multimode with IEC 60793-2-10 There are two types of construction Loose Tubes used in cable concrete trough, direct buried and other harsh environment Micro Tubes for less harsh environment The END NOTE :- Please comment if you wish to include Cable Voltage Drop Calculation , Stanadard conductor sizes and a sample cable plan
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CH15 | ROUTE RELAY INTERLOCKING
CONTENTS Introduction Push Button Interlocking Point Circuits Route Locking Signal Aspect Controls Route Releasing Overlaps Preset Shunt Signals DISCLAIMER : THIS ARTICLE DOSEN'T REFLECT THE CURRENT NSW PRACTICE AND MORE OVER ROUTE RELAY INTERLOCKING IS GETTING REPLACED WITH COMPUTER BASED INTERLOCKING.THIS ARTICLE IS PURELY FOR INTERLOCKING STUDY PURPOSE AND HELP TO GAIN KNOWLDEGE ON INTERLOCKING .CIRCUITS USED HERE SHALL NOT BE USED FOR OPERATING LINES WITHOUT CHECKING LATEST PRACTICE .THIS IS BASED ON AUTHORS UNDERSTANDING OF THE NSW INTERLOCKING INTRODUCTION 1.1 Development of Relay Interlocking Systems The earliest interlockings were mechanical, between the levers of a frame but, with the development of block systems and track circuits, electrical controls were added. Staffing levels could be reduced by displacing a number of small signal boxes by fewer, larger installations. Fully electrical interlocking systems became necessary to control power operated signalling equipment. Initially, miniature lever frames were used. There are however many advantages in using a control panel incorporating buttons and switches into a diagrammatic representation of the track layout. With panel operation, a totally relay based system was required. Many suppliers produced their own different systems which, although different in detail, followed similar principles of design and operation. The signalman's control device was usually a button or a switch. Unlike the lever, it was free to move at any time. The relays now provided the security of the interlocking system. Additional indications had to be provided to show the signalman the state of the interlocking. During a period of substantial investment in signalling modernization in the 1970's, design, installation and testing resources were in short supply. British Railways invited major suppliers to offer standardized interlocking systems which could as far as possible be factory wired and automatically tested. The contractors met the need with modular or "geographical" systems. The two main systems adopted, the Westinghouse "Westpac" and the AEI (GEC later acquired by Alstom) systems underwent several stages of development and were widely used. These systems certainly provided for very quick installation but there were disadvantages. The most important was that each development of a system was incompatible with its predecessors. The two systems were very different in their design philosophy. Westinghouse adopted a "one set per function" approach which led to high levels of redundancy, whilst GEC provided several sets combined as necessary for the function present. This approach increased substantially the quantity of inter-set wiring. Due to rising costs and problems of spares and modifications, geographical systems have fallen in popularity. State Railway Authority of NSW (later RailCorp, now TfNSW ) never adopted any form of geographical interlocking but has for some years purchased interlockings built to its own set of standard circuits. BR(British) also developed a set of typical circuits for free-wired interlockings which in operation would appear similar to the geographical systems and adopt a high level of standardization of circuit design. These were incorporated into a BR Specification (BR850) which is probably one of the most standardized, and probably the last, relay interlocking specification in widespread use. On BR the use of route relay interlocking systems has now been superseded by solid state interlocking for all new work. As most route relay interlocking systems follow similar design principles and methods of operation, the State Railway Authority of NSW(Railcorp/Currently TfNSW) circuits will be used throughout for reference. NOTE:- This article is purely written for study purpose and shall not be applied without checking current practice 1.2 Relay Types Various types of relay are used in the interlocking. As well as neutral relays, slow to operate, slow release and magnetically latched relays are used. It is important that the operation of each type is understood, and circuit symbols recognized, in order to follow the operation of the equipment. In particular, latched relays will remain in the last position they were moved to. 1.3 Diagrams All the circuit diagrams used for reference are based on the NSW (New South Wales. Australia) standard circuits. 1.4 Variations In practice, different installations may vary slightly in detail of circuit design. For example, full overlap swinging facilities may not always be provided. It is important to understand the relationship between the control tables and the interlocking circuits. The circuits must always be designed to function in the manner described by the control tables. 1.5 Relay Names and Functions 1.6 Control Panel Symbols Figure 1 to 4 below shows the detail of the panel push buttons. Of particular note to this course is the directional arrowhead on push buttons giving information about their use as the start (entrance) or finish (exit) of a route. The button surround may be of a different color according to the button function (main or shunt) and direction of traffic (up or down). 2. PUSH BUITON INTERLOCKING 2.1 Operation of Entrance-Exit (N-X) Panel In route control system of signaling, a route is set and the signal leading over it cleared by the signal man operating two pushbuttons which are located on a control panel .The first button operated is at the commencement(Figure 1) of the route and the second button at the finish(Figure 2) of the route .The finish button is generally at the next signal applying to the direction of traffic being dealt with, but in the case of a route which leads into a section, siding or terminal road the finish button is located in the section, siding or terminal road. Providing all conflicting routes are normal the push button operations are registered and any points in the required route which are not in the correct position will operate, then providing the line is clear to the clearing point, the signal will exhibit a proceed indication. If a conflicting route is set or a previous train is passing over points within the route and the points are out of position for the next movement, the button operation is not registered, and it will be necessary for the signalman to again operate the buttons when the route is free. To clear the next signal the last button operated which represented the finish of the previous route is again operated and acts as a commence button for the next route. A second button is then operated to locate the finish point for this route. After the passage of a train, or if it is required to cancel the route, without the passage of a train, the commence button for the route must be pulled to restore the route to normal. A signal cannot clear for a second train unless the route is normalized and then set again. If a button has been pushed as a commence button and for any reason the route which was to have been set is not required, the commence action may be cancelled by pulling that button. Only one button may be effectively pushed at a time and when operating a button as a finish button it should be depressed for approximately one second. The entrance-exit (N-X) push button panel is the standard type used by British and NSW Australia, and widely used elsewhere. The method of operation to set a route is: - Press the entrance (start) button and release it. The button will flash to indicate it is the selected entrance. The next button pressed must be the exit or finish of the route, and provided the route selected is both valid and available, the route will be set and locked, and white route lights will indicate the extent of the route set. The entrance button will display a steady white light. If an invalid exit or an unavailable route is selected, the route setting will be aborted. Because some buttons may function as both start and finish buttons (Refer Figure 5), and since the start signal may have several valid routes, each with a different finish, a constraint is imposed that only one route may be selected at a time. Where this would be over restrictive, several independent push button interlockings will be provided, one (or more) for each signalman's control area. In below figure 5, button for Signal 2 is a finish button for route from Signal 1 and is a commence button for route leading to Signal 4 and 6 LEGEND Note: When relay is energized (Up), front contact will be made(close) and back contact will be open, similarly when relay is de-energized (Down) front contact will be open and back contact will be made (Closed) Some relays will be Normally Energized (EG: USR, ALSR, Track Relay), whereas some relays will be Normally De Energized (EG: DR Relay for Control Signal to show green aspect) Non Vital Relay contacts are same ,but dot on each end of armature ,thats how we identify 2.2 Push Button Relays All push buttons on a panel have three positions, middle (denoted 'M'), pushed (denoted 'F' - meaning "from" the operator) and pulled (denoted 'T' - "towards" the operator). The button is sprung to return to the middle position after it is either pushed or pulled. Depending on the exact function of the button, relays will be provided to repeat the relevant positions or combination of positions of the button. Refer figure 5 for the most common circuit. To set a route from No .1 Signal to No.3 (M) Signal, button No 1 (for Commence) is pressed, energizing 1(FR) relay The diagrams show these relays wired direct to the panel button. This is the normal arrangement for the local interlocking at the control centre. For other interlockings, a remote-control link, normally TDM, is employed. Refer Figure 7. Any Telemetry such as Kingfisher also could be used. Traditionally control system is non vital system used for route request and indication of equipment status back from the track. 2.3 Push Button Checking Circuit At various stages in the route selection process, it is necessary to prove that only one button is being pressed. This is accomplished using the (R)PR circuit sample shown in Figure 8 The (R) PR provides the interlocking between buttons to ensure only one button operation is registered at a time. 1(F) R contacts make, energizing 1(R)PR relay. This lifts 1(R)PR contacts cutting out all other (R)PR Relays Back contacts of all following (R) PR relays are included in the negative side of each (R)PR relay to prevent it picking up if its button is operated (F) R energized whilst any following (R)PR is already energized. Thus, ensuring only one button operation will be registered at a time. 2.4 Pressing the Commence Button The interlocking will store the signal selected as the start of the route by holding the appropriate CeR up. Refer Figure 9 The CeR initiates the commence operation to set a route. 1(R)PR contact makes, energizing 1CeR relay. 1CeR stick contact is made and holds 1CeR relay energized when 1(R)PR drops out (ie when commence button No. 1 is released) via 1(N)R & FnJP2R contacts. 1(N)R contact is a route cancelling contact which energizes when a button is pulled and the FnJP2R contact (2nd repeat finish normally closed relay) forms part of the timing cycle circuit. These circuits will be covered later. As 3 buttons can be either a commence or finish button, 3FnR (Finish Relay) is proved de energized in its commence relay circuit. 2.4.1 Machine in Use The MUR sets circuit operation to ensure the next button operated initiates the finish operation for the route. 1CeR contact picks up and energizes the MUR relay whose contact turns MUR indication light on (flashing red). The FnPR contact ensures that the MUR is held energized to prevent the MUR from dropping out if a finish button is held longer than one second this would change the button from finish to a commence function (Refer Figure 13). In Figure 11 MUKR is a repeat of the MUR and the FnPKR is a repeat of FnR, Refer Figure 12 This gives a flashing red or green indication on the console which indicates to the signalman whether the machine is in use or finish. When FnJP2R relay drops out 1CeR relay de-energizes, thus extinguishing the MUR light. Refer Figure 14. 2.5 Pressing the Finish Button The finish of the route will be identified by one FnR energized. The combination of CeR and FnR uniquely identify the route. Refer Figure 7 above. To select destination (finish) in this case there is only one possible destination from Signal 1 which is No. 3 signal. Pressing button 3(M) for finish, energizes 3(M) (F)R. The (FM)R relay provides for emergency replacement of a signal after the route has been set. Refer Figure 12 for Finish Relay (FnR) Circuit The FnR initiates the finish operation to complete the route being set. When the 3(M) (R)PR energizes, and because the MUR relay is already held up via No. 1 CeR as shown Figure 10, 3FnR relay energizes for the period of time that the button is held in. As No. 3 button can be either a commence or finish button, both commence relay functions, 3 & 3 (S), are proved down in its finish relay circuit. This holds the FnR de-energized during the commence function when the MUR has picked up and while the (R) PR is still energized. 2.6 Push Button Circuit Normalization Approximately 1 second is allowed before the push button circuits are normalized, to permit setting of another route. This prevents preselection by the selection of the commence and finish signals before the route is available. Figures 13 & 14 show the normalizing circuits. The FnPR (finish repeat relay) initiates the timing sequence for a route to set. Refer Figure 13 for Finish Repeat Relay Circuit. When 3FnR contact makes, and energizes the FnPR relay the timing cycle, of approximately one second commences and, in this period the route must be capable of setting. If the route is not capable of being set within one second of the signalman operating the finish button, the action is not registered and the entire operation for setting a route must be commenced again. The FnJP2R and the stick function of the FnPR maintain the FnPR energized if the finish button is released before the timing cycle is complete to allow the timing cycle, once commenced to once completed. The FnJR & FnJPR's (finish timing relays) provide the timing sequence initiated by the finish function. The FnPR energizing starts the timing cycle through the slow to drop relays FnJR, FnJPR & FnJP2R. When the FnJP2R relay drops out a number of things occur: The CeR relay is dropped out via FnJP2R contact in the stick path circuit as shown on Figure 9. The MUR relay is dropped out via the CeR contact dropping out in circuit as shown on figure 10 The FnPR relay drops out if the finish button is released in circuit shown on figure 13, via the FnJP2R de-energized in its stick circuit. This timing sequence provides the non-storage feature of the system. (i.e routes cannot be pre selected until a train has vacated the route). 2.7 Effect of Pressing the Wrong Type of Button Pressing a finish only button at the start of route setting will have no effect because the signal does not possess a CeR. The circuits will normalize as soon as the button is released. Pressing the incorrect finish button will result in no route being set as the FnR and the CeR previously selected will not both appear together in the same route setting circuit anywhere in the interlocking. 2.8 Indications Panel To assist the signalman indication lamps are provided to show "machine in use" or "machine finish" to advise him of the current state of route setting. On many BR interlockings, the technician is provided with the facility to hold the equivalent of the FnJP2R energized to assist fault finding. This enables him to test circuits which otherwise may only be energized very briefly. This facility must be used with great care, and with the agreement of the operator, as it will inhibit the setting of any other route in the interlocking. 2.9 Route Control System-Route Setting Having now dealt with the operation and circuits associated with the control panel switches, we will now cover the operation and / or function of each relay, for the setting of the points to the correct position, locking them, setting the required route, and clearing the appropriate signal leading over that route which will include route locking and approach locking etc. Refer Figure 18 & 19 2.10 Route Setting When a button is pushed to select the commencement of a route (R)PR energizes & providing that no other button has been pushed a contact on the (R)PR closes the circuit for the 'commence relay' CeR, (Refer Section 2.4). A front contact of the CeR then energizes the 'machine in use' relay MUR (Refer Section 2.4.1). The MUR energized opens the pickup circuit for all CeR's & this determines that the next button to be pushed will function as a 'finish' button. The CeR for the button operated is held energized by a stick circuit which includes a front contact of the finish timing relay FnJR, back contacts of its own (N)R relay & its own front contact. When the next button is operated to define the finish point of the route to be cleared, its (R)PR is energized & because the MUR is up, the finish relay FnR (Refer section 2.5) for that button will energize for the period of time that the button is held in. A circuit is provided for the MUR via a front contact of FnPR, the finish relay's repeat relay, to prevent the MUR from dropping out if a finish button is held for longer than one second. (This would change the button from a finish to a commence action). Contacts of the CeR & FnR relays in series are utilized in the negative side of the route NLR delatching coil and drive the relay down this in turn closes the circuit for the route RUR and providing all locking and track circuit conditions are satisfactory the route will set and its signal clear. (Refer Section 2.12). The commence relay CeR and FnR (finish relay) remain energized for approximately one second after the finish relay has operated but long enough to allow the route NLR to delatch and the RUR to energize. This sequence provides the non-storage feature of the controls. That is, if the route is not capable of being set within one second of the signalman operating the finish button, his action is not registered & he must operate the buttons again when the route is free. The method of obtaining the one second timing period is as follows. When a finish relays FnR energizes, its front contact completes the circuit to the finish repeat relay FnPR (Refer Section 2.6) & a back contact of the FnPR opens the circuit to the finish timing relay (Refer Section 2.6). The three finish timing relays are slow-release relays & approximately one second after the FnPR has energized the FnJP2R opens its front contact to break the holding circuit for the commence relay network (CeR). (Refer Section 2.4). A stick circuit is provided to hold FnPR energized until FnJP2R is de-energized. This ensures that if the finish button is released before the timing cycle has been completed, the FnJP2R will still release & cancel out the CeR. When the commence relay releases & the finish button has been released the MUR releases, and with the timing relays energized & all button relays are in their normal position the system is ready for another route to be set or for another attempt to be made to set the same route. 2.12 Route Normalizing The (N)R Relay controls(Refer Figure Below) the normalizing of the appropriate route NLR, and when energized, drops the route reverse relay (RUR), and latches up the route normal relay (NLR). Refer below figure 20 for a typical (N)R Circuit Figure 20 Normalising Relay When No. 1 button is pulled, 1(N)R is energized and is held up by a back contact of the route NLR which is to be normalized, a back contact of 1 CeR and its own contact. The stick circuit will maintain 1(N)R energized until the route NLR circuit is completed by the signal returning to stop or the approach stick energizing as the case may be. The push button can therefore be released immediately after it has been pulled. Once the route NLR has latched up, its circuit is opened by the (N)R relay dropping & it is held in the up position by its magnetic latch. The back contact of the CeR in the (N)R stick circuit permits a signal to be recleared, if required, after it has been cancelled but the route has not normalized due to approach locking. A back contact of the (N)R relay is wired in the stick circuit of the relative CeR relay & this allows a button which has been incorrectly pressed as a commence button to be cancelled by pulling the button. 2.12 Checking Route Availability & Validity The Commence and Finish selected by the signalman, stored as CeR and FnR, must now be checked for validity - physical route possible - and availability. Normal lock and reverse route relays are provided for each possible route in the interlocking. These circuits comprise two parts. To the right of the relays (in the negative feed) are the relay contacts which respond to the push button circuits. The validity of a route is proven by the presence of a circuit with the correct CeR and FnR combination. Refer Figure 15 The availability of the route is checked via the positive feed where all locking is proved. An example of a route with points is shown on Figure 16. The points must be in the required position (NLR or RLR up) or free to move (WZR up). Providing the route is available the NLR will unlatch and allow the RUR to pick and stick. This operation must complete within the 1 second before the push button circuits normalize. In the event of the route not being available at the time of selection, or within one second, the push button circuits will normalize. The selection is not stored until the route becomes available but can only be acted on at the time of selection. 2.12.1 Route Lock Relay (NLR/RUR) Each route in the interlocking from signal to signal or from signal to section, siding or terminal road has a RUR to set points and clear the entering signal and a NLR which proves the route normal and is used in locking conflicting routes. The route lock relay circuits for No. 1 route are shown on Figure 15. The route RUR and NLR circuits are electrically interlocked with each other. Thus, 1NLR back contact is in series with 1 RUR operating coil and 1RUR back contact is in series with 1 NLR operating coil. The NLR is a magnetically latched relay and remains latched in its last operated position. It has two coils, one to latch the relay up, and another to latch the relay down. The operation is described on section 2.12.2 The route NLR when latched up is used to release conflicting routes, and proves that: - the signal has returned to stop the signal is not approach locked the route RUR is de-energized & is therefore not capable of setting points or clearing the controlling The route RUR when energized proves that the route NLR is latched down thereby checking that all conflicting routes and points are locked prior to the route setting. The interlocking between routes is carried out in the positive leg of the RUR relay in accordance with the locking table for the interlocking. If a route requires that certain other routes must be proved normal before that route can set, then normal contacts of the conflicting route NLR's are included in the positive side of the RUR for the route concerned. The interlocking between routes and points is also carried out in the positive leg where a contact of the points NLR or RLR is included and qualified by a contact of the WZR for the points concerned if the points are out of position but are free to move. Note: Positive leg of the route locking relay circuit included the interlocking function (Safety Function) and negative leg of the relay circuit included the route selection (Non Safety Function) Refer Figure 17 for sample control table. Routes and point conditions reflected are the logic for the positive limbs of the RUR/NLR circuits referred in Figure 15/16. Eg: Front contact of Route 9C NLR will be in series with 1NLR and 9BNLR, where 1NLR is qualified(parallel) by point 103NLR and 112 RLR and 9BNLR qualified by Point 112RLR. This will be in series with Points required/free to move Normal or Reverse 2.12.2 Route Lock Relay Circuit Operation (NLR/RUR) Refer Figure 15, when the commence and finish push buttons have been operated to clear No 1 signal, 1 CeR and 3FnR contacts will be up together as described in Section 2.4 & 2.5. This drives 1 NLR magnetically latched relay down and closes 1NLR contacts. This action then completes the circuit for 1RUR relay to energize via 1 CeR and 3 FnR contacts. Front contacts in the negative leg of 1 RUR circuit then close, and hold the relay energized via 1(N)R and 1 (FM) R normalizing contacts after 1 CeR and 3 FnR have dropped out at the completion of the timing cycle. The route will remain set until the commence button at No 1 signal is pulled, which will pick up 1 (N) R contact and open 1 (FM) R contact. If the ALSR relay is de-energized, ie, the route is approach locked, the route cannot be normalized to release the interlocking. However, it may be re-cleared for the train to proceed. 3 POINT CIRCUITS 3.1 Principles of point operation Points may be called to operate by one of two methods: - a) The setting of a route requiring the points to be moved using route setting buttons, or - b) The operation of a point key (lever) on the panel. At the time of calling, the points must be free of locking in their present position. Points may be locked by route locking, track circuit occupation, the point key having been turned, or another route having been set. The points must be free at the time of selection, and the selection must not be stored until the points become free, (anti-preselection). In the event of a power failure the last legitimately selected position of the points should be held, and, on restoration of the power, the points should not be called to another position due to the random recovery times of different relays within the interlocking. 3.2 Calling the Points Figure 21 shows a point Lock relay (NLR/RLR) circuit. It has two halves. Setting contacts are in the negative feed and locking contacts in the positive feed. At any one time only one Lock relay should be up corresponding to the position to which the points were last called. Unlike the route lock relays, both NLR and RLR are latched relays. There is no distinction in safety terms between the normal and reverse positions for points. Except in special circumstances, points controlled from a route setting panel are not returned to the normal position after use. 3.2.1 Lock Relays The points normal lock relay (NLR) and points reverse lock relay (RLR), perform route and interlocking functions associated with the points, they also control their operation. On operation of the control panel buttons to set a route, the RUR is energized, providing the interlocking is free. The RUR contacts then set all necessary point lock relays which in turn operate the points to line up the route. With point detection indicating that the points are in their correct position and providing that the track circuits concerned are clear the signal control relay energizes via contacts of the RUR and button normalizing relays. These relays are magnetically latched and remain in their last operated position. Therefore, before picking up one relay, it is necessary to energize the release coil of the other. This is accomplished by wiring the negative side of each release coil to the negative side of the operating coil of the other lock relay. As each lock relay operating coil is wired through a back contact of the opposite lock relay, one lock relay is proved down before the other lock relay can energize. Therefore, before a points lock relay can be energized to drive the points to the next position, the lock relay for the existing position is proved down ensuring that all routes which lead over the points in their present position are normal before the points can move. In the positive leg of the points NLR and RLR is the interlocking function between the points, and signals which lead through the points. Route (track) locking over the points, including selective overlap (tracks which will allow the points to operate to the vacant overlap), and back contacts of the opposite points lock relay, and detector relay, to prove those functions de energized before the lock relay concerned will operate. In the negative side of the NLR circuit are RUR contacts of all routes which will set the points normal in series with a contact of the points (C) R (Lever Centre Relay) and in the RLR circuit, RUR contacts of all routes which will set the points reverse, together with a point (C) R contact An alternative path is also provided for use when the points are to be operated under lever control for both normal and reverse operation. 3.2.2 Circuit Operation When a call is placed on 101 points to operate reverse, e.g., 3M(A) route has been called, 3(M)A RUR will energize closing the negative leg for 101 NLR release coil and 101 RLR operating coil via the lever centre relay (C)R, to negative. Providing the points are free to operate, i.e., 101 WZR (points free relay) is energized, indicating that the interlocking is correct, and the track circuits through the reverse route are clear, 101 NLR will be driven down via front contacts of 3M(B) & 3(S) B NLR, 3ATPPR and 3XTPR tracks (interlocking and track locking for the reverse route) 101 NLR and 101 WZR and WJR. This action closes a back contact of 101 NLR in the positive leg of 101 RLR operating coil, proving the NLR has de-latched and allowing the RLR to energize (latch up). A front contact of 101 RLR now connects the WZR to 101 NLR circuit in readiness for the points when called to operate normal. 3.3 Locking Circuits The positive feed to the NLR/RLR circuit checks the availability of the points to be moved, either normal to reverse or reverse to normal. The feed to the WZR can be obtained from either the normal or reverse branch of the circuit, dependent only on the present state of the NLR and RLR. If WZR is able to energize it shows the points are free to move from their present position. 3.3.1 Point Free Relay (WZR) & Point Timer Relay (WJR) The WZR or points free relay(Refer Figure 21) is a slow to release relay to prevent the RUR from dropping out during the operation of the points lock relays. It taps off the interlocking and track locking portions of both NLR and RLR. When the NLR is energized the WZR detects if the points are free to move reverse. When RLR is energized the WZR detects if the points are free to move normal. Thus, the WZR relay when energized indicates if the points are free to operate to the next position. The WZR is used to convey this information to the route RUR circuits which are allowed to energize if the required point lock relay is energized or if it is free to be energized. A point timer relay (WJR) is provided to ensure that the tracks have been free for a length of time to cover "bobbing" tracks. The WJR(Figure 21) is a slow pick-up relay, which together with the slow pick-up track repeat relays provides a two-stage timing before the points are free. The WJR is tapped off the points lock relay circuit, and a contact of this relay cuts the WZR. The WJR is provided in the NLKPR and RLKPR circuits. (Figure 21). A contact of the WZR is also used to illuminate the points free light above the centre of the lever and indicates to the signalperson when the points lever may be operated to drive the points to the next position. The only interlocking information not conveyed by the WZR relay is the point-to-point locking and this is added to the points free light circuit. The WZR relay and WJR point timer relay in conjunction with the transient nature of the button controls provides for non-storage operation of the points under route setting conditions. If when the buttons are pushed to set up a route, the point lock relays are not in the correct position or free to be operated to that position as indicated by the WZR relays for the one second period during which the button relays are energized, it will be necessary to operate the buttons again when the route is free. If a train were passing over points within the route in question the security of the points is dependent entirely on the track relays remaining down whilst occupied by the train. Therefore, if the track relay should "bob" during the one second which the button relays are energized the points would commence to move under the train. To guard against this event, track repeat relays are made 4 seconds slow operating so that local tracks in the point circuit must be clear for 4 seconds before the points become free to operate to the next position. 3.3.2 Lock and Detector Repeat Relays (LKPR) Refer Figure 21 .The circuits for these two relays which tap off the points lock relay circuits are the points Normal and Reverse lock and detector repeat relays (NLKPR and RLKPR). Contacts of these relays are used in the signal control circuits to provide proof that the detection and lock relays are in their correct position and that the operation of a route RUR has locked out the points lock relay for the movement to the next position before a signal can clear. The NLKPR taps off the normal lock relay circuit so that it includes all interlocking which prevents the points from driving normal. In the case of 101 points, 3(M)"A" and 3(S)"A" NLR's, proving that routes which require 101 points reverse are normal. It ensures that 101 NLR and 101 NWKR are normal. It also ensures that 101 ROLR, 101 RLKPR, 101 WZR and 101 WJR are de-energized by back proving contacts. The proving of 101 WZR de-energized is most important, and its function is as follows. With 101 NLR energized (latched up), and 3(M)"A" route is called, the release coil of 3(M)"A" NLR is energized when the commence (CeR) and finish (FnR) button relays are operated and when 3(M)"A" NLR makes its back contact, 3(M)"A" RUR is energized. When 3(M)"A" NLR opens its front contact the circuit for 101 WZR and 101 NLR is opened and 101 WZR drop contact makes to allow 101 NLKPR to energize and complete No. 3 signal HR circuit. Therefore before No. 3 signal can clear proof is obtained that 101 RLR circuit is opened via 101 WZR de-energized and therefore 101 points cannot be operated to the reverse position. 101 RLKPR taps off 101 RLR circuit and performs similar functions to 101 NLKPR, being utilized in signal control circuits which lead over 101 points in the reverse position. 3.4 Calling the points by route setting Energization of an RUR in the negative feed of the lock relay circuit will set the points provided the point key is central and the points are not locked by other routes, track locking or route locking. Energizing the RUR will have proved that the points are in position or available (e.g., NLR or WZR up for a move to the normal position). Contacts of the RURs for each route shown in the control tables to set the points will be wired in parallel in the NLR or RLR negative feed. Conversely the NLRs for the same route must be included in series in the positive feed of the opposite lock relay. 3.5 Moving the points using the point key/lever Relays (N)R and (R)R repeat the panel key/lever in the normal and reverse positions respectively. They allow the points to be moved individually. It is important to note, WZR must be up at the time of setting with the point key or the points will not move, even if any locking is later removed. Therefore, when moving the point key from normal to reverse (or vice versa), it must be held momentarily in the centre position to allow the WZR to reoperate. 3.6 Points in Overlaps There are situations, generally where swinging overlaps are involved, in which the simple use of route RLRs to call the points is not sufficient. Relays NOLR and/or ROLR may be provided to give the required point setting commands Refer Figure 22. Overlap relays automatically set facing points in the overlap of a signal to give a clear overlap for that signal. When a route which has facing points in its overlap is set and the points are lying so that the overlap over which the signal would clear is occupied, but an alternate overlap is clear and the points are free to operate to that overlap, the overlap relay OLR will energize and drive the points to that position. The controlling signal for the route then clears via the free overlap. The OLR relays are only energized during the one second period that the button relays are energized and thus comply with non-storage requirements. Protection against the OLR's causing the points to move if a track relay should bob under a train is obtained by wiring a contact of the relative point WZR relay in the OLR circuit, thereby ensuring that the points have been free for at least four seconds before they can be operated to another position. If the points in the overlap of a route are not free to move to an unoccupied overlap when the route setting buttons are operated, the route RUR will energize providing its requirements are met but the OLR will not be energized. Because of the transient nature of the button controls it will be necessary to either re-operate the buttons when the points become free or to set the points to the required position by operating the point lever. 3.7 Point Operation & Detection The position of the NLR and RLR in the interlocking must be translated into a command to the points to move to or remain in the corresponding position. This is done by means of a polarized circuit to the NWR & RWR at the location. A typical circuit is shown on Figure 23. Points control is affected through the NLR and RLR. Contacts of the relevant lock relay operate the normal points contactor (NWR) or reverse points contactor (RWR). The points contactors can be of the type that are mechanically interlocked with each other and are installed in the points locations, or polarity sensitive relays installed in the points location or provided within the point machine, and which will only energize if the polarity of the supply to the coils is correct. Relays installed in the points location are type QBCA1 and have two heavy duty front contacts that are capable of switching about 10 amps current to the point motor. Referring to the circuits above, each contactor is double switched by contacts of the points lock relay concerned. The points NLR when latched up will pick up the normal contactor and the points RLR when latched up, will pick up the reverse contactor. The opposing lock relay is proved down in the relevant contactor circuit in each case. This provides a measure of safety so that if both lock relays should be up at the same time or if either lock relay is unplugged both contactor coils are open circuited. When energized the motor will run either normal (if NWR energized) or reverse (if RWR energized). They are polarity sensitive relays, usually QBCA1 and will only energize if polarity i s correct. Positive to R1 and negative to R2 contact. These relays have two heavy duty front contact which are capable of switching about 10 amp current on and off to the point motor. They are energized by the NLR energized and RLR de-energized in the case of the NWR, and the NLR de-energized and RLR energized as in the case of the RWR. These contacts are double cut into the point relays, ie contacts in both positive and negative feeds. Each relay is also controlled by the opposite relay being de-energized, ie RWR proved down in NWR circuit and vice-versa. There are two contacts of each relay in the opposite circuit, ie: referring to the circuit diagram RWR A6/A5 and D6/D5 are in the NWR circuit. This is to prove that the whole relay is de energized as it is possible to have half the relay 'stuck-up' by failure of a set of contact strips. Relay rows A and B are operated by one strip from the armature and relay rows C and D from another. This then proves that the RWR is definitely de-energized before the NWR can pick and vice-versa. Once the points have moved to the correct position, a polarized detection feed comes back to energize NWKR or RWKR (Figure 24) provided the detection corresponds to the position required by the interlocking (NLR/RLR). The detector relay circuits (NWKR & RWKR) as well proving that the points have corresponded to the lever and are locked (via NKR or RKR), also proves the following:- The opposing WKR de-energized, via a back contact The corresponding NLR or RLR energized, thereby ensuring all interlocking functions are correct The LWR (E.P. points,NOT SHOWN ) or Isolating Relay (electrical points) de-energized via back contact . This ensures that the points cannot be operated, except under normal operating conditions. For EP. points (Not shown here) that the Plunger Lock has returned to the normal position (locked), via plunger lock normal contacts. This ensures mechanically that the points will not move should the control valve be falsely energized, or "creep" open due to worn equipment. For electrically operated points, a contact of the EOL is included to ensure that while the EOL is withdrawn from the lock for emergency operation of points, both WKR's are de-energized, thereby ensuring that the signals protecting the points cannot be cleared, or the points operated from the control With the points normal and called reverse, the points NLR is driven down and drops the normal detector (NWKR). A back contact of the NWKR closes the circuit for the points RLR, allowing the relay to latch up, providing that all interlocking functions are correct. This allows the points to then operate to the reverse position. The point operation circuit has the additional protection of the IR (isolating relay). Refer Figure 25. This proves all signals reading over the points normal, all direct locking tracks clear and no trains between the points and a protecting signal (unless moving away from the points. The WTJR (where provided) disconnects the circuit to the point contactors if the points have been running too long. This will avoid damaging the point motor or clutch if an obstruction in the points prevents them completing their movement. IR's (for electrical operated points) or LWR’s (for E.P. points) prevent irregular operation of the points should the point lock relay, contactor or control valve be falsely energized whilst a train movement is taking place over the points. The IR associated with electrically operated points can be of the neutral contactor type or a polarity sensitive relay and is installed at the points so as to be physically remote from the points contactor to prevent manipulation, or in the points location where the points contactors are of the relay type and thus sealed or located in the points machine. The LWR is associated with E.P. points and is normally located adjacent to the points. When energized the LWR unlocks the facing point lock via the plunger lock and allows the points to move. The IR or LWR check that the home signals protecting the points are normal and not approach locked, and that tracks from the home signals to the points, and the local tracks over the points are clear before the points can be operated to the next position. When the points have reached the required position the IR or LWR is open circuited by either a switch machine contact where the contactor type is used, or the relevant local detector relay (NKR/RKR) energizing where polarity sensitive relays are used and proved de-energized in the detector relay circuit (NWKR or RWKR). Where polarity sensitive relays are used, for electrically operated points a contact of the EOL (Emergency Operating Lock) is provided and when operated manually, the isolating relay is open circuited. The NKR and RKR (Normal and Reverse indicating relays), are located locally at the points and prove that the points have corresponded to the lever movement and are locked. They are divide into (2) two basic circuit types, those for E.P. points and those associated with electrical operated points. Figure 26 shows a typical NKR and RKR circuit used for electrically operated points using polarity sensitive relays. The points are proved Normal or Reverse and locked before the corresponding detector contacts are allowed to make. The opposing KR is also proved de energized via back contacts. 4 ROUTE LOCKING 4.1 Principles The control tables will often specify route locking to allow the route to be held in front of a train whilst being released section by section behind the train. This is effective as soon as the route is set and releases only after the passage of the train (or if no train has entered the route after the signal approach locking is released). 4.2 USR (Route Stick Relay) Circuits The relays used to lock each part of the route are called USRs, Route Stick Relays, which are energized when that section is free of route locking in the direction specified, and de-energized when route locked. A typical USR circuit is shown on Figure 27 The presence of the JR contact in the circuit will depend on whether the control tables specify a timed release. The route stick relay in route control systems of interlocking performs a similar function to those in conventional interlockings where it may be used to: maintain or hold the route locking to provide maintenance of selective overlap. hold the route locking where a train has passed an outer protecting signal which is interlocked with the points, and the signal normalized with a train occupying the track circuits ahead of that signal. qualify that portion of the route locking that would not be required where the route is signalled for both directions. The route stick relay is a normally energized relay with a stick function the relay being held energized by the signal concerned at Stop (ALSR Energized). The relay is de-energized when the signal is cleared and will remain de-energized with the track circuit ahead occupied although the route has been normalized. The USR is dropped by the ALSR down (signal cleared) and proved de-energized in the signal HR circuit. Under certain conditions the USR may be required to be timed out to release the locking, and where this is required a front contact of the track time limit concerned qualifies the stick function to allow the relay to energize at the completion of the timing cycle. An example of the function of a USR relay is shown in Figure 21 where 1 USR is used to hold the points lock relay de-energized for maintenance of selective overlap. 5 SIGNAL ASPECT CONTROLS 5.1 Aspect Requirements Once any route has been set, it must be proved entirely, including any overlap before displaying an appropriate proceed aspect and relevant route information to the driver. This may include track circuits and/or points depending on the type and geography of the route. 5.2 UCR Circuits The UCR proves continuously that all conditions are present for the signal to clear. A UCR will generally be provided for each route. The UCR relays are mounted in the main location and include all the functions normally placed in the HR circuits. In effect the UCR is an internal HR relay. The HR relays are located in the remote locations. The UCR drops the NGPR and then the USR and ALSR relays which are proved down in the outgoing HR circuit, in series with front contacts of the UCR. The UCR relays allow proving of internal relays. UCR circuits will generally contain the following controls: - a) A contact of the relevant RUR which only operates when the route is required to set. b) The SR, which allows the signal to clear for one movement only c) Track circuits proved clear by TPRs. For main routes, the tracks will be proved clear to the end of the overlap. Where facing points exist in the overlap, tracks beyond the facing points will have NWKR or RWKR contacts in parallel to exclude tracks when the points are set away from them . d) Points set and locked, using NLKPR and RLKPR contacts Also shown in the circuit examples are back contacts of the TZR (this will be present when automatic nominalization is required) and down proving of any track circuit timers which will be used to release route locking associated with the route. Refer Figure 28 for a Route Checking Relay which the UCR circuit for No.1 signal where the route is proved set by the RUR being energized thereby ensuring that all interlocking is correct and all relevant track circuits, including selective overlaps are proved clear (energized). This Figure 28 shows the UCR circuit for No3 Signal has four routes Main Route M(B) Shunt Route S(B) Main Route M(B) Shunt Route S(A) 5.3 Different Types of Routes Where controls are common between different types of routes (e.g., 3(M)A and 3(S)A), part of the circuit can be common to both UCRs. In the example shown on Figure 28, the main route UCR will include track circuits, but the shunt route will not. Such circuits can often be laid out in a geographical manner. The circuit designer should decide the most efficient layout by reference to the signaling plan and control tables. Where main and shunt routes exist from the same signal, the track circuits and overlap points will have to be separated out to appear in the (M)UCR circuit only. 5.4 Stick Relay The function of this relay is to maintain the signal at stop after the train has passed it. The signal will clear for one train movement only. Once the train has occupied the first track in the route, the stick relay can only be reoperated by normalizing and resetting the route. If automatic working is required, an (A)SR will be provided to maintain the SR circuit energized. The lever stick relay (SR) performs the same function as in a conventional interlocking. When a train passes a signal, the signalperson must pull the panel button to normalize the route before the signal can be cleared again. Referring to the circuit Figure 29, with the passage of a train passed No 1 Signal 1, SR is de energized by 1AT track dropping and will remain down after the train has vacated the track until No 1 panel button is pulled to energize 1(N)R relay, where a pickup circuit is established via 1AT and 1(N)R contacts. 1 SR is held energized via 1AT track contact and 1 SR stick contact when the route is set by the operation of the panel button. A front contact of 1 SR is included in No 1 signal control circuit (1 HR or 1 UCR if provided) and after the passage of a train past No 1 signal the SR contact prevents the signal from clearing again until the route is normalized and then re-set by the operation of the panel buttons. The (N)R contacts in parallel with the route NLR contacts allows re-energization of the SR relay should power failure occur when a train is approaching the signal and the signal is showing a proceed indication, under which conditions the approach stick down would prevent energization of the route NLR (approach locking) when the panel button was pulled and it would not be possible to energize the SR relay to re-clear the signal unless the timing period of the ALSR had elapsed. 5.5 Approach Lock Stick Relay (ALSR) 5.5.1 Approach Locking (Requirement) Approach Locking is achieved by means of an Approach Stick Relay (ALSR) and is provided on all controlled signals with the exception of certain starting signals. Its purpose is to hold the route locked, thus preventing the operation of points in the route and/or the setting of a conflicting route if the signal protecting the route has been returned to stop in the face of an approaching train. A route becomes approach locked once a driver has seen a 'proceed' indication or has seen an indication at a previous signal which would indicate to him that the next signal is displaying a 'proceed' indication. Where long sighting distances are involved, 600 meters is considered a suitable approach locking distance to the first warning signal. The approach stick relay is energized by front contracts of the NGPR i.e. signal at stop, and the approach track or tracks circuits to that signal unoccupied and will remain energized with the signal at stop via the stick path with the approach track occupied. The relay is de-energized when the signal for the route is cleared and will remain de-energized with the approach track occupied although the signal has been normalized. A front contact of the approach stick relay is included in the route NLR and prevents this relay from normalizing (latching up) when approach locking occurs as described above, thus preventing release of the interlocking. When a route becomes approach locked it is impractical to hold the route locked indefinitely once the train has come to a stand. To overcome this and the need for the signal electrician to provide a 'release', a time release relay is provided (ALSJR). The relay commences its timing cycles once the signal has been returned to the stop position (NGPR) energized. A timing cycle of 120 seconds is provided for main line running signals and is considered sufficient to ensure the train has come to a stand. For shunting signals a time limit of 60" is provided. A front contact of the time release relay is placed around the stick function of the ALSR and when energized allows the ALSR to energize. 5.5.2 Approach Locking (Operation) The circuit for 3 ALSR as shown on Figure 30,and its various circuit paths are as follows: PATH No 1:- 3 ALSR will energize (approach locking not effective) if No 3 signal is at stop, (NGPR energized) and track circuits approaching No 3 signal (1AT and 1BT) energized, with the approach track to No 1 signal (54.5B) included if No 1 signal has not normalized (1 ALSR down). This arrangement satisfies the condition where a driver has seen an aspect at a previous signal which would indicate to him the next signal is displaying a proceed aspect. The two-track occupation to release approach locking under normal running conditions is to overcome the problem of a track bobbing under a train thus releasing the locking. PATH No 2:- Allows for energization of 3 ALSR when a train proceeds past No 3 signal in the normal manner and allows a release of approach locking should a long train be standing with its rear on the approach locking tracks. To guard against a release due to an intermittent failure of 3AT, either 3BT or 3XT must be shunted at the same time. To guard against a premature release due to a power failure and restoration, which will cause the track circuit PR’s to drop and then pick up, a front contact of POJPR, a power off time delay relay, is included in the release path. The POJR, which is the parent relay for the POJPR, is wired directly across the AC supply and does not make. Its front contacts until 30 seconds after the supply is restored. PATH No 3:- The stick circuit holds the ALSR energized with the protecting signal (No 3) at stop and a train occupying the approach tracks. PATH No 4:- Energizes the time release which allows the release of the approach locking when the signal has been cleared and then returned to stop with a train occupying the approach tracks. 5.5.3 Approach Locking (Testing ) There are 4 approach locking test performed in the logic. Test1: Did the signal always stay at stop ?That is, signal not shown a proceed aspect .Then it is safe to normalise the route when controller cancels the route ,test passes and route get cancelled straight away Test fails if signal started to clear or made an attempt to show a route indicator and if lamp is failed ,driver believe a proceed aspect. Test 2 : Has no Train approached the signal ?Its is permitted to cancel (normalise) ,if approach tracks are clear (ie .Upto sighting distance of first caution-Comprehensive Approach locking ) .This means if no train approached test passes and route get cancelled .If Train has approached the signal and controller put signal to normal ,test fails .This is checked for the track status from concerned signal looking back to first caution aspect (Comprehensive approach locking ) .Test 2 is required only when route is not approach controlled for Red (MAR ) or an automatic working facility. Test 3:-Did Train enter the route ?If signaller cancels the route when train entered the route already ,test passes because sectional route locking will protect the train (USR relay above ) .Test fails when route is cancelled while train occupy birth track and just entered .Sectional operation of track is 1st track clear ,2nd track occupied ,AFTER first track occupied and second Occupied Test 4 : Did Train get Time to stop?.This is the last resort to normalise the route ,if all above 3 test fails when test sequence started and train seen a proceed or impression of proceed aspect .Then timer operates for the train to come to standstill OR have to wait for train to pass the signal for sectional route locking to release (USR) .In UK practice for a Main Signal Timer is 180 seconds for comprehensive approach locking and 120seconds for Signals get approach locking "when cleared" and in Australia it allow only 120 seconds for a train to come to standstill for main routes or 60 seconds for shunt route and approach locking get released after timer times out (Timer path on the ALSR relay ) .In nutshell for main line or loop line timer value is based on the class of route (Main Route /Diverging Main Route) ,which is 180 seconds for UK and 120 seconds in NSW ,Australia. If the route is Call on (Position Shunt Route ) ,irrespective of straight route or shunt route approach release timer value will be lesser .Check with your railway for these values ! 5.6 Signal Operation The UCR is at the interlocking. This will be used to operate a circuit to the HR at the location, controlling the clearance of the signal. Refer Figure 31 for a typical HR circuit for 1 Signal and 3 Signal where 3 Signal have both Main and Shunt routes. The HR (Signal Control) Relay operates the signal lights to show a proceed indication from the Stop position and is located at the signal location. The NGPR which proves the signal and train stop, if provided, have returned to normal, is proved de-energized in the HR circuit via back contacts. The ALSR and ALSJR are proved de-energized and ensures that the approach locking requirement is effective. The USR where provided is back proved thereby ensuring that the route locking is effective. The UCR proves that all points are detected in the correct position and that the track circuits are unoccupied for the route set. Refer Figure 32 for a Typical Signal Control Relay (DR) for green aspect. The DR Relay when energized provides the full clear (green) indication in the signal. The relay is energized by a front contact of its own HR and the HR for the signal in advance. Where single light colour light signaling is used, a front contact of the ECR for the signal in advance is included. This ensures that if a lamp fails in the signal in advance, the signal will only display a caution indication. The VRR of the signal in advance is also included when train stops are provided. To prove that the signal has responded to the interlocking control, an NGPR (signal normal - at stop) and an RGKR (signal cleared) are fed from the signal location back to the interlocking. The NGPR (Normal Signal Repeat Relay) proves that all signal control and operating functions, ie:- UCR's HR's and train stop if provided, have returned to the normal position, signal showing stop indication. The NGPR conveys this information via a front contact to the ALSR for the necessary proving, and the stop indication for the signal repeater in the diagram. It is also proved de-energized in the signal HR circuit. The (RGKR) Reverse Signal Indicating Relay, indicates that the signal has been cleared and is energized by front contacts of the HR relay concerned. This relay provides the clear indication for the signal repeater. Refer Figure 33 for Normal /Reverse Signal Repeat Relays Proof of signal normal is vital in the approach lock release. Both relays are used to provide control panel indications. 6 ROUTE RELEASING 6.1 Principles Route releasing comprises the following sequence of events: - A) Initialization of route release - signalman pulls the signal button at the start of the route .If automatic normalization is provided, this will be initiated by the train passing the signal. B) Release of approach locking on the signal - the train is proved past the signal (this may occur before or after (A)), the train has come to a stand at the signal or there is no train approaching. C) Release of the route up to the rear of the train, if any, known as sectional route release. 6.2 Approach Lock Release As far as the circuits are concerned, the first step must always be to operate the ALSR. The circuit is shown on Figure 30 above. Three separate circuit paths are provided to pick the ALSR according to whether the train is entering the route, the train has come to a stand at the signal or there is no train approaching. The arrangement of this circuit will be determined by the "approach lock tracks" and "approach locking released by" sections of the control table. Once operated, the ALSR will remain up until the signal is ready to clear again for another movement. 6.3 Route Release Picking the ALSR will allow the NLR to latch up (Refer Section 2.12) provided the route has actually been cancelled. This, in turn, will allow the USR (or the first USR if there is more than one) to pick as soon as the tracks are clear. It can therefore be seen that the route locking will always release behind the train. If the route is cancelled with no train, the USRs will pick up immediately as all the tracks are clear. 7. OVERLAPS 7.1 Principles Interlocking circuits are considerably complicated by overlaps. The controls must be accurately specified in the control tables and then translated into additional calling and locking circuits. The circuits must ensure that any route requiring an overlap has that overlap (or an acceptable alternative) maintained as long as the route is set or there is a train in the route. 7.2 Aspect Circuits The UCR circuit will include the additional point detection and track circuits in the overlap. For signals with facing points in the overlap, all valid overlaps will be included, with the necessary conditions of the position of the facing points. The facing points themselves will only be detected where they are set and locked to prevent a confliction with another route. 7.3 Route Locking of the Overlap Where an overlap is provided, route locking must extend to the overlap. Normally a timed release is necessary to prove the train has come to a stand and allow the overlap to release when no route has been set forward from the next signal. The USR will then include a TJR contact for the last track in the route in parallel with the TPR front contacts. 8 PRESET SHUNT SIGNALS Occasionally a preset (or facing) shunt signal is positioned within another route. It may either be operated on its own as a shunt signal or cleared by setting the main route (presetting). Once a train has entered the main route, the preset shunt must remain off until the train has passed it. The preset shunt may be replaced to danger in emergency, but this will not permit any release of the main route beyond the preset shunt. The examples do not include a preset shunt signal. The main points to note are as follows: - a) Main routes which require the shunt signal to be preset will first prove that the corresponding routes from the shunt signal are not in use (and vice versa). b) Setting of the main route will initiate the presetting process. c) The main signal will prove the shunt signal cleared in its UCR circuit. Pulling the signal button of the preset shunt will therefore return the main signal to stop. d) Route locking for the main route will be effective to the end of the route, not just to the preset shunt signal. Once the train has entered the main route, it cannot be partially released beyond the preset shunt . A train cannot therefore be re-routed by cancelling the preset route and resetting for another route.
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CH20 | INTERFERENCE & IMMUNISATION-PART 1 (ELECTRICAL)
CONTENTS Introduction A.C Traction Systems D.C. Traction Systems Mixed Traction Systems Protection Against Other Forms of Interference Future Developments and Problems 1. INTRODUCTION Electric traction is being used as an economic means of operating a railway. Electric power is taken from the national or regional supplier. Due to the scale of production this can normally be produced at a lower price than with individual power units on each locomotive. Electric locomotives are much simpler and cheaper to control maintain and operate than their diesel equivalent. Electric traction brings with it a number of additional problems for the signal engineer. Signaling equipment, in most cases, operates at relatively low voltages and currents compared to locomotives and multiple units. Unavoidably, traction circuits run alongside and even share conductors with signalling circuits. Interference, usually through direct contact or induction must be anticipated and equipment protected from its effects. Even without electric traction, signalling equipment may still be subject to interference from adjacent power supply lines or lightning storms. Similar techniques are often employed to protect against the likely effects. The objectives of protection (or immunisation) can be summarised as:- a) Prevention of failures, particularly wrong-side failures. b) To ensure the safety of staff working on the equipment under normal operating conditions and, as far as possible, under traction fault conditions also. c) To prevent the effects of traction currents from damaging or destroying signalling equipment. 1.1. Electrification Systems The effects of electric traction and the precautions which must be taken will depend on the type of traction supply. The electrification system chosen for a railway will, in tum, depend on a number of factors: - a) Type and frequency of traffic. b) Availability of Power Supplies and the locations at which they can conveniently be brought to the lineside. c) Local geographical and climatic conditions. d) Structural clearances (for overhead wires etc) e) The overall length of the line. f) Whether the line to be electrified is an extension to an existing electrified line or a completely new traction system As a result many different systems are in use. Once a system has been selected, it is very expensive to change due to the investment in traction units and power supplies. The systems can generally be divided into two groups: - A.C. systems, normally employing a high voltage (typically 25kV) with overhead current collection. D.C. systems, generally at much lower voltages (600 to 1500 volts). The method of current collection is either overhead (requiring a much heavier conductor than for a.c.) or third rail. Low voltage systems tend to be d.c. because this makes the traction equipment on the trains much simpler. Early electrification did not have the benefits of modern power semiconductors to control traction motors so the power was at a voltage which could be applied directly to the d.c. traction motors. The cost penalty is a large number of substations and heavy feeder cables. The use of a.c. traction permitted substations to be spaced considerably wider apart and the overhead conductor to be used as the main feeder. No separate feeder cable is necessary. The equipment on board the train is generally more complex. The high supply voltage must be transformed to a usable voltage and rectified to d.c. for the traction motors. Modern variable frequency a.c. drives have now made the induction motor a viable alternative to the series wound d.c. motor. 1.2. Modes of Interference In most cases one or both running rails are used for the return current path. To some extent earth currents will also be present. In all types of track circuits there is the risk of interference by direct contact, traction currents passing through the signalling equipment or between the signalling equipment and earth. There is a high risk that traction currents could cause wrong side failures. The incidence of right-side failures could affect reliability to an unacceptable level. Other circuits are generally not in direct contact but there is still the possibility of induced interference. insulation could also break down causing direct contact with the traction supply. Protection must also be provided against dangerously high voltages (either induced or through direct contact) which could injure or kill personnel working on signalling and communications equipment. 2. A.C. TRACTION SYSTEMS 2.1 Feeder Arrangements BR's 25KV 50 Hz AC. traction system is typical of those on many other railways and is usually derived from the 132 KV National Grid system. The supplies are taken as single phase through 132/25 Kv transformers provided by the Supplier. The supplies are taken to feeder stations at usually the same phase. Adjacent feeder stations are not parallel (i.e. not necessarily from the same phase and usually not from the same source. This enables the Electricity Authority to balance the railway load between the 3 phases of its system. The distance between feeder stations is dependent on traffic and power requirements. An electric locomotive accelerating with a full load may take a current of the order of min 270 amps. Neutral sections are provided between feeder stations to separate the supplies and track sectioning cabins (TSC) are located at intermediate points and the boundaries of feeding sections to provide alternative feeding and track isolation facilities in the event of failures or isolation for maintenance. 2.2 Return Paths for Traction Current 2.2.1.Rail Return This is the simplest form of traction current return. In most cases, only one running rail is used. Modern Track Circuits allow dual rail traction return. On plain track this is normally the cess rail, but in complex layouts the location of return rail changes in order to meet track circuit requirements by cross-bonding. The problem with rail return is that the rail is not well insulated and return traction current will leak to earth over the length of the return rail. This produces a large imbalance between the current in the overhead wire and that in the rail, which dramatically increases the level of induced voltages in lineside cables (see later sections for details). 2.2.2 Return Conductor If the traction return path is well insulated, the current imbalance in the circuit is reduced, hence interference is minimised. A return conductor can therefore be provided to carry the traction return current. The current carried is in anti-phase to the current in the contact and catenary wires, and similar in magnitude, therefore the net electro-magnetic inducing field is reduced. The reduction over rail return, where signalling circuits are placed in the optimum position for minimum interference, is of the order of 45%. 2.2.3 Booster Transformers Some of the return current will still flow via earth instead of the return wire. This will be in inverse proportion to their relative impedances between the train and the substation. The cost of a conductor large enough to eliminate the earth currents would be prohibitive. To force the majority of the return current into the return conductor, transformers are provided in the traction circuit as shown on FIGURE1 The return conductors are bonded to the traction return rail midway between booster transformers. These are located at 3.2 Km intervals, the primary winding connected in series with the catenary and the secondary winding in series with the return conductor. The effect of the booster transformer is to produce a current in the return conductor which approximates to that in the catenary but is 180â—¦ out of phase. Maximum suppression of induction is achieved with the traction load at the mid-point connector (95% reduction) and the minimum when at the booster transformers. This alone does not immunise signalling and communications circuits. As the two conductors are widely separated, induction will still occur whenever a signalling circuit is nearer to one conductor than the other. The relative positioning of the overhead conductors, the return conductor and the signalling cable route must be such as to place the cable route equidistant from the traction conductors. 2.3 Modes of Interference and Immunisation Techniques Of the three possible modes of interference, conduction, electrostatic induction and electromagnetic induction, induction is the dominant mode in a.c. traction areas. Conduction, by direct contact between the power line catenary and the rails, equipment or cable conductors happens rarely. If unprotected, it is usually fatal to personnel and equipment. The only safeguard is the provision of contact circuit breakers in the traction supply. Due to bonding of overhead structures direct to the traction return rail, the risk of a traction fault raising the rail potential, and that of track circuit equipment, to dangerous levels is small. Track circuits will however be subject to false operation due to the multiple return paths created by return conductors and earth bonding and extra precautions must be taken when setting up and adjusting track circuits to ensure safe operation. Due to the distance between the traction equipment and the signalling equipment and the low supply frequency of 50Hz., electrostatic effect are usually negligible. Electromagnetic induction is caused in one conductor by current flowing in another. This action is similar to a transformer, the traction conductors being the primary winding and the signalling conductors the secondary. The voltage is induced along the conductor (not between the conductor and earth). The effect is maximised when conductors run parallel to each other. The induced voltage increases with length of conductor and decreases with separation of the conductors. As signal engineers we are concerned with the following aspects of a.c. induction into our lineside cables a) voltages produced under normal operating conditions that could affect line circuits and tail cable circuits and result in the malfunction, failure or damage to equipment and/or hazard to staff working on the equipment. b) Higher levels of AC. voltage induced in our cables as a result of traction system. c) Any increase of these effects due to disconnection, earthing or other failures within the signalling equipment Dangerous levels of induced voltage are possible if the traction current rises rapidly (short circuit or traction flash over). Telecommunications circuits are susceptible to all normal levels of induced voltage on unprotected cables from the traction system. Therefore, special measures have to be adopted to protect these systems from the 50 Hz base and odd harmonics of this frequency. 2.3.1 Positioning of Cable Route The simplest method to reduce the levels of induced voltage is the physical location of the cable route, as described earlier. The best position for the cable route is approximately equidistant from the two traction conductors (where booster transformers and return conductors are provided). This distance will obviously vary for different designs of overhead equipment, for multiple track lines where two or more return conductors are mounted together, and for lines without booster transformers and/or return conductors. On most circuits which form a loop (one conductor out and the other return), this provides an adequate level of protection for normal operation provided the signalling equipment is in full working order. Both conductors are subject to the same electromagnetic fields which tend to oppose each other. Under fault conditions (earth faults and/or disconnections) further protection will be required. 2.3.2 Electro-Magnetic Screening With any wire installed parallel to an A.C. electrified line, a reduction in interference is obtained by the screening effect of earthed conductors in its vicinity. This includes cable sheaths, metal pipes and running rails. All communications cables are provided with a screening sheath (usually aluminium) with up to a maximum of 4 steel tapes around the sheath. These are connected to a good earth of no more than 4 ohms located every 1000 m. Signalling cables run in the same cable routes as screened telecomms cable and therefore benefit from the mutual screening effect between the cables. Where existing cables, not to electrification standards are to be retained, it may be possible to immunise them by a separate screening conductor. This is a large cross-section copper wire run along the length of the cable route and earthed as above. Cable sheaths should also be earthed. 2.3.3 Immunisation of Relays Relays for d.c. circuits can be designed to withstand substantial a.c. voltages without energisation. The relays have copper slugs fitted over the cores, near the pole pieces and a magnetic shunt is fitted between the cores above the copper slugs. When a DC voltage is applied to the winding the DC flux is produced as normal and attracts the armature. Some of this flux is diverted via the magnetic shunt and therefore greater power has to be supplied. However, when an AC voltage is applied, the AC voltage has difficulty in establishing itself across the air gap due to the large copper slugs and it tends to short circuit the air gap via the magnetic shunt, thus the AC flux plays little part in the operation of the mechanism. Refer Figure 2 &3 showing the principle. 2.3.4 Choice of Operating Frequency Where d.c. circuits are not practical, equipment should be designed to operate at frequencies other than the mains frequency and its harmonics. Filters can be used to keep signalling and traction currents separate Although the mains frequency is normally very accurate, fluctuations can occur and these must be allowed for in the design of systems. Fluctuations of ± 0.5% are typical so the design may allow for 1% maximum error to give a degree of margin for error. It should therefore be evident that the available bandwidth between successive harmonics decreases by 2Hz each time and above 1kHz no bandwidth is available. In practice, harmonics of this order are very small and frequencies above 1.5 kHz are successfully used for track circuits. For additional safety, two or more frequencies are used together so that traction faults could not generate both together. 2.2 Practical Immunisation of Signalling Equipment 2.4.1. Limits Appreciable earth currents may still flow for some distance away from electrified lines and protection must be extended sufficiently far for their effects to become negligible. At the limits of electrification, or where a non-electrified line leaves the electrified lines, experience has shown that signalling equipment should be immunised for 800m from the electrified line. 2.4.2. Line Circuits These should be DC circuits using AC immunised line relays. The length of line must be limited to 2km to ensure that the induced voltage from the traction system does not exceed limits for electrical safety (maximum induced voltage of 110v). Circuits required to cover greater distances must be repeated by means of a relay and new power supply. Where circuits run along non-electrified branches, they must be cut 800m from the electrified line. Where circuits from the same supply feed in opposite directions, care must be taken to ensure that the total length of parallel circuits is less than 2km. All vital line circuits are double cut to reduce possible false operation and hazard to staff where an earth fault is present. 2.4.3.Track Circuits The traction return current can usually be carried satisfactorily by only one of the running rails. Single rail DC track circuits, immune to the highest AC voltage that could occur, are used. It has been found desirable to limit the track circuit length to a lower value than on non-electrified lines to prevent the combined effect of a return conductor and a broken rail providing an alternative path to the train shunt. The track feed set must be designed to prevent a significant DC voltage being applied to the rails as a result of rectification of AC from the traction current. The track relay must also be immune to AC in the same manner as those used for line circuits. Reed track circuits operate at frequencies clear of 50Hz harmonics and may also be used in AC electrified areas as may most modulated audio frequency track circuits such as the TI21. When little used and therefore rusty rails require track circuiting there are two solutions - a welded stainless steel strip on the top surface of the rail or a high voltage impulsing type of track circuit. The Jeumont track circuit is immune to false operation by AC traction current, but its length is severely limited in a.c. traction areas. This may not be a serious disadvantage, as it is mostly used for point and crossings into sidings and loops. There are few types of track circuits mentioned here, Signal Engineer must check with the product owner and manual for other types of track circuits. 2.4.4 Signals Colour light signals use tungsten filament lamps, which operate readily on both AC and DC. Therefore the only method of preventing induced AC lighting the lamp is to limit the length of the circuit between control relays and lamps and employ as high a voltage as practical. The usual practice is to have a transformer for each aspect in the signal head to reduce the voltage to the 12 volts or less required to operate the lamp and supply 110V AC over the contacts of the controlling relays. In this case the maximum parallelism allowed is 183m, i.e. a signal head must be less than 183m from its controlling relays. If the overall maximum distance between signals fed from the same location or relay room supply exceeds 183 m, then an isolating transformer is required to feed signals on one side of the location or relay room. By limiting the length of signal lamp circuits, it is unnecessary for them to be double cut. Shunt signals generally operate on 110-volt lamps so no transformer is necessary. Searchlight signal operating mechanisms must be immune to false operation. This immunity is achieved by the use of chokes mounted as close as possible to, and in series with, the d.c. searchlight mechanism. An a.c. searchlight signal could be operated at a different frequency (e.g. 83.3Hz) For LED Signals (current practice), signal head has AC/DC rectifier unit and few types are mentioned below 1)Aldridge RL 400 ,120V Main Line Signal 212mm /127mm.Cable to signals shall be less than 750m for single cut circuits .Between 750m to 1500 meter for single cut circuits with two bleed resistors fitted in the signal head .Fit a 130VAC ,20mm varistor across each LED unit. 2) Aldridge Tunnel Signal 120V AC 127mm.Cables to Signal less than 1200 mm for single cut circuits and less than 2000m for double cut circuits .No varistors required for surge protection. 3) Alstom Mark 2 120V AC Mainline out door signals 212mm.Cable to signal less than 750m for single cut circuits ,between 750m and 1500 m for single cut circuits with two 4K7 6W bleed resistors fitted in the signal head .Less than 2000m for double cut circuits .No varistors required for surge protection ,but must have some surge protection to earth on each leg of 120VAC supply at power supply locations . 2.4.5.Points Electric point machines are immunised by using permanent magnet machines. These ensure that, even if there is an AC voltage at the terminals, the motor's field will remain uni-directional and although there will be considerable vibration, there is no resultant torque and therefore the motor will not move. Electro-Pneumatic point machine valves must be immune. These valves are immunised in a similar manner to that outlined for relays. Electro-Hydraulic (clamp lock) point mechanisms were found to have inherent immunity and can be used with no special measures being taken. Mechanical Points should have insulation inserted in the point rodding at the ends adjacent to the lever frame and the points. This is to prevent stray voltages causing electric shock to personnel. Detection of all types of points is by polarised relays. These relays are fully immunised in the manner already described. 2.4.6.Level Crossings Where lifting barriers are used it is desirable to position the barrier so that, if it is knocked over no part of it shall come closer than 150 mm to the overhead line equipment. If other positioning requirements make this impossible, then the barriers should be made of metal or have a continuous metallic strip of adequate section along its length. The barrier or metallic strip should be bonded to a traction return rail or cable. Control circuits must be immunised as already described. Where closed circuit television is used special precautions must be incorporated into the design. 2.4.7. Remote Control Immunisation of remote control systems is dealt with in the separate notes covering remote control. This mainly involves line isolation at regular intervals. 2.4.8.Power Supplies Although the safety precautions in force for higher voltage power supplies would cater for the possibility of induced voltages at the levels expected, care must be taken when working on supplies which are switched off to avoid the possibility of high induced voltages. As the cable may not be sectionalised in the same way as vital signalling circuits, dangerous voltages could occur on long power feeders. 3. D.C. TRACTION SYSTEMS Because of the low traction voltage, traction currents are high, typically, 2,000 - 3,000 amps per train during acceleration. Due to the large DC return currents present in the earth near to and in the return rail, there are problems caused by corrosion, and problems caused when insulation, equipment and cables break down. A d.c. supply will normally produce no inductive interference effects other than from switching transients and ripple at harmonics of the mains frequency from an unsmoothed power supply. The main hazard is from conduction (to which track circuits are particularly exposed) by direct contact or via earth faults. The main precautions therefore comprise operation of equipment from a.c. supplies, effective insulation arrangements and earth leakage detection. 3.1 Limits Earth currents from d.c. supplies are much more troublesome than for a.c. They may propagate over much longer distances and immunisation should be provided for at least 3km from a d.c. electrified line. 3.2 Line Circuits DC circuits are used which are not sectionalised other than for volt drop purposes. It might initially seem unwise to. use d.c. but the reasons for accepting d.c. line circuits on B.R. in most situations are as follows:- a) All cables have non-metallic sheaths and are therefore less likely to pick up DC potentials along their routing. b) All cables are tested to a rigid specification c) Additional insulation is provided by terminating cables on non-metallic materials d) All line circuits are double cut to ensure that an earth fault in one leg of the circuit cannot cause false operation e)Additional earth leakage detection equipment is used f) In most dc. traction supplies there is a significant a.c. ripple from the rectifiers. Use of a.c. circuits would not provide immunity from the effects of this ripple 3.3 Track Circuits AC track circuits use vane relays which have self immunity to the effects of DC. Even where the traction supply may contain a.c., the two element vane relay will only operate if the interference signal is at the correct phase relationship. The relay will not respond to higher harmonics unless these are contained in the correct proportions in the supply to both coils. Reed and TI21 track circuits and several other audio frequency track circuits are also immune to the effects of DC. 3.4 Signals No special precautions are taken for signal lighting circuits. These are usually a.c. fed with signal head transformers. Search-light signals are operated by a.c. vane type mechanisms, which, like track relays, have self immunity to DC. 3.5 Points No special precautions are taken in the choice of machine for the control of points. Although a.c. machines might be considered to have better immunity, d.c. operation is normally acceptable at higher voltages (110-130 volts) together with adequate earth leakage detection. Clamp locks, having separate valve and motor circuits can only be falsely operated by two simultaneous faults. Detection of points may be achieved with a.c. circuits using vane type three position relays. Alternatively, 110 volts a.c. may be used from the detector to the location where each individual circuit is transformed and rectified to operate a 50 volt d.c. line relay. 4. MIXED TRACTION SYSTEMS There are some areas where both traction systems are in use, either using the same tracks or an adjacent track. This means that the signalling system has to be immune to both AC and DC traction supplies. Circuits used must operate on a frequency distinct from the mains frequency and its harmonics. 4.1. Limits Although the d.c. immunity is required for a greater distance, it is normal to dual immunise for approximately 3km. 4.2 Line Circuits Circuits are usually d.c. with precautions taken as for a.c. and d.c. lines. 4.3 Track Circuits Formerly, the most popular method was to use a.c. two-position vane relays but operated from an independent power supply at a frequency different to the mains supply and its harmonics (normally 83.3 Hz for 50 Hz mains). The additional power supplies were a significant added cost. Modern audio frequency track circuits such as the TI21 are immune to both d.c. and a.c. traction. 4.4 Signals The circuits are identical to those used in AC traction areas. Searchlight signals are immunised by using an AC vane type mechanism with a maximum length of 55 metres for the feed circuit. If signals are more than 55meters apart on either side of the relay room or location an isolating transformer must provided in the feed circuits of all signals on one side of the relay room/location. 4.5 Points The methods of operating points are identical to those used in areas of AC traction. Detection of all types of point is either by a two element vane relay from an independent supply at a separate frequency, or by a filtered circuit such as the GEC vital Reed system. Frequencies 477.5 Hz and 414.75 Hz (type RR 4000) have been allocated for use with point detection circuits on B.R. and are employed for Normal and Reverse detection respectively. 5. PROTECTION AGAINST OTHER FORMS OF INTERFERENCE Apart from traction, adjacent power supplies and lightning are the main sources of electrical disturbance. Power transmission lines generate the same type of interference as a.c. traction and protection is therefore similar. Lightning protection is a very specialized area and will only be covered in outline. A direct lightning strike produces voltages far higher than the worst traction fault conditions. It is virtually impossible to provide effective protection. When lightning finds a path to earth, it will however raise the voltage at the point at which it enters the earth. Currents will flow which, although of extremely short duration, can induce large voltages in adjacent equipment. The danger is that the voltages may be large enough to find a path through the signalling equipment or break down insulations in cables etc. Fortunately, protection can be provided against this effect by the prov1s10n of surge arrestors. The most common type is a gas discharge tube which has three electrodes, one connected to each leg of a loop circuit. and the other to earth. For the normal operating voltages of the signalling circuit, it will appear as an open circuit. When the voltages induced by lightning are sufficiently high, an arc will form in the gas discharge tube which will provide a low impedance, high current capacity path to earth for the duration of the lightning strike. When the current is insufficient to sustain the arc, the surge arrester will revert to its open circuit state. Semiconductor devices are also available to operate in a similar manner. Their switching time is much faster than a gas discharge tube, giving quicker protection, but their current capacity is much lower. They provide better protection against moderate strikes but may not be able to handle the current caused by severe strikes. Both types of device are also suitable for protecting vulnerable equipment (e.g. electronic track circuits) from traction fault conditions where required. 6. FUTURE DEVELOPMENTS AND PROBLEMS We have concentrated so far on interference caused by the supply. This is always at the mains frequency or harmonics of it and these frequencies can therefore be filtered or avoided as appropriate to the type of equipment. Electric traction units have in the past invariably employed d.c. traction motors. These do not produce any different interference to that already produced by the power supply. Many railways are now employing traction units either with d.c. motors and variable frequency thyristor chopper controllers or more recently, ac. variable frequency induction motors. The control equipment for these types of traction unit can produce interference at many (often continuously variable) frequencies over a broad spectrum. It is impossible to detail specific problems but the signal engineer must be very careful in the future as new traction units are introduced that he is aware of their possible effects on the signalling equipment. In Part 2 we will cover Electronic Interference
Read Full Article- Posted 4 years ago
CH21 | AXLE COUNTERS-SECONDARY TRAIN DETECTION SYSTEMS
Introduction Principle of Operation General Description of Equipment Wheel Detection Transmission Indoor Equipment Failure of Equipment Disadvantages of Axle Counter System Vs Track Circuit Type of Axle counters Australian Standard 1 ) Introduction For many years, the normal means of train detection has been carried out by Track Circuits. A wide variety of track equipment is available to cater for different types of Tracks and traction equipment. However, there remain some situations where the track circuit cannot be used or unreliable in operation. Some of the situations are listed below: - A) Ballast conditions, the ground conditions or sometimes the structure upon which the track is laid, may be electrically unsuitable for a track circuit Eg: Track directly fixed to a steel bridge, use of steel sleepers, ground conditions varying between totally dry and waterlogged making satisfactory adjustments impossible. Environmental conditions could impact the Rail To Rail and Track to Earth resistance. During wet conditions or poor insulation on the track bed to rail, Track to Earth resistance could go below the required value of certain track circuits ( Eg: FS3000 track circuit expect resistance not less than 7.5 ohm-km ) B) Length of Track Section. For very short track circuits, there is a minimum length below which some vehicles may not be detected. It may therefore be impossible to provide track circuits sufficiently short in length through some crossings (Eg: Large angle between tracks of multiple track lines crossing each other). Conversely, there is also maximum length over which any track circuit will operate. Multi-section track circuits can be very expensive to provide C) Access to Line. A track circuits line will need access at regular intervals for maintenance. In long tunnels and in areas where the terrain makes safe access difficult, its undesirable to locate any equipment which requires regular maintenance D) Power Supplies. On long sections of rural railway, its expensive and therefore undesirable to provide line side power supplies. E) Some audio frequency track circuits such as Siemens FS2550, FS3000 have null zone around the boundary of track circuits where train will not be detected. Sometimes this undetected gap can go above 2m. F) Track circuits requires (depends on configuration and type) IRJ’s which is expensive to install and maintain. G) Track circuits on electrified lines requires traction bonding & impedance bond. H) Rusted sparingly used tracks can go trains undetected, also flooding, contaminated ballast, or other problems with insulation between the rails can bypass the circuit; I)crushed leaves or ice on the rails can form an insulating layer that makes the circuit unreliable. J) Sometimes a train sprinkles anti-slip sand to aid deceleration while braking, but the sand contaminates the rail, and the track circuit stops working Axle counter can assist in solving these problems. 2) Principle of Operation As its name suggests, the axle counter operates by counting the number of axles entering and leaving a track section, as opposed to a track circuit which proves that an entire track section is clear of rail vehicles. Provided there is a reliable, fail-safe communication link between the ends of the track section and the interlocking, its length may be as small or as large as required. Eg: An axle counter can go up to around 10,000 metres (33,000 ft) from the evaluation unit when connected directly. However, with the addition of an Ethernet network, the distance is limited by the transmission system. However latest development is inclusion of optical fiber network which makes larger distance possible. It is not constrained by minimum vehicle lengths or the electrical characteristics of the track bed. When the section is clear, the count of axles in the section must be zero. A train runs into the section, each axle is counted in and added to the total. When the train leaves the section, each axle is counted out and subtracted from total. A zero total is therefore equivalent to a track circuit clear, a non-zero total is equivalent to a track circuit occupied. The detection and counting equipment must be capable of bidirectional operation. Even if its not required for normal traffic, but the absence of this facility will prevent return to normal use without manual intervention after an emergency or irregular train movement. 3) General Description of Equipment German manufacturer ,SEL(Standard Elektrizitätsgesellschaft) was one of the earlier popular axle counter equipment .SEL later merged with Alcatel and now its part of Thales. I thought it would be good to discuss with original SEL system for understanding the Axle Counter Concept. The following description relates to their systems. Current version of Thales Axle counter and Frauscher axle counter will be mentioned later stage of this article, as these are proprietary products, and the owner of the system provides manuals and are also widely available on internet. Refer Figure 1 for a SEL Axle counter general arrangement The equipment can be divided into three basic parts:- The trackside (Including Track Mounted) equipment which detects the passage of the wheels of the train. This in turn controls a.c signals to the interlocking or other suitable central location A transmission link from the trackside to a suitable location or equipment room. The equipment has the facility to transmit to two separate destinations to enable one set of track equipment to serve the boundary between track sections in different interlockings. The “Indoor” equipment which processes the signals from the various trackside detection points and convert them into “track clear” or ‘Track Occupied” data in a form that can be used by the interlocking and Train control system (or ATS Mimic/HMI) to display the status. It also monitors the trackside equipment for correct operation. 4) Wheel Detection The track mounted equipment must reliably detect the passage of each axle as it passes the detection point. It must be able to detect the direction of travel of the train. It must also operate over the full possible range of train speeds. An electronic system is being used. A detector consists of a transmitter on the outside of each rail and a receiver on the inner (running) edge of the rail. Two of these detectors are mounted in close proximity (170-200mm) measured along the track). To detect the direction of the movement, the two detectors are staggered .The stagger distance must be large enough to detect the direction of movement at high speed but small enough to ensure detection of only one axle at a time .Older systems were mounted on each rail ,newer systems are constructed so that all track equipment can be mounted together on one rail ,simplifying the cable connections .The distance is normally set about 170mm .Both detectors can therefore be mounted in the same sleeper bay . Both the transmitter and receiver consist of coils wound on to a magnetic (ferrite) core. The transmitter coil is adjustable to suit different magnetic characteristics of various rail profiles. Its continuously fed with an a.c signal. Refer Figure 2 for a Rail Mounted Transmitter and Receiver. Refer Figure 3 for SEL Axle Counter Track Equipment Diagram Although the magnetic field surrounding the transmitter ,receiver and rail is complex ,the simplified diagram assumes two components of flux Φ1 and Φ2 linking the two coils .With no wheel present Φ1 is greater than Φ2.The wheel and flange have greater effect on Φ1 reducing it almost to the level of Φ2 in older systems and due to different frequencies used ,below the level of Φ2 in later systems .The induced voltage therefore reduces almost to zero (older systems) or reverse phase (later systems) .By processing the outputs from the two receivers the number of axles passing the detection pint ,and their direction can be determined. Alongside the two rail mounted detectors is a junction box containing the electronics and power supply. It is connected to the location case or equipment room by a two-core balanced cable (four wire system was used in first versions). As the wheels pass over the detector, each receiver experiences a reduction or phase reversal of voltage in each receiver. On earlier systems the outputs were each amplified and transmitted to the indoor equipment. Later system perform more processing at trackside .The Tx/Rx board for Tx/Rx1 produces a continuous 30KhZ signal .TX2 7 Rx2 operate at 29kHz.The receiver signal is then compared with the transmitted signal .As the wheel passes ,the phase of the receiver signal will reverse .This causes the d.c output of each Tx/Rx board to switch between a high voltage (logic 1 -no wheel present) to a low voltage (logic 0-wheel present) and back again .These two pulses are modulated on to 5060Hz and 4150 Hz signals respectively. These are then transmitted together with a supervisory signal at 2530Hz over a single cable pair to evaluator. Referring to Figure 4 shown above, if we assume that the track section is to the right of the detectors, a vehicle passing from left to right (into the track section will produce a pulse on Rx1 first .A vehicle travelling from right to left will produce a pulse from Rx2 first. 5) Transmission This consists essentially of line matching at each end and a twisted cable pair between the trackside and the location or equipment room .The length of the transmission circuit is limited by the attenuation of the audio frequency signal which imposes a practical limit of about 20km .A lower transmission frequency can be used to increase this distance if required If the d.c supply to the junction box is fed via transmission line (to avoid a separate power cable ) the maximum circuit length for 0.9mm diameter conductors is about 4km .It can therefore be seen that there is potential capability for axle counters to cover much longer track sections than track circuits 6) Indoor Equipment To operate the equivalent of a track circuit, data is required from two (plain line) or more (points & crossings) detection points. More complex track layouts can be provided for by the use of additional detectors. By provision of appropriate wiring between the inputs and counters, overlapping track sections could be catered if required. The evaluator requires an input for each detection point and a counter for each track section. The received a.c voltages are filtered and amplified and converted to a d.c pulse for counting. Axles can now be counted “in” and “out” with respect to a nominated reference direction (equivalent to up or down traffic). An “in” count for example may add to or subtract from the count for a section depending on whether the detector is at the up or down end. Pulse 1 is used as a gate pulse which must be present for any count to register. Pulse 2 count in or out depending on whether the falling or rising edge falls within pulse 1 Each counter is a binary counter capable of counting up to 511,1023 or more axles as appropriate to the traffic needs. If the count is zero, a ‘track clear’ output is produced, if a non-zero, a ‘track occupied’ output is produced. On a plain line track section, in pulses from one input will add to the count while out pulses from the same input will subtract from it. The converse will apply for the inputs from another detector. To ensure fail safe operation the equipment also includes several checking and monitoring circuits at each stage of the counting process. There will not be opportunity to cover these in detail. In essence, however, a correct count ‘in’ or ‘out’ will only be registered by the combined signal from both rails. Any irregular signal from one or the other of the detectors will invalidate the count, raise an alarm and show the equivalent of a ‘track occupied’ indication. 7) Failure of Equipment The outputs from the counters are in the form of ‘track occupied’ (count non-zero),’track clear’ and ‘alarm’. To correctly give a track clear indication to the interlocking, the track clear output must be present and the other two absent. All other conditions will produce a track occupied indication to the interlocking Circuits are provided to monitor all counts into a track section. Any failure in counting out will automatically result in a track occupied indication because the resultant axle total will not be zero. A significant change in the gain of any amplifiers in the system will result in the count in not being valid, due to change in received voltage. Any change in the frequency of the oscillators and /or filters will result in the absence of an input a.c signal thus initiating an alarm. If a failure occurs, the counter can be reset to zero by the maintenance technician after remedial action has been taken. Great care must be taken to ensure that the section is clear of all trains when this is done . Axle counters can therefore be used as an effective substitute for track circuits where track circuits would be either impractical or unreliable. 8) Disadvantages of Axle Counter System Vs Track Circuit Even though axle counter eliminates many of the track circuit disadvantages, there are disadvantages for axle counter compared to Track circuits. Track circuits can recognise rail breaks under certain circumstances where as axle counter cannot detect the breaks in rails. For various reasons, such as a power failure, axle counters may 'forget' how many axles are in a section. A manual override is therefore necessary to reset the system. This manual override introduces the human element which may be unreliable. An accident which occurred in the Severn Tunnel is thought to have been due to the improper restoration of an axle counter. That was not proven during the subsequent inquiry, however. In older installations, the evaluators may use 8-bit logic, causing numeric overflow when a train with 256 axles passes the axle counter. As a result, that train would not be detected. That imposes a length limit of 255 axles on each train. More modern systems are not restricted by train wheel numbers. Where there are interlocked turnouts, an axle counter unit needs to be provided for each leg of that turnout. On lines with non-interlocked/hand-operated switches, detection of the switch points would have to be monitored separately, whereas on track-circuited lines misaligned points can be set to automatically break the track circuit. Axle counters have problems maintaining correct counts when train wheels stop directly on the counter mechanism. That is known as 'wheel rock', and can prove problematic at stations or other areas where cars are shunted, joined and divided. Also, where main lines have switches to siding, spur or loop tracks, extra counters will need to be deployed to detect trains entering or exiting the line, whereas the same infrastructure using track circuits needs no special attention. Magnetic brakes are used on high speed higher speed trains with a maximum speed greater than 160 kilometres per hour (100 mph). These are physically large pieces of metal mounted on the bogie of the vehicle, only a few centimetres above the track. They can sometimes be mistakenly detected by axle counters as another axle. This can happen at only one end a track block, because of magnetic field curvature, defects of track geometry, or other issues, leading the signalling system to become confused, and also requiring reset of the detection memory. Modern axle counters are 'eddy current' brake-proof and the magnetic effect of the braking system as described above is overcome, with count information remaining stable even when a vehicle fitted with magnetic brakes is braking whilst traversing the detection point. 9) Type of Axle counters Thales and Frauscher are the two major manufactures of axle counter system .Below is the list of some manufactures .If you know any other manufactures and wish to include in this list .Comment below 9.1 Frauscher(Austria) -ACS2000(Fail Safe Relay interface suitable for relay based interlocking 9.2 Frauscher(Austria) - FAdC (Fail Safe Serial Inetrface ) can do relay interface as well 9.3 Frauscher(Austria)-FAdCi-Failsafe Serial interface 9.4 Thales(France) - AzLM (Fieldtrac 6315) 9.5 Thales(France) -AzLS 9.6 Voestalpine(Austria)- UniAC[1] 9.7 Voest Alpine (Austria)-UNIAC[2] 9.8 Central Electronics Limited (India)-SSDAC-710P/HA SSDAC-720P/MSDAC-730P 10) Australian Standard Refer AS 7651:2020 (Rail Industry Safety And Standards Board ) Rollingstock compatibilty ,reset procedures (Train Sweep / Manual Reset) are operational specific to individual railways and shall be performed according to your railway operating procedures . Note :- Purpose for this article is to brief the Axle counter system .You selected suppliers will provide Installation Manual ,Operation & Maintenance manual .There for all these topics are out of scope for the article
Read Full Article- Posted 4 years ago
CH23A | PRINCIPLES OF TESTING-FIXED BLOCK
CONTENTS Introduction Competence of Testing Staff Documentation Testing of Equipment Rooms and Location Cases Testing of External & Lineside Equipment Remote Control Systems Control Panels Power Supplies Functional Test of System Other Important Considerations Maintenance Testing Conclusions Note: While these notes are based on the authors' understanding of current railway signalling practice in the United Kingdom/Australia and elsewhere, they must not be taken to modify or replace any existing rules, instructions or procedures of any railway administration. Where any apparent conflict exists, reference should be made to the appropriate documents produced by the administration of your organization or the operator of your railway. 1. INTRODUCTION These notes deal with the principles of testing a new or altered block signalling installation. It is not possible to cover in detail the testing of specific types of equipment. It must be stressed that these notes must not be taken as any form of testing instruction. The instructions and procedures issued by your own administration must be observed. Verification & Validation requirement of modern signalling system (Eg:Communication Based Train Control ) are more stringent and shall be referred to CENELEC -EN50128 - Railway applications - Communication, signalling and processing systems - Software for railway control and protection systems & CENELEC -EN50129-Railway applications - Communication, signalling and processing systems - Safety related electronic systems for signalling. This article cover the site specific testing V&V Process for modern signalling will be published in another article. 1.1. Why Do We Test? It is vitally important for the safety of the railway that a signalling installation operates correctly. Prior to installation, the signalling equipment will have been specified and designed to sound signalling principles. At each stage the specification and design should have been checked. The installation should therefore be carried out to a correct and consistent set of drawings. When installation is complete, a thorough test must be undertaken to ensure that the equipment as installed is correct to the drawings and also that it actually performs to signalling principles and basic safety rules. It must be stressed that this is the last opportunity to uncover any errors in specification, design or installation before the equipment goes into service. The testing must therefore be done correctly and completely. 1.2. What is to be Tested? For a new installation, the answer to this question is simple - everything. There will also be interfaces to existing equipment. These too must be tested. For an existing installation which has been modified, it is not always so clear as to what requires testing. Obviously, all circuits and other equipment which are shown as altered on the drawings must be tested. However, it may be necessary to test some parts of the installation which remains unchanged. Although the work may have been confined to a small portion of the equipment, it may have been possible for an installer to have interfered with working circuits which were not part of the equipment to be altered. 1.3 Who Will Test? It is vital that all staff who undertake testing are competent to do the job. There must also be one person in overall charge of testing who will define the tests to be carried out in the form of a testing plan and ensure that the progress of testing is properly monitored and documented. The testing must be carried out independently of the design and installation. Persons who have participated in the design or installation process must never test their own work. It is generally acceptable for those who designed or installed the equipment to be involved in an assisting capacity. Where testing is carried out by contractors or other external testers, the standard of testing must be maintained. The employing company must be satisfied that such testers are of the required standard. The competence of testing staff will be covered in more detail in section 2 of this article. This is a matter for the tester's skill and judgement. He must take into account the type of equipment and the environment in which the work is carried out. The limits of the testing should then be clearly defined in a testing plan. 1.4. Where to Test? In many cases equipment can only be tested on site. This is particularly true of alterations. However, where new equipment is factory wired and delivered complete to site, it is very often easier to carry out some of the testing before it leaves the factory. The continuity of through lineside circuits may often be tested before the equipment is connected at either end. 1.5. When to Test? A basic rule which should always be followed is to test as much as possible before commisssioning. New installations may often be tested complete using suitable simulations for external equipment and interfaces to existing signalling. Even with alterations, it is generally possible to reduce the amount of testing at the commissioning by testing any complete new circuits beforehand. It is often desirable to take this into account at the design stage. It may be better overall to replace a circuit which would be extensively altered with a complete new circuit rather than cut into the existing circuit in several places. The testing workload on commissioning may then be substantially reduced. It should be remembered that testing staff are often under pressure at a comm1ss1oning. Testing staff are always the last to finish and they may well have been delayed by earlier stages of the work taking longer than planned. However great the pressure to do so, equipment must never be handed over to the operators until it has been fully tested. Testing as much as possible beforehand can help to reduce such pressures. 1.6. How to Test? - The Management of Testing This will be covered in detail in sections 3 onwards. A good tester is thorough and methodical. He works efficiently but does not rush. Testing does not only involve proving that what does happen should happen. It is much more important that the tester ensures that what should not happen does not happen. One person must be appointed in overall charge of testing. He should first of all prepare a testing strategy. This should be done at an early stage. As the strategy adopted for testing and commissioning any project can have a significant bearing on costs, the testing strategy will need to be considered before financial authority is given for the project. The testing strategy should cover as a minimum the following matters:- a) What will be tested? b) How many staff, and with what specific skills, will be required to undertake all testing? c) How long will the testing take, both before and during commissioning? d) When will the equipment be available for testing and when is it required to be in service? e) In what order should the tests be carried out? f) What additional resources (equipment, transport, staff ) will be required and for what period. This testing strategy must then be developed into a full testing plan detailing a programme of tests to be carried out (including those associated with the commissioning) and the individuals responsible, preparatory work required, possessions required, equipment, temporary work (simulations etc.), methods of working, methods of communication, and methods of recording. This plan must then be thoroughly discussed with all those involved. It must also be independently checked. Once it has been agreed and approved, the testing plan must be communicated to everybody involved in the testing and commissioning programme. 2. COMPETENCE OF TESTING STAFF To be effective, testing must be carried out by competent staff. It is therefore the responsibility of each railway administration to ensure that all staff who are entrusted with any part of the testing are competent to carry out their delegated tasks. There are generally two ways to deal with competence of testers. a) The duties of testing are included in the job specification and are implicit in taking up the post. The tester's ability will be known by his superior on appointment to the job and will be monitored by normal managerial processes. Suitable action must be taken by the manager (training, discipline, restriction of duties) if the tester is found to be deficient in any part of his work. b) Formal processes of ensuring competence of testing staff may involve periods of instruction and/or experience in an assisting role (or under supervision), will usually require some form of examination, and will enable individuals to be certified. This certification will be required either as a qualification for a particular post or to permit the individual to perform specific duties. A specific time limit on the certificate should be considered, after which retraining and/or re-examination will be required. British Rail originally adopted the informal approach. With the greater variety and complexity of equipment, faster changes in technology and the need to attain the highest standards of quality and safety, the emphasis has now changed to a much more formal system of training, examination and certification. Sufficient staff must be trained and certified to carry out the required amount of testing, ensuring that testing remains independent of the design, checking or installation. Larger companies can usually justify the employment of specialist testing staff. Even so, there will be peaks (e.g. major commissioning stages) which require additional resources. Suitable design staff may obviously be employed but it is important to ensure the independence of all testing carried out by careful allocation of tasks. Smaller companies with limited numbers of staff will obviously require their staff to be more versatile. It is even more important in this case to ensure independence of testing. It is a natural preference for railway companies to prefer to carry out their own final acceptance tests for equipment from external suppliers. However, if independence of testing cannot be ensured it may be better to employ suitably qualified contractors or consultants to undertake all or part of the testing. 3. DOCUMENTATION At each stage of testing it is important to document precisely what has been tested and by whom. Ideally a signature should be obtained from the person carrying out each part of the test although in practice it may not be possible to do this for some remote tests until some time after the test has been carried out. To aid the tester full use should be made of check lists and other similar reminders. The person in charge of testing should ensure that a single log book is provided in which to document all queries and faults found. It will be necessary to provide multiple copies of entries in the log book so that these can be passed on to designers, installers or contractors (as appropriate) to take any action necessary and then reported back to the testers after corrective action has been taken. Test certificates should be provided for each part of the work. These are then summarised into the required parts, building up to a master test certificate to cover the complete project. All testers must adopt a standard method of marking diagrams and control tables so that there will be no ambiguity in the record of testing if one person has to take over from another. These standards should be issued as standard instructions or incorporated into the testing plan. 4. TESTING OF EQUIPMENT ROOMS AND LOCATION CASES 1.1. General Inspection Before testing individual circuits, an inspection should be carried out to ensure that the correct equipment is in place and properly identified. This inspection should include the following items:- a)Location cases are correctly labelled . b) All equipment is installed as specified on the drawings, to the correct layout and actually present. c) All equipment which is pin coded or otherwise uniquely configured to its mounting (e.g. signalling relays) is of the correct configuration. d) Cables and wires are of the correct size and type, correctly terminated and properly secured where appropriate. e) Equipment is Where initial testing takes place off site, this check to be carried out again when the location or other equipment is installed on site. 4.2. Wire Count The inspection above should have proved that the equipment is in place as specified. The next group of tests must prove that the circuits are wired as specified. As well as proving that each circuit exists as shown in the wiring diagrams, it must be proved that there is no electrical connection between circuits. The presence of a wire forming part of a circuit can be proved by a continuity test (see 4.3.). The absence of any other wires will not necessarily be shown by a continuity test. By counting the number of wires on each terminating point of all affected items of equipment, the presence of unwanted connections between circuits can be proved. If all wires have been installed according to the diagram, the wire count will correspond to the contact or terminal analysis for each item of equipment in the circuit. Any unwanted connection to another circuit will be evident by an additional wire or wires to those shown. 4.3. Continuity Test Using a bell or buzzer connected to a low voltage power supply, the continuity of each wire in each circuit should be checked. Where practical (e.g. new installations) all relays, fuses and links should be removed. On working installations, it may be necessary to test an unterminated wire. In this case the wire must be suitably labelled. On commissioning, it must be checked that the wire has been terminated on the correct terminal. 4.4. Circuit Test (Strap & Function Test) Persons carrying out this test must have a knowledge of the function and operation of each circuit being tested. To ensure that any earth faults are detected and eliminated, earth leakage detection is advisable on each leg of the supply for the duration of the test, if this is not already incorporated in the permanent power supply. The object of this test is to ensure each circuit operates as intended. Each circuit will normally have an end function (e.g. a relay) which operates when the circuit is fully connected. The equipment should be set up so as to operate this function. The voltage and polarity at the operating terminal (e.g. relay coil connection) should be observed using a meter or other suitable measuring instrument. Having proved that the circuit operates when it should, we must now break each switch, fuse, contact or link in the circuit, in turn, to prove that the relevant control is included. If there are controls in both legs of the circuit, each leg must be tested. The contact should be broken by energising or deenergising the relay or operating the switch (as appropriate) and the change in voltage noted. The broken contact should then be strapped out and the voltage observed to return to its original value. Where there are parallel branches of a circuit, all possible circuit paths must be completely tested. It is important that any straps used for such tests are not left behind after the testing is completed. To avoid this possibility, a set number of straps shall be provided, identified and numbered. Only these straps shall be used for circuit testing and they shall all be accounted for at the end of each testing session. 4.5. Other Tests Other tests may also be required to ensure the correct functioning of equipment. Included in these are:- a) Continuity, earth, and insulation tests on all cables. b)Adjust and/or set all Where seals are provided, these should be in place before testing is complete. c) Test all power supplies - see section 8. 4.6. Other Precautions If a test panel or other temporary wiring is used to simulate external functions, all circuits must be fully documented and must be re-tested after removal before an installation is fully brought into use. All redundant wiring to be removed must be distinctly identified (e.g. by tapes or labels of a specific colour). It may be desirable not to remove the wiring until the testing is complete. If this is the case, all removed wires must be completely insulated on disconnection until the wiring is removed. If possible, redundant wiring must be removed before the equipment is brought into use, otherwise as soon as possible thereafter. 5. TESTING OF EXTERNAL & LINESIDE EQUIPMENT Section 4 has dealt with the general method of testing the controlling circuitry. In addition, each item of external equipment must be tested to ensure its correct operation and that controls from and indications back to the interlocking function properly. The most common items of equipment are detailed below. Only general guidance can be given here. Additional tests may be necessary for specific types of equipment. In general, it will be necessary to have one or more persons on the track to observe the operation of the external equipment and its controlling relays and circuits. Another person will be required to operate the signalman's controls and observe indications. Suitable communication must be provided. Alternatively, it may not be possible for various reasons to use the controls from the interlocking. In this case, a temporary feed must be provided at the location to enable all local circuitry to be tested. The through circuits must be tested at a later stage when they are available. If it is not possible to carry out a complete test this must be recorded on the testing documents to ensure the remainder is subsequently tested. 5.1. Power Operated Points A general inspection should be carried out to ensure that the points are correctly installed and labelled and that all cables are secured clear of moving equipment. Toe points should be operated by hand to ensure that they move freely, each switch rail fits correctly against its respective stock rail and there is adequate clearance when the switch is open. A wire count should be carried out on all terminations. Before commencing the test, the tester on site and the tester at the control panel should confer to check that the site tester is at the correct set of points (name runing line and position relative to other equipment etc.). When describing the position of the points, the term "left (or right) hand switch closed" should be used rather than normal or reverse. Toe person at the control panel should then check correspondence with the controls and indications. Earth leakage detection should be operative during all electrical tests. Operate the points under power from the control panel to confirm detection at the location and the signal box, the panel indications and all controlling relays correspond with the position of the points. On 4-wire detection circuits the opposite circuit to that under test should be monitored to ensure that no irregular voltages appear during the operating cycle. For each position of the points break each detection contact of each end of the points to ensure that the detection relay de-energises and the panel indications extinguish. Any supplementary detectors must also be included in this test. Check that the clutch (where provided) slips at the correct current when an obstruction is placed in the switches and that the cutout timer operates correctly. On multi-ended points check for correspondence. For example, if the points are normal, move each end to reverse in turn to ensure that detection is lost in each case. Check all possible permutations of normal and reverse to ensure that normal detection is only obtained when all ends are normal. Each supplementary detector, if provided, must be separately included in this test. Repeat for reverse detection. 5.2. Signals Firstly, visually check the signal to ensure that the profile of the signal is as shown on the signalling plan and agrees with all documented sighting requirements. The correct identification plate must be fitted and other items such as signal post telephones and emergency replacement switch (if provided) should be correctly fitted and labelled. If possible the signal post telephone(If present) should be in working order so that it can be used for the test. Where the facility is provided, the signalman's telephone equipment should indicate the correct signal to which he is speaking. Check inside the signal head that the lamps are of the correct type, close-up segments are correctly positioned and filament changeover relays(if incandescent signal used ) are present. Check for correct alignment and sighting of the signal. Carry out a wire count on all terminations. Check by operating the control relay(s) that the correct aspects and route indications are displayed. All routes must be tested. Check each main aspect lamp in turn to ensure that only the main filament illuminates and that filament changeover relays and associated indications function correctly when the main filament fails. Lamp proving should continue to operate when the main filament fails. Check its correct operation by simulating failure of both filaments. For LED signal lamp proving relay shall be tested (ECR) .Some modern interlocking products have direct signal driving card ,capable of detecting current for lamp proving functionality .This shall be tested for the functionality when LED signal fail. Where junction or route indicators are lamp proved, test that the failure of the required number of bulbs maintains a red aspect in the signal. Check for the correspondence of indications to the aspect(s) displayed for all indicated signals. Where the signal is not indicated (automatic signals) test the aspect lines to the signal in rear. 5.3. Automatic Warning System, Trainstops and ATP Systems On many British Rail main lines, the electro-magnetic Automatic Warning System (AWS ) is still fitted as standard. The following procedures apply to testing the track mounted equipment. Inspect the track mounted equipment for correct layout, height relative to the rail and distance from signal(s). Check that the internal links in electro-inductors are correctly connected for the supply voltage used. Test each permanent magnet and inductor with a strength and polarity meter. The electro-inductor should be tested for each aspect of the respective signal(s) and should only be energised for a green signal. Suppressor inductors should respond to the controlling relay. Where other similar warning or automatic train protection equipment is provided, its correct operation in conjunction with the signals must be tested. For a trainstop, inspection should check that it is securely fixed to the sleepers in the correct position relative to the signal. Height relative to and distance from the running rail must be within tolerance. The arm must be checked in both raised and lowered positions. The arm should not be bent or otherwise damaged. Setting of indication contacts must be checked for tolerance. A wire count should be carried out on tail cable terminations. Depending on the type of trainstop, the lowering mechanism or circuit should cut off and the holding device should operate at the end of travel when lowering. Disconnection of the operating circuit should result in the trainstop returning to the raised position. Normal and reverse indication circuits should be checked for correct operation via the allocated contacts. The operation of the trainstop with the signal may be checked at this stage or when performing the aspect sequence test. Energisation of the signal operating relay (HR or equivalent) should cause the trainstop to lower. The signal should remain at danger until the trainstop is fully lowered. Locking the trainstop arm down should prevent the signal in rear from clearing when the signal is at stop. Unless the controls specify otherwise, the signal in rear should be able to show a caution aspect when the signal associated with the trainstop has cleared again.For ETCS Level 1 ,Transponder position shall be checked against transponder layout plan /signalling scheme plan. functionality shall be tested to ensure signal aspects are replicated and signals transmitted to train antenna to capture. 5.4. Track Circuits The full length of the track circuit must be examined to ensure that its limits agree with the bonding plan, all bonding (including traction bonding) is in position and correctly secured to the rail and all block joints and track circuit interrupters (where specified) are present. Staggering of block joints, spacing of adjacent block joints, clearance points and track circuit minimum and maximum lengths must conform to laid down requirements. The lineside/location equipment must be inspected to ensure that the correct equipment has been provided and that it is compatible with all adjoining and parallel track circuits. A wire count should be carried out at all disconnection and termination points. Check the required voltages/currents to ensure that the track circuit has been correctly set up and test for correct operation by shunting the track circuit at several places, including all extremities. On jointless track circuits ensure that the actual limits of the track circuit are as specified. If all or part of the track circuit has excessively rusty rail surfaces, the drop shunt test should be repeated after the rails have been cleaned sufficiently by-passing trains. With all adjacent track circuits energised, disconnect the feed and check that the relay de-energises. This ensures that cables are not transposed and/or voltages are reaching the track relay from adjacent feeds via the rails. Any residual voltage on the rails should be below a specified safe level which will not under any circumstances energise the relay. Check polarities for staggering with respect to adjacent tracks and test that the correct indications operate when the relay is deenergised. All sections of a multi-section track circuit must be tested. 5.5. Axle Counters The full length of the axle counter section must be examined to ensure that its limits agree with the bonding plan. A wire count should be carried out at all disconnection and termination points. Axle counter power On test shall be performed and section occupancy and clearance shall be checked by running Trains . Axle counter data upload check list shall be completed with file name ,version CRC and shall be maintained with relevant signatories signing the form. Counter RESET functionality shall be carried out to check the counts “forgotten’ are reset via train sweeping and counter reset to be tested . 5.6. Level Crossing Equipment Check that the layout of the equipment corresponds to the drawings and all equipment is of the correct type. Telephones where provided should be operational and give the correct indication to the signalman when in use. All indications (e.g. road signals, barriers, power supply) should be tested for correct operation. To test the operation of the crossing equipment, the same tests should be applied to the controlling equipment as those specified for locations and relay rooms (section 4). 6. REMOTE CONTROL SYSTEMS The main test of any remote-control system is that each output responds to its associated input and does not respond to any other input. This is best done for TDM equipment by first checking at the inputs and outputs of the TDM equipment itself and then testing between the signalling input and the corresponding signalling output. For FDM systems, each receiver should respond only to its associated transmitter. Where several parallel systems are in operation tests should be made to ensure that crosstalk is within safe limits. Line voltage levels should be checked to the equipment specification. Where automatic line or system changeover is provided, simulate a failure to ensure that the changeover operates correctly. Check that all system alarms operate correctly. Check that the failure of a TDM system produces the correct warning indications on the control panel. If the remote-control system performs any button or indication processing, outputs should be tested individually to confirm that they are only produced by the correct combination and/or sequence of inputs. 7. CONTROL PANELS It is vitally important that the control panel (or VDU graphic display) represents accurately the layout of the track and signalling. It should be checked to both the signalling plan and the panel drawing. Check that the correct relay(s) or remote-control input(s) respond to buttons and switches. Check that incoming indication circuits illuminate the correct lamp(s) on the panel. Indications which are combined at the signal box (e.g. point indications in route lights and track indications over points) should be checked for correct operation. Check that the correct indications are shown under remote control failure conditions. 8. POWER SUPPLIES Before testing any power supplies ensure that the correct safety precautions are taken for the highest voltage likely to be present. The main tests which could have serious implications for safety are the polarity of each supply and the operation of earth leakage detection. Other tests are mainly concerned with the reliability of the supply and its ability to carry out its required function. A wrongly rated fuse for example may not cause a wrong side failure but could cause serious disruption if a cable bums out. Measure all voltages to ensure that they are within 10% (or other specified tolerance) of the required value. In particular check the voltage at the supply point under light load conditions and the voltage at the end of each feeder under maximum load to ensure that these tolerances are not exceeded. Check all fuses are of the correct rating and that there is the correct fuse discrimination. Where equipment is commissioned in stages, power supplies should always be re-tested whenever the addition or removal of equipment significantly alters the electrical load. Because of interaction between the various electrical loads and the distribution system, final adjustment of power supplies may not be possible until all other equipment has been connected. 9. FUNCTIONAL TEST OF SYSTEM Many separate parts of the signalling system will have been tested beforehand. It is important that, before any equipment is brought into use, the signalling is tested as a complete working system. If it has not been possible to do so beforehand, each through circuit must be tested complete to ensure that all controls and indications operate correctly to the correct function. The signalling must then be tested to ensure that it conforms to the control tables and to signalling principles. It is possible to carry out both these tests at the same time as described below. The aspect sequences between all signals must also be tested by observation of each signal. 9.1 . Through Circuits All circuits, whether direct wire or via a remote control or data link must be tested to/from the controlled function. Where cables are terminated intermediately, the polarity is to be checked to confirm that there are no crosses in the circuit. Polarised circuits are to be tested to ensure that they only operate on the correct polarity of supply. 9.2. Control Tables Test This test ensures that the interlocking performs according to the control tables. It must always be remembered that we are testing that unsafe situation will not occur rather than looking for the expected clearance of signals and movement of points. Therefore, as an example, when testing the controls on a signal, the route should first be set and the signal cleared. Each individual control must then be removed in tum to prove that the signal will return to danger each time. Similarly, route locking should be retested as the train clears each track circuit. A test panel, wired to a bank of switches to disconnect each incoming indication circuit, is the normal means of testing that items such as track circuits, point detection and lamp proving are included in the appropriate controls. It is vitally important that the test panel wiring itself is documented and tested on its installation and again on its removal. Generally, the tester in charge of this test will require an assistant to operate the various functions from the test panel, If a principles test (see 9.3) is carried out at the same time he must also have an assistant to mark off each item on the control table as it is tested. The main tests to be carried out are listed below although this is not an exhaustive list. 9.3. Principles Test As previously stated, this can generally be carried out at the same time as the control tables test. The tester must request all controls from his knowledge of signalling principles, not by reference to the control tables. He must not be led by the checker, who is recording the progress of the test on a copy of the control tables. The checker should only intervene if the controls have not been completely tested. In this case the checker and tester must resolve any discrepancies before proceeding. Remembering that any redesign must be independently checked and tested, testing staff should not become involved in the detail of any circuit alterations required as a result of incorrect controls discovered during testing. Where circuit alterations are necessary, all previous tests should be repeated on the affected circuits before continuation of the principles test. As well as tests between conflicting routes and points, the tester should also attempt to test as many other routes and set up as many other independent conditions as possible during testing to prove the integrity of the signaling. 9.4. Aspect Sequence Test Although the individual signals will have been tested to their controlling relays, this is a vital test which ensures the correctness of all circuits between signals so that the correct aspect is displayed to the driver. The control tables may be used for this test but it is often easier and more efficient to use an aspect sequence chart. Signalling plans should not be used alone unless they show complete and unambiguous aspect sequence information. All signals should be cleared to all possible aspects for each route. The aspects of all signals which are dependent on that aspect are to be observed and checked for correctness.Lamp proving controls should be tested. For automatic signals, the presence of all track circuit controls should also be tested. Trainstop proving controls should also be tested where appropriate. 10. OTHER IMPORTANT CONSIDERATIONS It has been stated previously but it will be repeated here that all redundant and temporary test wiring is best removed before the signalling is brought into service. If this cannot be done, wires to be removed must be insulated at both ends and suitably identified. The removal must take place as soon as possible after testing. The removal of temporary wiring will require a further possession. The circuits affected must be fully tested. Effective communication is vital to efficient testing. All instructions and messages must be clear and concise. Standard forms of messages should be used where possible. Messages should be repeated where necessary. Where radio or telephone communication is used, each person must be clear whom he is speaking to. When requesting an action, confirmation that it has been done should be obtained before noting the results of any test. Consistent terminology should be used throughout Examples are:- a) Relays - "up" or "down", "normal" or "reverse". b) Points - "left hand switch closed" or "right hand switch closed" c) Signals - state lamps illuminated, not meaning of aspect (e.g. "yellow",not caution or "two green lights", not clear). Give number, letter or position for route, junction or turnout State whether or not marker lights are illuminated and if the main signal red lamp(s) remain alight when the subsidiary signal is in use. Trainstop position (where fitted) should also be stated. d) Track circuits - "clear" or "occupied". There are many advantages to running a test train as an additional final test. Finally, however thorough the test there are likely to be some further adjustments (e.g. power supply voltages, signal lamp voltages) necessary after commissioning. Remember that the equipment is now working and possessions will have to be requested and arranged. 11. MAINTENANCE TESTING All of the preceding paragraphs refer to the testing required for new and altered signalling installations. The high degree of testing is necessary because the equipment has not been used in service before or its controls have been altered. Testing is often necessary during maintenance activities, either as part of the routine replacement of equipment for servicing or during the rectification of a fault. In general the scope of testing under these circumstances is much reduced. This can be justified provided the work comes within any of the following categories:- a) like for like replacement of equipment. The signalling controls and the function and arrangement of all circuits are unaltered. When the work is complete, existing wiring diagrams are still valid. b) Circuit diversion. Re-routing part of an existing circuit through another identical item of equipment, e.g. bypassing a faulty cable core or relay contact. The function of the circuit is still identical. The form of the wiring diagrams is unaffected but allocations will change and suitable record must be made of the alteration, whether temporary or permanent. c) Temporary disconnection of a circuit and its subsequent reconnection in the same form, to enable engineering works to take place (e.g. the disconnection of track circuits or the removal and replacement of a trainstop while permanent way renewals are carried out). If the work affects the form or function of a circuit (e.g. track circuit bonding changes) tests must be carried out as for new work. Under the above conditions, a detailed test of all controls is not necessary because the majority of the circuits have not been altered. The purpose of testing under these conditions is to prove that the replacement equipment has been correctly connected and is in working order, a diverted circuit is connected in the same manner as the circuit replaced or disconnected equipment has been replaced in its original state. It is not possible to give comprehensive rules to cover all known situations but the following principles should provide useful guidance. 11.1. Preparation and Planning Even with the smallest job, adequate preparation and planning can assist in the prompt execution of a job and its completion without any mistakes. It is often useful to identify the tasks involved and write them down as a check list. In effect, this is a simplified form of the test plan used for new works. If wiring is to be removed and later replaced, the wiring should first be checked to ensure that it corresponds to the wiring diagrams (e.g. by wire count) and any affected wires labelled. Before any work is started, replacement equipment should be inspected and, where possible tested, to ensure it is of the correct type and in full working order. Where more than one item of equipment is involved, all equipment should be available at the site of work. Cable cores and other wiring to be used for diversion of circuits should be tested for continuity and insulation to earth. Contacts on relays should be checked that they are of the same configuration as the faulty contact (i.e. front or back). If a component or module to be replaced has any variable settings, a note should be made of the existing settings for later reference (e.g. track feed resistor/capacitor, power supply transformer tappings). This will aid setting up but does not avoid re-testing of circuit values and adjusting as appropriate. 11.2. Execution of Work Make the necessary arrangements for possession of the affected equipment and ensure that the appropriate rules have been complied with before commencing work. Take the necessary steps to ensure staff safety by switching off power or disconnecting circuits as appropriate. As the work progresses, check that each step has been carried out before proceeding to the next. Where wiring has to be replaced, check that the termination point of each wire conforms with the labelling and carry out a wire count when all wires have been replaced on their terminations. Depending on the type and scale of the work, it may be better to test in stages or to carry out a single final test. Do not hand back equipment to service until testing is complete. 11.3. Testing on Completion Ensure that equipment is correctly fitted and secured. Carry out a wire count on all terminations where wires have been removed and/or replaced. Carry out any earth or insulation tests according to the type of equipment. Perform any mechanical adjustments of equipment (e.g. point machines) before applying power. Test for the correct operation of the new or replacement item of equipment in the existing circuits. Full circuit tests should not need to be carried out on parts of the circuit which have not been affected. Ensure that equipment is labelled correctly. 11.4. General Precautions Although it is important that persons do not test their own work, the strict requirements for independence of new works testing are not necessarily appropriate for maintenance testing. Much work, particularly fault rectification, will be done by a small team of perhaps two or three staff. One of these may need to perform lookout duties. It is therefore permissible in most cases for one person to direct and test the work provided he does not participate in the detail of the installation. It is essential when carrying out any work that complete current circuit diagrams are available. If an alteration to equipment allocation is necessary, this should be noted on the wiring diagrams and (if permanent) arrangements made for the records to be amended. 12. CONCLUSIONS Following testing, the equipment is brought into use. It will now be used to control real trains. Rigorous design, checking and installation procedures, together with the tester's skills must have eliminated any remaining errors in design and installation. The only acceptable level of accuracy is 100%. Testing is the last defence against any previous errors. The safety of the railway depends on it. NOTE : Read Chapter CH23B | "Verification ,Validation and Trial Run " detailing a CBTC system approach testing model
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CH2 | BASIC SIGNALLING PRINCIPLES | PART 2
CONTINUED FROM - SIGNALLING BOOK | CHAPTER 2 | PART 1 SIGNALLING BOOK | CHAPTER 2 | PART 2 CONTENTS 1. Introduction - In Part 1 2. Signal Aspects - In Part 1 3. Signalling Principles - In Part 2 4. Drawing Standards - In Part 2 5. Interlocking Principles - In Part 2 6. Train Detection & Track Circuit Block - In Part 2 7. Colour Light Signals - In Part 2 8. Control Panels & Other Methods of Operation - In Part 2 9. Colour Light Signalling Controls - In Part 2 3. SIGNALLING PRINCIPLES Any railway administration must have a set of rules which determine the basic principles of design and operation of the signalling system. In some cases they may be carefully documented in detail, in others they may have developed through many years of custom and practice. At the very minimum, the operating rules must define the meaning of each signal aspect. British Rail has a set of Standard Signalling Principles. They will not be referred to directly in this course but many of the BR principles are similar to SRA (TfNSW) practices. 4. DRAWING STANDARDS Most readers will be involved in the design, specification, installation, testing or maintenance of signalling equipment. They will inevitably have to convey large amounts of technical information to others. This is usually done by means of drawings. It is important to appreciate the impact of drawings on engineering activities. They are used to:- Specify a signalling system Agree the specification with the users (operating department etc.) Agree the specification with suppliers/contractors Estimate costs Order materials Construct and install equipment Test an installation for correct operation Maintain the equipment Locate and rectify faults The information included in the drawing will vary considerably according to which of the above purposes it will serve. A drawing will generally need to be read by someone other than the person who produced it. In the same way that persons talking to each other need to speak the same language, engineers need to use common conventions and symbols in their drawings to convey the necessary information. If a drawing is not understood, it is of no practical use. SRA (TfNSW) have developed a large range of standard schematic and circuit symbols to depict signalling equipment and electrical circuits. A copy of these symbols is provided with these notes. Also provided are the main schematic symbols likely to be used on British signalling plans. These are incorporated in a British Standard - BS 376. As this has not been revised for a number of years, certain additional symbols not included in BS376 are now in regular use. In most cases, each railway administration will have its own company standards for the production of technical drawings. It is nevertheless important that the signal engineer should be able to ensure that each drawing is responsible for issuing, conveys the necessary information. This may include the use of symbols and terminology peculiar to the signal engineering profession or even to a particular company. 5. INTERLOCKING PRINCIPLES To understand some of the basic principles of interlocking, it is best to start by referring to a simple mechanical signalbox. Most Readers will not be engaged in the installation of mechanical equipment, but many mechanical signalboxes still exists and it provides a simple example which demonstrates the general principles. Each lever in the frame has a normal and a reverse position. For signals, the normal position is always associated with the danger or stop aspect. Moving the lever to the reverse position operates the signal to the proceed aspect. With points, normal and reverse have similar status, each being associated with one of the two possible positions of the points. However, the normal position is generally used to set points for main routes. This terminology has been carried forward to electrical signalling although levers are no longer used. lt is therefore essential to remember the association between the terms normal and reverse and the state of the equipment. In the mechanical signalbox depicted below, the levers are mechanically interlocked. A signal lever cannot be moved from normal to reverse unless all point levers are in the correct position. Once the signal lever is reversed, the point levers for that route and any which provide additional protection, cannot be moved from their current position (normal or reverse). Signal levers reading over the same portion of route in opposite directions cannot be reversed at the same time. Starting signals 3 & 7 when operated will not lock any other lever. They will however be electrically controlled by the block equipment to the adjacent signal boxes. Home signal 2 requires points 5 normal. Conversely, 5 points reverse will lock signal 2 in the normal position. Signal 2 need not directly lock signal 4 as the signals require 5 points in opposite positions. To reverse 4 signal lever requires 5 points reverse AND 6 signal normal. To reverse distant signal lever 1 requires signals 2 AND 3 reverse. In colour light signalling practice there is no equivalent to this type of interlocking as distant signals will usually work automatically, controlled by the aspects of the stop signals ahead. It should be evident from the above examples that all basic interlocking is reciprocal (ie. if 4 reverse locks 6 normal then 6 reverse must lock 4 normal, if 5 is required reverse to release 4 then 4 in the reverse position will lock 5 in the reverse position). The reciprocal nature of locking is inherent in the construction of any mechanical system but in electrical systems the engineer must ensure that it is provided. It is a useful check to ensure that for each basic interlocking control the converse is also specified. It is possible to produce a complete set of interlocking controls (the locking table). However, as modern signalling systems do not employ lever frames, the format shown is no longer used. As its main purpose is to specify the construction of the mechanical interlocking, the converse of the releases is not shown, neither is the locking applicable for moving the lever from reverse to normal. Although adequate for this purpose, it is totally inadequate for an electrical system. RELEASED BY (Req Lever Reverse) LEVER NUMBER LOCKS (Req Lever Normal) 2.3 1 2 5 3 5 4 6 5 2.8 5 6 4 7 8 5 7.8 9 release the lever in the left hand column, the other levers must be in the position shown. REQUIRES LEVERS LEVER NUMBER N -> R R ->N 1 2R.3R 2 5N 1N 3 1N 4 5R.6N 5 2N.8N 4N.6N 6 4N.5R 7 9N 8 5N 9N 9 7R.8R 6. TRAIN DETECTION & TRACK CIRCUIT BLOCK The development of a safe, reliable means of detecting trains, the track-circuit, allowed a major advance in safety and ease of operation. The earliest application of track circuits was to prevent a signalman forgetting a train standing at his home signal, and giving permission for another train to enter the section. To do this, the berth track circuit bolds the block instrument at "train on line". The use of track circuits was then extended to cover sections of line which were out of sight of the signalman, and also to lock facing points while trains were passing over them. It was soon realised that a track-circuit could be used to ensure that the whole of the section was clear. There would then be no need for signalmen to supervise the entry and exit of trains, to ensure the section was clear. "Track-Circuit Block" was thus created and block instruments could be dispensed with. With track-circuit block, the rear signalman does not have to ask permission to send a train forward, be can do so whenever the track circuits are clear up to the end of the overlap. 7. COLOUR LIGHT SIGNALS The development of track-circuit block made it possible for signals on plain line to work automatically - a signal could show clear when the section track circuit was clear, and stop if otherwise. Normally, no action would be necessary by the signalman, other than to observe that the trains were running normally. This, in turn, made it economic to have short block sections, allowing increased line capacity, as a signalbox was no longer needed for each block section. The use of automatic colour light signals soon became widespread. The signal aspects were and still are as described in section 2. Usually the appearance of the signal is slightly modified to identify it to the driver as an automatic. This may be by means of a sign or as in SRA (TfNSW) practice, by offsetting the upper signal head to the left for double light signals and by offsetting the marker light towards the track for single light signals. The manner in which colour light signals are used will differ according to the required capacity of the line. 7.1. 2 Aspect Signalling This is a direct colour light replacement for the mechanical distant and stop signals. The block section is the length of track between two successive stop signals. Each stop signal will have an associated distant signal at least braking distance from it. Block sections will generally be long, typically several times normal braking distance. 7.2. 3 Aspect Signalling To increase the frequency of trains on a line, the block sections must become shorter. When the length of the block section is not significantly greater than normal braking distance, 3 Aspect signalling economises on the number of signal posts required by combining the two signals at the same position along the track. Each stop signal also displays the distant aspect for the next stop signal. Each signal can display stop, caution or clear. The length of the block section must always be greater than or equal to braking distance. 7.3. 4 Aspect Signalling On high speed lines or those with a high traffic density, it is often necessary to have block sections shorter than braking distance. It is then necessary to give the driver an earlier caution indication, as he has insufficient distance to stop between seeing the caution and arriving at the stop signal. In such cases, the signal in rear of the caution shows a medium aspect as a Preliminary Caution. This is a "4 aspect" signalling system as each signal can display four distinct indications to the driver. Although, in theory, capacity could be increased further by the introduction of additional aspects, few railways have found it necessary to do so, unless associated with the introduction of automatic train control. It is likely that too many different aspects would lead to confusion. If the total length of two adjacent block sections is less than braking distance due to signal positioning requirements, it would appear that a further aspect is necessary. SRA (TfNSW) practice however is to repeat the medium caution in this situation. British practice is to ensure that signals are suitably spaced to avoid this situation. Note that the additional "low speed" aspect used on many SRA (TfNSW) signals is not for increasing headways at normal speed. Although it often forms part of the normal sequence of aspects to bring a train to a stand, its overlap generally coincides with the caution aspect and does not affect the overall line capacity. Its purpose is to allow trains to close up to each other after their speed has been safely reduced. It is particularly useful in the vicinity of stations to minimise the effects of station stops on line capacity, although its inclusion in the normal aspect sequence can be restrictive if a full overlap beyond the signal at stop is available. This will be covered in more detail in later sections. The existence of low-speed aspects does not need to be taken into account in determining the capacity of a line for through running at normal line speeds, unless the low speed overlap lies beyond the caution overlap for the signal in the rear. 8. CONTROL PANELS & OTHER METHODS OF OPERATION The introduction of colour-light signals, and power-operated points, in tum allowed the bulky and cumbersome lever-frame to be replaced by modem signalboxes with panels. The standard British type of panel has for many years been the "Entrance-Exit" (N-X) type, with push-buttons for setting routes. Each button has 3 positions: middle, pushed and pulled. The button is sprung to return to the middle position after it is either pushed or pulled. To set a route and clear a signal, the entrance button corresponding to that signal must first be pushed and released. This button will flash, to indicate it is the selected entrance. Toe next button pressed is taken to be the exit or destination. Provided the route between the two buttons is both valid and available, then the route will set, the entrance button will change to a steady white light, and in addition white route lights will illuminate to the destination. With the route set, any points will move to the required position automatically. Provided the route is clear, the signal will then clear. To restore the signal to red, and release the route, the entrance button is pulled. If required, the points can be controlled manually from the panel. Each set of points is provided with a three position switch for this purpose. With the switch in the central position, the points will move automatically as routes are set. Alternatively, it may be turned either left to move the points normal, or right to move them reverse. The position of all trains in the panel box area is indicated by red track circuit lights on the panel, normally appearing in the same aperture as the route lights. Indications are also provided for each signal, and each set of points. Unlike a lever frame, where the signalman can only pull a lever if it is safe to operate that signal or set of points, with a push-button panel the signalman is always able to operate the buttons or switches - but the trackside equipment will only respond provided it is safe to do so at that time. The "interlocking" is used to ensure this safety. Conventionally, the interlocking has been done with relay circuits, a typical panel signalbox requiring many thousands of relays. Relay technology, although very reliable in operation, is now being replaced on many railways by electronic or processor based systems. British Rail, in conjunction with Westinghouse and GEC, have developed "Solid State Interlocking" (S.S.I.), which is now being used in a large number of installations, achieving significant savings in space and cost. SSI also has the advantage that alterations to the signalling controls do not require extensive alterations to physical wiring. Most of the controls are stored as data which can be prepared off-site beforehand. Improvements in technology have not only revolutionised the interlocking equipment. Attention has also been given to the interface with the signalman. Although S.S.I. may be operated from a conventional control panel, it is becoming more usual to use video display units (VDU's). These can either be used as a direct replacement for the control panel or as part of a much larger integrated system for providing all train running information to signalmen, passengers and other operating staff. As an example, the BR IECC (Integrated Electronic Control Centre) includes a train describer system, automatic route setting to a stored timetable, train reporting, passenger information systems, communication with adjacent signal boxes and extensive monitoring facilities. If all trains are running normally, the signalman can sit back and watch the trains go by while the Automatic Route Setting does most of the work. SECTION OF A TYPICAL CONTROL PANEL (BR Style) 9. COLOUR LIGHT SIGNALLING CONTROLS This section describes the normal controls which would be found on a modem colour light signalled layout operated from a control panel. 9.1. Types of Route A ROUTE is the section of track between one signal and the next. All routes have an entrance and an exit. A signal may have more than one route if there are facing points ahead of it. Although the exit is usually another signal, it may be a buffer stop (terminal platform or siding) or an unsignalled portion of the railway (depots, yards or sidings). A route is uniquely defined by the number of the entrance signal, a suffix defining the direction of the route (in order from left to right as seen by the driver - normally a letter although some railways use numbers) and, where necessary, a letter denoting the class of the route. The type of the route is determined by the purpose of the train movement. In British terminology these are known as classes of route. Each class of route will have different controls applied. SRA (TfNSW) does not use the term class; however, there are three general types of route (four in British practice). A signal may have more than one type of route to the same exit. 9.1.1 Main Routes A main route is from one main running signal to the next. The signal proves all track circuits clear and points correctly set and locked up to the next signal, which is proved alight. In addition, a further distance beyond the exit signal is also proved clear with points set. This is known as an "OVERLAP". The purpose of the overlap and the determination of its length will depend on the type of railway, the setvice operated and the provision of any protective devices to prevent a train running past a signal at danger. In the Sydney metropolitan area, trainstops are provided which are set to operate a tripcock on the train's braking system if a signal is passed at danger or in some cases approached at too high a speed. In this case the length of the overlap should be sufficient for a train which bas been tripped to stop within the overlap. Overlaps distances may therefore vary for each signal according to line speed and gradients. Under present day operating conditions, the worst case overlap would be of the order of 830 metres for a line speed of 115 km/h on a 1 in 50 (2%) down gradient. Overlaps may often be longer than the signal sections. Elsewhere, trainstops are not provided. Unless and until some form of automatic train protection is provided, there is no certain means of ensuring that a driver will not inadvertently pass a signal at danger. The driver bas the final responsibility for obeying the signals. So, whatever the length of overlap, there is no guarantee that it will be adequate for all situations. It can therefore be considered as a margin for error if the driver misjudges his braking or the train braking system does not perform adequately. A nominal 500 metres is the present standard. This has been shown by experience to be adequate for most situations. In special circumstances, the overlap distance may be reduced. The end of the overlap is indicated on signalling plans, and often on the signalman's panel. Routes giving a low speed aspect may also be classified as main routes although the overlap will be much shorter (often 100 metres or less). Some caution or low speed aspects may be conditionally cleared (i.e. approach controlled) to permit a shorter overlap to be used at the next signal. In British practice such routes would be defined as "warning" routes. SRA (TfNSW) does not make such a distinction. 9.1.2. Calling-on Routes Some railways prefer a separate type of route for passenger trains running into occupied sections (e.g. bringing a second train into a partially occupied platform. This is known as a calling-on route and will require a distinct aspect, the main aspect remaining at stop or danger. Although calling-on signals exist on SRA (TfNSW), there is now no distinction between calling-on and shunting routes. Calling-on moves will be made under the authority of a subsidiary shunt signal (see 9.1.3.). 9.1.3. Shunt Routes A shunt route is used for low speed (usually non-passenger) movements, e.g. into or out of sidings or for shunting between running lines. Any move into a line which is not proved clear, e.g. a siding, and any move from or up to another shunt signal or "limit of shunt" is classed as a shunt move. Shunt routes may be from dwarf or position light shunt signals or from a main signal, using a subsidiary signal on the same post. Route indications are provided where required. For a shunt route, the signal proves all points correctly set and locked. Proving of track circuits will depend on the policy of the railway concerned and local operating requirements. If it is regularly required to shunt into an occupied line, track controls should not be provided. Some sidings, of course, may not even be track circuited. Where a shunt move is made using the subsidiary aspect of a main signal, the train should first come to a stand. This can be partially achieved by using only a short range signal. However, some railways require the subsidiary signal to be approach controlled by timed track circuit occupation. For a shunt move from a ground-shunt signal there is no requirement for approach control, although it is sometimes provided. The train should either be approaching at low speed anyway or it will have set back behind the signal and must first stop before reversing direction. Where propelling moves (i.e. with the driver at the rear of the train) are regularly made past a shunt signal, some railways employ "I.AST WHEEL" replacement of the signal aspect so that the signal does not go back to danger until the driver has passed the signal. In such cases the signal continues to show a proceed aspect,·even, when the train occupies the first tracks beyond the signal, and is only replaced when its berth track clears. 9.2. Approach Locking When a route is set, the interlocking will lock all the points in the correct position, and lock out any conflicting and opposing routes. The signalman must not be allowed to restore the route, and release this locking, with a train approaching the signal. This is called "APPROACH LOCKING". Once a signal has cleared, its route cannot be released until either:- the train is proved to have passed the signal a suitable time delay has elapsed, allowing an approaching train to see the replaced signal (or any cautionary aspects leading up to it) and be brought safely to a stand without any risk of passing the signal at danger. there is proved to be no train approaching ("Comprehensive Approach Locking") Proving that the train has passed the signal is done by monitoring the sequential operation of the track circuits immediately beyond the signal. 9.3 Point Controls Although the signalman has a switch for manual control of each set of points, they are normally controlled automatically by the setting of routes. The points are then locked by the route set over them. The points are also locked by the track circuits over them, so that they cannot be moved under a train. Where a set of points has more than one end, then they are locked by the tracks over all ends. If a track circuit adjacent to the points is positioned so that a train standing on one of the diverging tracks could be foul of a movement over the other track (a "foul" track circuit) it must be proved clear before the points are allowed to move to the position which would allow the fouled movement. 9.4. Route Locking Once a train has passed a signal, its route can be restored but any points, conflicting routes, etc. ahead of the train must remain locked. This is done by the "route locking", which is indicated by the line of white lights on the signalman's panel. The release of route locking must first be preceded by the release of approach locking (i.e. it is safe to start releasing the route. If the route is cancelled after the train enters the route, the white lights extinguish behind, releasing the points for other moves. The white lights always remain alight in front of the train, holding the points ahead locked. If there is no train in the route at the time of release and the approach locking has proved that there is no train approaching or it is safely at a stand, the whole of the route will release immediately.
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CH3 | SIGNALLING A LAYOUT | PART 2
CONTINUED FROM - SIGNALLING BOOK | CHAPTER 3 | PART 1 SIGNALLING BOOK | CHAPTER 3 | PART 2 CONTENTS 1. Introduction - In Part 1 2. Headway - In Part 1 3. Positioning of Running Signals - In Part 2 4. Types of Signal - In Part 2 5. Points and Crossings - In Part 3 6. Track Circuits - In Part 3 7. Identification of Signals, Points & Track Circuits - In Part 3 8. Examples - In Part 3 3. POSITIONING OF RUNNING SIGNALS If we are starting with a blank track layout, we need a logical method of setting out the signals in order to produce a signalling plan. As running signals must fulfil the needs of both headway and braking distance, it is usual to position running signals first. Shunting and subsidiary signals are dealt with after the running signals have been placed. 3.1. Headway Constraints First plot the positions of the running signals on the principal running lines. Then proceed to lower priority lines in order of importance. It is often difficult to decide where to start. If a station exists on the layout it is usual to start by plotting platform starting signals, and then continue by plotting the signals in rear and in advance to be within the tolerance of minimum separation (for braking distance) and maximum separation (for headway) for the type of signalling to be employed. It is also desirable to position signals close to facing points at junctions so that the driver does not have a long distance to travel to the turnout. This will minimise delay to traffic and reduce the possibility of signals being misread. Service braking distance must be provided from the first cautionary aspect to the signal at danger in every case. The maximum distance from the first caution to the red is set by the headway requirement. Ensure that any large excess over service braking distance is within acceptable limits. 3.2. Other Constraints In addition to headways, other constraints must be borne in mind. These include junctions, further stations, tunnels, viaducts and level crossings. There may be many places where it is required to stop a train for operating reasons. The engineer will have very little choice in the position of these signals. Conversely there are also many places where it is highly undesirable to stop a train and signals must be positioned to avoid these. Other factors to consider are the visibility of the signal to the driver, the practicality of installing a signal at a particular site and ease of access for maintenance. Signal A has been placed at a distance greater than service braking distance from B because it would have otherwise stood on the points 201. If a line is generally signalled with 2 or 3 aspect signals and it seems necessary to have two signals spaced closer together than service braking distance, remember that the next signal in rear must be capable of displaying a medium aspect to give an earlier warning of the need to stop. If a suitable signal does not already exist, an additional signal will have to be introduced. A signal should be positioned so that a train stopped at that signal does not: Stand in a tunnel (wholly or partially). This would not apply to underground passenger railways where special provision has been made in the design of the trains and/or tunnels to ensure the safe evacuation of passengers in an emergency. Stand on a viaduct, unless special provision has been made for safe evacuation of passengers and/or access by emergency services. Foul a junction. Stand partially at a platform (unless the passenger doors can be kept closed by the train crew). There are occasions when, due to local circumstances, these requirements cannot be wholly met but every effort should be made to comply. Additional considerations will apply on electrified lines to ensure that trains are not brought to a stand in neutral sections or gaps in the conductor rail. If possible, heavy freight trains should not be stopped on steep falling or rising gradients, especially in combination with sharp curves. 3.3. Examples of Standage Constraints Ensure maximum length train can stand in platform. Ensure that maximum length train can stand clear of junction fouling point to allow other movements to pass behind it. Signals on adjacent running lines should as far as possible be placed opposite each other. This minimises the chance of drivers reading the wrong signal. It also simplifies the design and installation of power supply and location equipment. 4. TYPES OF SIGNAL Running signals may be divided into three general groups according to the form of control exercised by the signalman. 4.1. Automatic Signals Automatic signals are designed to operate only according to the presence of trains on the track circuits ahead. The signalman does not have to set the route for each train. Usually he will not have any facility to set routes but may in certain circumstances be provided with a switch or button to replace the signal to danger in emergency. A signal may be shown as automatic if all the following conditions can be met:- No points in the route to the next signal. No points in the overlap beyond the next signal (Exceptionally, simple facing points may be allowed). No directly opposing routes within the route or the overlap. No ground frames, controlled level crossings or other equipment with which the signal must be interlocked. The normal aspect of an automatic signal (i.e. the aspect shown when there are no trains present) will generally be a proceed aspect. As the signalman is not directly concerned with the operation of automatic signals, some other facility may be required to stop trains in an emergency. If it is decided that this facility is required, options available are:- Individual replacement switches or buttons on the signalman's panel (for all automatic signals or selected signals only) Grouped replacement switches or buttons. Each one can replace a group of consecutive signals on the same line to danger) A replacement switch mounted on or near the signal, usually requiring a special key to operate. Earlier British practice was to provide replacement facilities on automatic signals in any of the following circumstances:- Controlling the entrance to a section in which a level crossing equipped for automatic operation is situated. Controlling the entrance to a section in which a tunnel or viaduct is situated. Controlling the entrance to a section in which an electrical traction break is situated. If not otherwise required, on at least every fifth signal in any section of automatic signals and at any other signal where required for operating purposes. Since the accident at Clapham Junction in 1988, this policy has now changed to one of providing some form of emergency replacement for all automatic signals. On all new installations, this is by means of an individual button on the panel. Automatic signals must be recognisable as such to the driver. In the event of a signal failure and loss of communication with the signal box at the same time, the driver can then pass the signal at danger and proceed at extreme caution to the next signal (prepared to stop short of any obstruction). Some railways identify their automatic signals by a different style or colour of identification plate. SRA (TfNSW) practice is to offset the lower signal lights (or marker light) 204mm to the right of the upper signal lights. If clearance constraints make this impossible, and the lights must be in line, a separate plate with a white letter A on a black background is provided. 4.2. Semi-Automatic Signals In many areas signal boxes or local ground frames are provided which provide access to sidings or are not continuously manned. Signals must be provided which permit through traffic to operate when the frame or signal box is unmanned. These signals have to operate automatically for much of the time but must also be capable of control by the local operator. Semi-automatic signals are provided in this case. When the signal box is switched out or the local frame is locked normal, the signal functions as an automatic signal. Once again, the driver must be aware of the correct action to take in the event of failure. A distinct identification plate is used on some railways. The driver must then confirm that the ground frame or local signal box is not in use before treating it as an automatic signal. SRA (TfNSW) practice is to provide an internally illuminated "A" indication below the semi-automatic signal (which otherwise has the same appearance as a controlled signal). The "A" is illuminated only when the signal is working automatically. It may also be necessary to divert any signal post telephone circuit to another supervising signal box when the local control is not in use. 4.3. Controlled Signals Any signal other than those described above will be a controlled signal. It must be controlled from a signal-box (other than by Emergency Replacement). It will usually require a lever, switch, button, key or plunger to be operated for each movement. Controls may be provided to allow a controlled signal to operate automatically (i.e. without re-setting the route for each movement). It must be decided whether this should be apparent to the driver. BR practice is not to provide any distinct identification on the signal. The driver will always treat it as a controlled signal, even when working automatically. This is because it is unsafe to adopt any form of "stop and proceed" working where there are points ahead of the signal which may be moved. A signal will normally be "Controlled" if there are points and/or conflicting routes in the route or overlap. 4.4. Signal Identification Plates All stop signals and/or all signals provided with a signal post telephone should be provided with an identification plate. As stated earlier this may be of a different style according to the type of signal (controlled, automatic or semi-automatic). When talking to the signalman, the driver should be able to identify where he is, even if this information is also shown on the signalman's telephone equipment. Other signals (e.g. distants and repeaters) may also be identified for maintenance requirements. Some controlled signals in particular positions may need to be specially identified. For example, the "accepting" signal on approaching an interlocking from a section of automatic signalling is specially plated to remind the driver that he is no longer under the control of automatic signals. 4.5. Signal Aspects Having decided on the necessary position of each running signal on our plan, we must now ensure that we depict each signal to show the correct combination of signal aspects. This will be governed by two main factors:- the type and distance of the next signals ahead; whether it is necessary to give warning to the driver to stop. whether the signal or the next signal ahead is a junction signal. Any signal that has more than one running route is a junction signal and, as such, must provide the driver with the appropriate turnout aspect and/or route indication when required. For all junction signals on the signalling plan, an adjacent box should be included showing the signal number, description of each route, its exit signal, and the route indication (if any) displayed. 4.5.1. Aspects for Through Running (Single light) In single light signalling areas a stop signal must at minimum have a head with a red and a green light, with a marker light below. If the next signal ahead is a stop signal at greater than braking distance, a caution aspect must be provided. If the signal ahead is at less than braking distance and the following signal section is also shorter than braking distance, the signal must display also a medium (pulsating yellow) aspect. The signalling plan will show a letter "P" against the signal to indicate that it displays a pulsating yellow aspect. If there is braking distance between the next two signals, the medium aspect is not required. Single light signal heads capable of showing only two lights should be shown on the signalling plan with the actual colours required (R for red, Y for yellow and G for green). Distant signals will usually display caution and clear only. If there is inadequate braking between the next two adjacent stop signals ahead, a medium aspect (pulsating yellow) will also be required. 4.5.2. Aspects for Through Running (Double light) All double light signals have two signal heads, one below the other. The top head is the stop signal and the lower head is the distant for the signal(s) ahead. If the signal is not a junction signal, the upper head will be red & green only. The form of the lower head will depend on whether three or four aspects are required. The, rules are exactly the same as for double light but the aspects are different. The lower (distant) signal head will display red and green only for 3 aspect signalling, red,yellow and green for 4 aspect. The provision of low speed aspects is dealt with elsewhere in these notes. 4.5.3. Junction Signalling (Single Light) The indication of a route diverging from the main line will be by either a turnout signal (maximum of one route to left and/or right) or a multi—lamp route indicator in conjunction with the main aspect (more than one route to left or right). When the turnout signal is used, the main signal remains at red. All turnout signals must be capable of displaying a caution (three steady yellow lights) and may display a medium turnout aspect (three pulsating yellow lights) when the signal ahead is showing a proceed aspect. When a route indicator is used, the main signal will display a steady or pulsating yellow (according to the next aspect ahead) and the appropriate route indication will be displayed. Clear signals are not given through turnouts. 4.5.4. Junction Signalling (Double Light) The indication of a turnout on a double light signal is by a yellow in the upper signal head. All junction signals therefore require an upper signal head with red, yellow and green lights. The lower signal will still indicate the state of the signal ahead, red if the first signal ahead past the junction is at stop, yellow if the next signal displays a proceed aspect. If a junction signal is set for the turnout (either caution or medium) the previous signal will display a medium aspect, never a clear. Route indicators may be used where two or more turnout routes exist. 4.5.5 Low Speed Signals Section 2.5 has dealt with the main situations in which low-speed signals are used. Firstly, check whether a conditionally cleared caution could adequately fulfil the operating requirements (the overlap should be at least 100m for a conditional caution). Low speed signals are not necessary for normal through running. They should, however be considered in cases where reduced overlaps can aid the regulation of traffic and/or headway for stopping trains and a conditional caution is not appropriate. It is also recommended to provide low speed signals through any area where a track circuit would otherwise control three or more running signals. This is to localise the effect of failures by restricting the number of handsignalmen required in the event of track circuit failures. 4.6 Shunting and Subsidiary Signals Having catered for all running moves, we must now provide for shunting and other non-running movements. Before starting to place signals on the plan, make sure you know exactly what movements are required. Any movements which are not signalled will have to be authorised by handsignals. Handsignalled movements on lines where the majority of trains are properly signalled are disruptive to normal traffic and allow the possibility of human error. Conversely, signals provided for movements which are never used are an additional and unnecessary initial cost to the project. They also represent a continuing maintenance cost and a potential source of additional failures. Although the terms are often used interchangeably, there is a distinction between shunting and subsidiary signals. Subsidiary signals are part of a main running signal. Shunting signals are independent. Subsidiary signals therefore only need a proceed aspect - the main signal provides the stop aspect. Shunting signals must display both stop and proceed aspects. Subsidiary signals can broadly be divided into the following functions:- To shunt from a running line (in the normal direction of traffic) into a siding. To move forward from a running signal into an occupied section. A main route to the same destination may already exist. The provision of both main and shunt routes could assist operations in critical areas during track circuit failures. On a multiple track line, to shunt on to another line in the opposite direction to normal traffic. To move forward to a shunt signal facing the normal direction of traffic. On absolute block lines, to permit a train to pass the starting signal at danger for shunting purposes only. The shunting movement must return behind the starting signal Unless the movement is on to a section of line which is not fully signalled there will need to be an exit signal to limit the extent of the movement. Independent shunting signals can broadly be divided into the following functions:- To shunt between running lines from a position where no main signal is provided. To enter, leave or shunt between sidings. To shunt in the opposite direction to normal traffic. To limit the extent of any shunting movement (including "shunting limit" boards). The diagram below shows examples of some common applications of shunting and subsidiary signals. 4.6.1. Calling-on and Subsidiary Shunting Signals This will be provided where a movement must be authorised to pass a main signal at danger to enter a section which is or may be occupied. An example would be the coupling of two portions of a train in a station platform. Signal 4 on the diagram has a subsidiary provided for this purpose. The usual aspect displayed is a miniature yellow. Some older double light signals display an internally illuminated "CO". 4.6.2. Dead End Signal This will be provided where a movement must be authorised to pass a main signal at danger to enter a dead end siding via a facing turnout from the main line. It displays a miniature yellow light and is offset to one side of the signal post (according to the direction of the movement). Signal 5 is an example of this. 4.6.3. Shunt Ahead Signal Used to shunt ahead of the starting signal and mounted below the main signal. Used on single light signalling only and displays a pulsating miniature yellow light. Signal 7 is provided with a shunt ahead signal to enable long trains to draw forward past the signal before shunting back over the crossover. This type of signal will normally be found in single light signalling areas only. 4.6.4. Dwarf and Position Light Shunt Signals Shunting signals normally have two aspects - stop and proceed. The stop aspect is two red lights and the proceed aspect is a single yellow light. This instructs the driver to proceed at caution. It does not guarantee that the line ahead is clear. SRA (TfNSW) uses both dwarf and position light shunting signals. The difference between the two types of signal is the orientation of the lights. The choice of signal type will depend mainly on lineside clearances. On a position light signal, the two red lights are side by side, the yellow light is above. A dwarf signal has the three lights vertically arranged; the red lights are at the top and bottom with the yellow light between. Route indications are provided where required, particularly where wrong line movements are signalled. 4.6.5. Shunting Limit Boards Effectively a shunting signal fixed at danger, a shunting limit signal faces in the opposite direction to normal traffic and is used to limit the extent of a wrong line shunting movement. An example is shown on the down line. This would enable trains to shunt out of No. 1 siding on to the down line before proceeding forward. Without the board there would be no signal to prevent the wrong line movement continuing indefinitely on the down line. 4.6.6. Facing (or Preset) Shunt Signals Occasionally, shunting movements in the normal direction may be required to start from a position where a running signal is not provided. Such a shunt signal must therefore be passed by normal running movements. To avoid the driver seeing a yellow light after he has just passed a main signal showing clear (and possibly braking unnecessarily) "facing" shunt signals are provided with an additional green light to show clear when the previous main route is set past the shunt signal and the signal is showing clear. Signal 55 is a facing shunt signal. 4.6.7 Point Indicators Point indicators should be provided on any points (whether facing, trailing or catch points) where the driver is responsible for observing the position of the points before proceeding over them. The points will usually be hand worked, as shown in siding 1 on the example. Where regular shunting takes place without the need for the signalman to set the route for every move, point indicators will be displayed. These will be selected by a separate button on the signalman's panel. This is preferred to providing two shunt signals with opposing locking removed as it avoids the possibility of two trains approaching each other both under proceed aspects. 4.7 Trainstops SRA (TfNSW) provides trainstops on most of the Sydney metropolitan area. Double light signalling is normally provided. All electric multiple unit trains are provided with tripcock equipment which will apply the brakes if a train passes a raised trainstop. The trainstops are provided at each main stop signal and in certain other locations (e.g. exits from depots and sidings) to prevent a rear end collision with another train. If the signal is at danger, the trainstop will be raised. A train irregularly passing a signal at danger will be tripped and brought to a stand within the length of the overlap. Where trainstops are to be used, the engineer must ensure that the length of each overlap is adequate for emergency braking at the highest speed at which a train is likely to pass the signal. Obviously the trainstop cannot ensure total safety if all trains are not fitted but it can make a major contribution to safety in areas where trains regularly run at close headways. An important part of the preparation of the signalling plan is therefore to decide where trainstops are to be positioned. This is closely associated with the calculation of overlaps. It may often be more important to accurately position the overlap for track circuit clearance purposes, then work back to the position of the signal and the trainstop. A low speed signal tells the driver that there is little or no overlap beyond the exit signal. Running speeds will be low (normally less than 35km/h). The low speed overlap will be based on the passing speed of the low speed signal. However, the driver could fail to brake, or even accelerate after he has passed the low speed signal. This would leave an inadequate low speed overlap. Intermediate trainstops are therefore often provided between a low speed signal and the next signal, to be lowered only after sufficient time has elapsed for the train to have reached the trainstop at or below the correct speed. The following general rules therefore apply to the positioning of trainstops:- A trainstop is required at all stop signals. It must always be on the same side of the line. SRA (TfNSW) provides trainstops on the left hand side, London Underground and British Rail use the right hand side of the track. Additional trainstops may be required on the approach to stop signals with a reduced overlap where a speed reduction has already been enforced at a previous signal. As an example, a low speed signal reading into a station platform could have a low speed aspect to allow early entry of following trains. The overlap associated with this may be 100 metres or less, even reducing almost to zero. To ensure that a train does not accelerate to a speed which would render the overlap inadequate, an additional trainstop is provided on the approach to the exit signal after the low speed signal. The lowering of this trainstop is timed to trip a train which is running above the permitted speed. The positioning of trainstops therefore has to take account of the braking and acceleration characteristics of the train and the length of the overlap. The calculation can become very complex so a simple example is used here to illustrate the possibilities. In the following diagram, the two stop signals are 200 metres apart. For headway and/or junction clearance purposes, it has been decided that only a 50 metre overlap is available beyond the second signal. We will assume that the train passes the first signal, displaying a low speed aspect at 27 km/h or less (otherwise it would have been tripped). This example will assume a typical service braking rate of 0.9 m/s 2 , an emergency braking rate of 1.4 m/s 2 and an acceleration rate of 0.55m/s 2 . These are typical of those which have been used for SRA (TfNSW) signalling for electric multiple units although the actual performance of the trains which will use a line must always be confirmed, and gradients taken into consideration. The train should under normal circumstances brake to a stand at signal 2 along or below curve A. The trainstops should ensure that the train will come to a stand within the overlap, should the driver fail to take the correct action to control his train. There are various possibilities which may arise. The signal engineer must decide whether to allow fully for all of these or whether circumstances will permit some relaxation. After passing signal 1 at the permitted speed (27 km/h in this example) the driver could totally fail to brake. Even worse, he could accelerate after passing signal 1. We could assume either no acceleration, acceleration due to gradient only or acceleration under full power. Whichever is chosen, the overlap should be greater than the emergency braking distance from signal 2. If this is not the case, an intermediate trainstop (labelled ITS) must be provided which should be timed to lower just before the train reaches it on a normal service braking curve (point X on the diagram). Curve D shows the effect of this trainstop on a train accelerating under full power. With the intermediate trainstop having been passed at the correct speed (lowered before the arrival of the train), the train could then accelerate at full power towards signal 2. In this case the trainstop at the signal will ensure the train stops within the overlap. Curve B shows the likely speed profile of the train in this situation. Even with these safeguards it is possible that a train could stand just past signal 1 on the timing track circuit for the intermediate trainstop. The trainstop would lower after the prescribed time interval and the train could then accelerate under full power towards signal 2 without the protection of the intermediate trainstop. It will be seen from curve C that the train will overshoot the overlap, passing the overlap joint at up to 9 m/s. To overcome this, the length of the timing track circuit for the intermediate trainstop must be limited such that an accelerating train could pass signal 2 at a speed no higher than that possible on curve B. Alternatively, an additional intermediate trainstop could be provided to check the train speed at an earlier point. SRA (TfNSW) practice in open (i.e. above ground) areas is to allow some margin for possible acceleration but not the deliberate full acceleration of curve C. In tunnel sections where the driver's perception of speed and distance may be affected, the positioning of trainstops and their associated timing track circuits should cater for all possibilities. It should be noted that due to the introduction of newer trains with better acceleration characteristics, the protection provided by certain older sections of signalling is now reduced. It will still protect against most normal occurences other than the deliberately malicious driver intent on overriding the protection of the signalling equipment. Typical distances for open areas are as follows:- For following trains and overlaps less than 50 metres, the intermediate trainstop is positioned 100 metres from the end of the overlap with a timing track circuit between 80 and 220 metres in length. For overlaps clear of fouling movements, the overlap should be at least 100 metres and the intermediate trainstop 200 metres from the fouling point. In addition, any previous signal whose full overlap extends beyond the fouling point should be conditionally cleared to caution. This arrangement is not recommended where signal spacing exceeds 500 metres. TO BE CONTINUED - SIGNALLING BOOK | CHAPTER 3 | PART 3...........
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CH3 | SIGNALLING A LAYOUT | PART 3
CONTINUED FROM - SIGNALLING BOOK | CHAPTER 3 | PART 2 SIGNALLING BOOK | CHAPTER 3 | PART 3 CONTENTS 1. Introduction - In Part 1 2. Headway - In Part 1 3. Positioning of Running Signals - In Part 2 4. Types of Signal - In Part 2 5. Points and Crossings - In Part 3 6. Track Circuits - In Part 3 7. Identification of Signals, Points & Track Circuits - In Part 3 8. Examples - In Part 3 5. POINTS AND CROSSINGS Although, in general, the siting of points and crossings on an existing railway will be dictated by permanent way design considerations, it is left to the Signal Engineer to determine the operation of the points. Furthermore, the Signal Engineer may require additional trapping protection to be provided on occasions and such cases must be referred back to the Permanent Way Engineer or other responsible engineer. In the case of combined track remodelling and resignalling projects, it is sometimes possible to provide simpler or improved signalling controls by minor alterations to the track layout. Close co-operation between the Signal Engineer and the Permanent Way Engineer is essential if the optimum results are to be achieved. 5.1. Position and Numbering of Points Any set of points will be defined as lying in its Normal position for one route and its Reverse position for the other route. The Normal position of the points will be shown on the signalling plan as follows:- A similar convention applies to switched diamond crossings, if used. Points should be numbered in such a way that any point ends required to work simultaneously carry the same number. To localise failures, it is not advisable to number more than two ends to work together. In addition, Solid State Interlocking (SSI) equipment is normally only configured to operate single and double ended points, although in certain circumstances three ends can be accommodated. For control purposes, each end has to be identified separately (A or B) but this may not need to be shown on the signalling plan. A convention must be determined for identifying A and B ends (e.g. A end nearest control centre or A end at lowest reference distance etc.) and strictly observed. On TfNSW network a down train will meet the A end first. 5.2 Ground Frames Ground frames control infrequently used points, usually outside interlocking areas. Although referred to as ground frames, they may equally well be locally operated control panels. In its most common form the ground frame consists of just 2 levers, the point lever and a release lever (which will also work the F.P.L. if the points are normally facing). Movements over the points during shunting are usually controlled by handsignal, although extra levers may be provided to control or slot signals which the train must pass during shunting. Note the use of separate releases where the ground frame controls more than one function. Instead of providing an electric lock on the release lever, a separate key is electrically released when the signalman operates the release button. This key is then used to release the ground frame release lever. It remains captive until the ground frame is normalised and can then be returned to the instrument to give back the release. 5.3 Trapping Protection It may be necessary to request trap points (normally known as catch points on TfNSW) to be provided at certain locations: At the exit from sidings, where they lead on to running lines, catch or trap points must be provided to prevent an unattended vehicle running away or a shunting movement overrunning and fouling the running line. Where a full overlap cannot be obtained and movements are required to closely approach a converging junction, catch or trap points leading away from the running line can be used as an overrun in place of the normal overlap. On railways where a distinction is made between passenger and non-passenger lines, trap points may be used where the non-passenger line joins the passenger line. Where trap/catch points occur in track circuited lines many railways employ a track circuit interrupter to ensure a derailed vehicle which is still fouling the track, although not standing on the rails, remains detected. The track circuit interrupter is normally insulated from the rail on which it is mounted and bonded in series with the opposite rail. 6. TRACK CIRCUITS Track circuits shall be provided in a manner which permits maximum flexibility with minimum expense and complexity. 6.1 Overlaps Running signals should, in general, be provided with separate berth and overlap track circuits, the berth track circuit terminating immediately beyond each signal. This will ensure the signal is replaced to danger at the earliest opportunity after the train passes. Where more than one overlap is required, a joint must be provided at the end of each overlap. Calculation of overlaps has already been covered in the earlier sections dealing with the positioning of signals and trainstops. However, old TfNSW practice on overlaps is summarised below. 6.1.1. Where Trainstops Are Not Fitted The overlap is a margin to allow for braking errors. There is no positive means of stopping the train if a driver completely misses or misreads a signal. As such it is an approximate distance based on experience, rather than one which has been calculated on any scientific principle. The standard SRA overlap is 500 metres. This may be smaller or greater than the actual braking distance. Where speeds are low, this is sometimes reduced. Recommended overlap lengths are:- 6.1.2 Sydney Metropolitan Area (Open Sections) If trainstops are fitted,(recently ATP rollout has taken place which will remove the significance of mechanical trainstop) the overlap must be based on emergency braking distance for the prevailing speeds and gradients. The following example shows how this may be calculated. To simplify the calculation when dealing with gradients it is often easier to express the braking rate as a percentage of the acceleration due to gravity (g = 9.8m/s 2 ). Braking distance = v 2 /2a (where a = braking rate, v = train speed) = 100v 2 /2g(%B + %G) %B = Braking rate as a percentage of g %G = Gradient as a percentage (down gradients negative) If the line speed is 25 m/s, the braking rate is 10% and the gradient is 1% down, the emergency braking distance and hence the overlap will be:- 100 x 25 2 / 2 x 9.8 x (10 - 1) = 62500 / 176.4 This would give a minimum overlap of 354 metres. This would probably have to be increased by a suitable margin to allow for less than 100% braking performance (e.g. some brakes isolated, wet or greasy rails, delay time for brake application). Where available, braking tables or curves should be used. If the full overlap is foul of junctions or station platforms, a reduced overlap should be considered. The train speed would have to be suitably reduced by a low speed or conditional caution aspect at the previous signal. 6.2 For Points and Crossings The positioning of track circuit joints to prove clearance will depend on the dimensions of the rolling stock in use. One must first determine the difference between the maximum vehicle width and the width of the widest vehicle in service. The position must then be found where the rails leading away from the crossing are at least this distance apart (normally adding a small safety margin). At this point, the extreme ends of vehicles on the adjacent tracks will not be foul of each other. Measuring away from this position, the joint must be located at a distance greater than the maximum end overhang of any vehicle. This is obtained by measuring from the centre of the outer axle to the extreme end of the vehicle. Track circuits should allow maximum flexibility of use of the layout. In particular, where the track layout permits parallel moves, the signalling must not prevent them. Joints should be positioned to achieve the earliest release of points after the passage of a train consistent with safety, economy and practicality of installation. Example 1 Joint A allows simultaneous moves over both ends of crossover normal. Joint B allows points to be moved as soon as train leaves points. Joints C, at clearance point, allow movements across crossover with Tracks X and Y occupied. It is common on plans to place joints C opposite the tips of the points. Example 2 Joints A allows parallel moves. Joints B allow points to be freed as soon as junction cleared. Joints C are set back at clearance point. These may also be the overlap joints for signals approaching the junction. Joint D will be dependent on factors other than the requirements for operation of the junction, eg. the position of the protecting signal. 7. NUMBERING OF SIGNALS, POINTS AND TRACK CIRCUITS To enable all signalling controls to be specified, each signalling function must be uniquely identified. It aids design, testing and fault location if this is done in a logical and orderly manner. In particular, confusion is avoided if different types of functions are numbered in different number or letter series. The main functions which need to be numbered are:- Main Signals Shunt Signals Points Track Circuits Ground Frame & other releases Separate number series should be provided for each type of function (points, signals etc.). Main and shunt signals may be numbered in the same or separate series. Lines for each direction of traffic are normally designated UP and DOWN. Signals reading in the Down direction normally carry odd numbers with the lowest number at the Up end of the control area. Signals reading in the Up direction normally carry even numbers, again with the lowest number at the Up end. Points will be numbered with the lowest number at the Up end of the control area. Where possible suitable gaps should be left in the numbering sequences in anticipation of future alteration. Distinct branches should be numbered in separate series. Historically, several different conventions have been used for identification of track circuits. Each has advantages and disadvantages. One common method is to use a simple numbering sequence. The disadvantage of numbers is that, on a large installation, very large numbers or duplicate number sequences need to be used (with greater risk of errors in design and testing). Another alternative which has been used is to number track circuits based on the distance along the line. This results in track circuits in one locality having long and very similar numbers. Again confusion and errors may result. TfNSW uses a system based on the signal numbers. The first track past signal 5 would be 5A, the next 5B and so on. The suffix E is not normally used. The BR standard is now to use letters. Each track circuit indicated to the signalman should be identified using two capital letters, arranged alphabetically in a logical sequence. Letters I and O are not used. Where a number of track circuit sections have a common indication they should have the same identity plus an individual suffix number, eg. AA1, AA2 etc. This arrangement is simple but does not give any indication of the relative locations of tracks and signals. 8. EXAMPLES A few examples are now given of some of the more commonly found track layouts and suggested arrangement of signals. They do not cover all situations. In practice, different requirements will conflict. The signal engineer must resolve these conflicts in the most effective and economic manner 8.1 Junctions The logical arrangement at a junction is for the protecting signal to be as close to the junction as possible. For diverging movements this ensures that trains are not checked too far from the junction, while for converging movements it reduces the chances of trains being checked due to conflicting moves on the junction. The signal next in rear of the junction cannot be cleared unless the section is clear up to the junction signal, and the overlap beyond. It is preferable that the overlap is not fouled by conflicting moves, so ideally the signals protecting the junction should be placed overlap distance in rear of the junction. If large overlap distances make this impractical, a reduced overlap clear of the junction should be considered. In this case the signal in rear should have its full overlap clear of the junction. 8.2 Station Platform with Loop It is usually desirable for headway reasons to site a signal at the end of the platform. This provides a platform starting signal and also protection for any level crossing at this point. In situations where there is no platform starting signal, there is a risk that a station stop will divert the driver's attention sufficiently for him to forget the aspect displayed by the previous signal. After restarting his train, he could approach the next signal at an unsafe speed. In the above example, signal A is located at full S.B.D. from signals B & C. For main line running this is satisfactory, but for a move into the loop the time taken for the train to slow to 25 km/h over the crossover may adversely affect the headways. A better arrangement is shown below : Signal A is moved closer to the turnout. If this results in inadequate braking distance from signal A to signals B & C, signal D must be a 4 Aspect signal. Signal A would only need to be 4 aspect if the signal ahead of C was less than braking distance. 8.3 Terminal Stations The station throat track capacity must be at least double that of the approaching line. This is because some arriving and departing movements will completely block all other routes. The signal reading into the platforms should be as close as possible to the plaforms. If this signal does not have an overlap clear of points, the signal in rear should do so to allow trains to approach unrestricted. Signal spacing on the approach to the station should be as close as possible consistent with standing room and headway requirements. The first signal leaving the station should be as close as possible (whilst retaining necessary train standage clear of the pointwork), to allow the best possible aspect on the platform starting signals. The standage requirement may have to be reduced to maintain adequate line capacity on the departing line. The platform entry signal 3 exhibits stop and caution aspects only for running moves. Buffer stops are equivalent to a signal permanently at stop. Subsidiary shunt signals are provided to enable trains or locomotives to enter occupied platforms. If wrong line shunting is required, a shunt limit board must be provided at a position which permits adequate standing.
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CH25 | Modern Train Control Systems
1. ‘Communication’ Based Train Control Systems Train control has advanced from conventional fixed block system to moving block technology during the last three decades. Many highly dense cities realized the need for moving commuters in a large scale, especially during peak hours and frequent train operation under 3min or less is inevitable.. This is often referred as Headway among the Technical diaspora. Headway can be put it in simple words “The theoretical time separation between two Trains travelling in the same direction on the same track. It is calculated from the time the head-end of the leading Train passes a given reference point to the time the head-end of the following Train passes the same reference point” It is possible to achieve tight 3 min headway with the help of automatic signalling ,however this is possible at the expense of large amount of signalling asset maintained and less safety .Not forgetting the fact that Automatic train Protection system can be implemented for safety enforcement but large number of standalone ATP system are getting obsolete (Example :Hitachi L10000) within next decade .large scale greenhouse gas emission reduction also boosted the requirement for large scale passenger and freight movement with utmost safety and energy efficiency .Thanks to Paris agreement often referred as Paris Accords adopted in 2015 .Headway requirements are relatively low for Mainline and freight however the need for enhanced safety and efficiency are ever growing .There are pros and cons for the most popular train control systems . 2. Communication Based Train control System (CBTC) As per Institute of Electrical & Electronics Engineers (IEEE 1474 ) CBTC is a “A continuous automatic train control system utilizing high-resolution train location determination, independent of track circuits; continuous, high capacity, bidirectional train-to-wayside data communications; and trainborne and wayside processors capable of implementing vital functions” They do possess the following characteristics Determination of train location, to a high degree of precision, independent of track circuits. A geographically continuous train-to-wayside and wayside-to-train data communications network to permit the transfer of significantly more control and status information than is possible with conventional systems. Wayside and train-borne vital processors to process the train status and control data and provide continuous automatic train protection (ATP). Automatic train operation (ATO) and automatic train supervision (ATS) functions can also be provided, as required by the particular application It is not necessary that train shall be in unattended mode (driverless) to be identified as a CBTC. Any system that performs the above-mentioned functionality can be identified as a Communication Based Train Control System. Even though IEEE definition didn’t expect the need for a secondary train detection system, majority of the suburban network make use of a secondary Train detection system such as Track circuits or Axle counter for the degraded mode of operation in case the complete loss of communication. CBTC is mentioned as the train control system used in urban mobility per IEEE definition rest of this article ,even though European Train Control System or Positive train Control System also based on wayside to train communication. 3. Utilisation of Moving Block in CBTC Conventional railway system works on Fixed block system where each blocks are defined and separated with safe distance(braking distance) with safety margin and only one train possible in a longer block at a time and the leading train has to clear the block before following train can occupy the block.Where as in moving block train as a “moving block “ maintain safe distance based on braking curve with a safety margin .Refer below figure to identify the difference. There is an article with comparison is posted in RailFactor ,and detailed comparison between traditional fixed block and moving block is out of scope for this article. Fig 1: Traditional Fixed Block System Fig 2: Moving Block System 4. European Train Control System (ETCS) Evolved from the need for economic integration of the European Union for inter operation of their Trains .There were different signalling principles ,’non standardized ‘ signalling equipment existed in conventional system giving nightmare to operate between boarders with multiple train borne systems(Turn off and Turn On ) to cross the boarder ,even crew were needed to change during boarder crossing irrespective of same gauges between boarder. A technical specification for interoperability was embraced by the European parliament and the council of Union on the interoperability of the European rail system in accordance with the legislative procedure. Major Rail System providers from Europe known as Unisig companies under European Union Agency for Railways jointly produced the rules described in ‘Subsets” .So far ETCS has five levels (Application Level 0, Level NTC ,Level 1 ,Level 2 and Level 3) as described in SUBSET-026-2 . Global System for Mobile Communications-Railway(GSM-R) is the mode of data transmission between train and regulation centres (Wayside and Train borne) for ETCS Level 2 . Considering the fear that next 10 years will phase out the GSM-R and various ETCS Level 2 implementation planned will impact.It could be implemented with Long Term Evolution (LTE) digital Radio System.LTE is normally regarded as 4G protocol and the Future Railway Mobile Communication System (FRMCS) is considering to move something similar to 5G. Note :- European Rail Transport Management System ( ERTMS) include ETCS +GSM-R 5. Positive Train Control System (PTC) A communication-based Train monitoring and control system with a train protection system originated for the North America. As defined by AREMA (The American Railway Engineering and Maintenance of Way Association ) a Positive Train Control System has the primary characteristics of Safe Train Separation to avoid train collision ,Line speed Enforcement ,Temporary Speed Restrictions ,Rail worker safety and Blind spot monitoring .As published in Digital Trends PTC work by ” combining radio, cellular and GPS technology with railway signals to allow trains to identify their locations relative to other trains on the track “Concept wise” in a way PTC and ETCS are same. 6. East Japan Train Control (EJTC) Classified as four levels from Level 0 to Level 3 .Level 0 make use of an Automatic Train Stop device (ATS-S) to prevent collision .This has been replaced with Automatic Train Stop device Pattern Type (ATS-P) in Level 1 addressing the weakness in ATS-S .New development with EJTC Level 3 make use of radio transmission and train itself detect its location and communicate with other trains .EJTC Level 3 is named as Advanced Train Administration and Communication System ( ATACS ) using Autonomous Decentralized System (ADS) Technology. ADS technology is considered as most innovative modern technology for smart trains by Dr. Kinji Mori from Japan. This decentralized system composed of modules designed to operate independently capable of interacting each other to achieve the over all goal of the system. This innovative design enables the system to continuously function even when the event of components (modules ) failures .This plug and play module also enable to replace the failed module while the overall system is still operational. Refer Figure 3 for message passing in an autonomous decentralized system. Fig 3: Architecture for Autonomous Decentralized System ADS is a decoupled architecture where each subsystem communicates by message passing using shared data fields .Uniqueness of ADS system is that it doesn’t contain a central operating system or coordinator. Instead of that each subsystem manages its own functionality and coordination with other subsystems. When a subsystem needs to interact with other subsystems it broadcasts the shared data fields containing the request to all other subsystems. This broadcast does not include the identification or address of any other subsystem. Rather the other subsystems will, depending on their purpose and function, receive the broadcast message and make their own determination on what need to be done with it or ignore. Data transmission can be carried out by Enterprise Service Bus (ESB) .It operates in the autonomous decentralized system 7. Chinese Train Control System(CTCS) Largely based on ETCS except CTCS has Six Levels. China has a large rail network constitutes of several types of rail network such as High Speed, conventional, passenger and freights and realized the dire need for standardization, that is the basis for CTCS. Like ETCS ,CTCS also make use of balises on CTCS Level 2 and Level 3 .However Wuhan -Guangzhou high speed line uses ETCS Level 2 .China From year 2016 onwards all metro lines in China are required to utilise LTE as the basis of their communications network. 8. Definition Standards for CBTC and ETCS This section depicts the major requirement specification for both the technology .Its recommended to refer these standards . In further chapters will cover case study and standard references for subsystems for each elements to build a ETCS and CBTC systems . ETCS (All Levels) -ERA UNISIG EEIG ERTMS USERS GROUP SUBSET-026 - System Requirements Specification SUBSET -027 - FIS Juridical Recording SUBSET -034 - Train Interface FIS SUBSET-035 - Specific Transmission Module FFFIS SUBSET-036 - FFFIS for Eurobalise SUBSET-037 - EuroRadio FIS SUBSET-038 - Offline Key Management FIS SUBSET-039 - FIS for RBC/RBC handover SUBSET-044 - FFFIS for Euroloop SUBSET-047 - Trackside-Trainborne FIS for Radio infill SUBSET-056 - STM FFFIS Safe time layer SUBSET-057 - STM FFFIS Safe link layer SUBSET-058 - FFFIS STM Application layer SUNSET-098 - RBC-RBC Safe Communication Interface SUBSET-100 - Interface "G" Specification SUBSET-101 - Interface "K" Specification SUBSET 114 - KMC-ETCS Entity Off-line KM FIS SUBSET-137 - On-line Key Management FFFIS ERA_ERTMS_015560 - ETCS Driver Machine Interface CBTC IEC 62290-1:2014- Railway applications - Urban guided transport management and command/control systems - Part 1: System principles and fundamental concepts. IEC 62290-2:2014-Railway applications - Urban guided transport management and command/control systems - Part 2: Functional requirements specification. IEC 62290-3:2019- Railway applications - Urban guided transport management and command/control systems - Part 3: System requirements specification. IEEE 1474.1 - Communications-Based Train Control (CBTC) Performance and Functional Requirments IEEE 1474.2 - User Interface Requirments in Communications-Based Train Control (CBTC) Systems IEEE 1474.3 - Recomended Practice for Communication-Based Train Control (CBTC) System Design and Functional Allocation IEEE 1474.4(Draft) - Recomended Practice for Communication-Based Train Control (CBTC) System IEEE 1482.1 - Rail Transite Vehicle Event Recorders IEEE 802.11 - IEEE Standard for Information Technology — Telecommunications and Information Exchange between Systems Local and Metropolitan Area Networks — Specific Requirements IEEE 29148 - Systems and software engineering - Life cycle processes - Requirements engineering - IEEE Computer Society IEEE 828 (Configuration Management ) - IEEE Standard for Configuration Management in Systems and Software Engineering IEEE 12207 (Software Life Cycle Process) - Systems and software engineering — Software life cycle processes IEEE 15288(System Life Cycle Process) - Systems and software - Systems life cycle processes IEEE 24748 (System &Software Engineering ) - Systems and software engineering — Life cycle management IEEE 802.3 (LAN Interface) - IEEE Standard for Ethernet 9. Comparison between CBTC and ETCS As mentioned before CBTC is based on Institute of Electrical & Electronics Engineers (IEEE) defined requirements where as ETCS is based on Subsets from ERA * UNISIG * EEIG ERTMS USERS GROUP.Refer Below table for some of comparison between CBTC and ETCS solution. 10. Selection of System (food for thought !) It is also vital to select the best suitable solution for the rail network, especially brownfield based on your operational needs, track alignment, type of rollingstock operating on the line, whether track is laid on viaduct /at grade or Tunnel. Sometimes it could be tricky. Let me explain a complex scenario of a suburban network, with varying distance between stations which could be in between 1km to 25km .It is currently operating with a fixed block system operating 12 Trains Per hour during peak time and 7 Trains per hour on non-peak hours and the future patronage for the next 50 years are identified as 25- 28 train per hour during peak and 12 Trains during non-peak hours . Track is laid on Tunnel for some sections, and majority are either on ground or viaduct /bridges. Network need to operate long freights on non-peak hours which cannot be fitted with trainbourne equipments .It also shares main land trains with similar scenario on non peak hours . Network has active level crossing through out the network .Below table detail some of the ideal solution in terms of cost (especially when many long sections are present ,implementing and maintaining a DCS /Wifi will be expensive ) .What do you select as the ideal solution in this scenario ?
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CH26 | Automatic Train Supervision (ATS) System
The system that monitors and provide necessary commands to direct the operation of Trains in order to maintain the schedule in the required traffic patterns to minimise the effect of Train delays. Automatic Train Supervision System (ATS) is part of the Automatic Train Control (ATC) along with Automatic Train Protection System(ATP) .Refer Figure 1 for the relation .Primary Function of an Automatic Train Supervision(ATS) is to control and monitor the train operation .ATS is managing the train operation according to a time table or a specific headway or based on human (operator ) interaction on degraded mode of operation. A modern day ATS includes Automatic Train Regulation (ATR) and a Schedule Compiler. Figure :1 Block Diagram of Automatic Train Control ATS provide interface for the operator to supervise and manage trains. It provide real time status of the operating positions of the trains. ATS control launching of trains from storage facility to the commercially operating line and return the trains from operating lines to storage facility. It can also send manual and automatic commands to initiate and terminate train operations with override automatic command functionality. Modern day ATS system has some of the following functionality. Operator can create and modify timetables. Operator can monitor and control real time trains Automatically send route call requests according to train operation requirements Automatically send traffic locking calls based on train operation requirements Automatically regulate train movements based on regulation needs. Allow restore services and re-establish train operating patterns due to service delays and breakdowns Tracking Train Positions in all signalled area. Manage events and alarms from signalling system, trains and other signalling subsystems. Supply real time train information to Public addressing system. Generate performance reports for train service availability and train service quality factor. Allow automatic and manual taking over of control centres, if have multiple control centres for redundancy purpose. Capable of remotely control to open and close train doors and Platform Screen Door, if present Can build centralized functionality and localized functionality. Can build Train Description Facility for tracking and maintain a record of each train. Functionality of a Modern Automatic Train Supervision System Automatic Train Regulation (ATR) Modern Day Train Automatic Train Supervision can regulate the train based on various train regulation strategies under automatic mode .It can dynamically regulate trains to maintain headway and time table .Automatic Train regulation allow operator to configure the level of service deviation and raise an alarm to the operator if the service deviation exceeds the set level.ATR functionality within ATS can incorporate some of basic train regulation strategies. 1.1 Timetable Adherence ATR feature of the ATS monitor real time train services to ensure they are on schedule. It recover services making use of corrective actions to maintain time table .Corrective actions can be taken by adjusting station dwell times ,run times ( within the maximum speed of the track ) , coasting and adjustment of headway 1.2 Headway ATR monitor the train services to ensure constant headway is maintained. Best suitable headway is selected based on number of trains allocated by operator (without exceeding the maximum number of trains possible within designed headway, usually 88 seconds or more ) .ATR can even alert the operator if he /she allocate more trains or headway if he/she exceed the limit . Automatic Route Setting (ARS) facility ATS is capable of automatically make route calls based on train location ,time table and route strategies .ATS will not send a route call repeatedly to the interlocking if the route is not available .Rather it check route is already set by other request ,route is blocked or the requested route can cause a dead lock around terminal or turn around areas .Unlike any other Entry-Exit ,or one control switch control system ATS can store any route request and retry only when the route become available. In case the controller block a route and next controller try to set the route, ATS can alert the operator with the reason. ATS can automatically identify conflicting routes and when two trains approach a station can give priority to the train set which is scheduled to use the station first. ATS can allow override of automatic route setting for a manual route setting when needed. ATS ensures it will not set a route for an arriving train at a junction or a station platform which restrict a departing train to leave the platform Manual Route Control facility ATS can allow to perform manual route setting which can override the auto route setting. On modern day ATS you can find blocking and unblocking of signals and points for easy maintenance purpose with controlled reversal to avoid human error. Station Dwell Time adjustments As required operator can adjust station dwell time which can override the dwell time proposed by the timetable or ATR without affecting the T minimum Dwell time. Platform Hold ATS can allow the operator to prevent trains from station and hold. ATS can allow auto hold as well based on known scenarios. As an example, if at a time two trains cannot be at same place in one tunnel ventilation system, ATS apply hold functionality to halt one train at a station ,not allowing two trains comes to same section at same time. Similarly, when there is a fire in some tunnel section ATS can apply hold for trains in adjacent station. Train Hold ATS can allow the operator to hold and release a Train at a station with status display on the workstation Skip Stop ATS can allow operator at an operating terminal to make a train skip a station or more as desired. When a hold command is applied before skip command, hold command take precedence over skip command as hold command could be more related to fire and other safety related issues . Train Control ATS allow Train controller to manually control the train by sending remote wake up and sleep command. Command are normally issued automatic based on schedule or manually by an operator from a control terminal. ATS can also send command to reset train borne ATC equipment. Operator can also send emergency brake release command. This will be ensured when ATC confirm its safe to release. Operator can have a remote command to open and close a train door when the track side equipment confirm its safe. In case there is a Platform Screen Door present in the system , command is simultaneously issued to Platform Screen Door controller to open /close concurrently with the train door.ATS can also send a creep mode command to a train in automatic mode but suffers a ATO failure. Temporary Speed Restriction (TSR) An operator from his control terminal can apply and remove temporary speed restriction with immediate effect for any direction of travel in entire signalled area .It can also apply for a track circuit section .TSR can vary from 0Km/Hr to the maximum design speed of the system. Mode Inhibition ATS can allow the operator to inhibit the mode of operation such as Automatic Mode(Grade of Automation 3/4) ,Manual Mode(Grade of Automation 0 or 1) ,Semi Auto Mode(Grade of Automation3) for a train or entire fleet along the signalled area . Time Table ATS allow the operator to modify the running time table as per demand with in allowed headway and maximum trains possible in the line for that headway .Operator can create ,modify or delete a time table ,service ,trip or suspend a time table .In case by mistake operator suspend a time table ,ATS can allow through a command to resume from the current time. Controller workstation allow to load new time table ,modify origin ,destination ,dwell time ,arrival /departure time ,inter station run time ,coast level with recording of the controllers action on parameter change with time stamp. Operator can download the time table and send for printing. Energy Optimisation Based on traction power supply limitation ATS can optimise energy consumption by avoiding waking up trains at same time from same power zone to minimise rise in current. Alarm and Events ATS can alert the operator on any faults as needed by the operator through alarm ,warning based on the severeness of criticality .Pre-emptive warning of any subsystem can also be alerted .Various railways use different background /text colour code for severity level .Severity level are configurable based on needs .ATS can also log and record events with time stamp and link with maintenance facility Automatic /Manual Take Over Depends on railway requirement there could be Main Control Centre ,Back up Control Centre ,Station Control Centre ,Depot Control Centre etc .In case of failure of Main Control Centre ATS server or Loss of communication between Station and Main Control Centre ,Station ATS can take over control to avoid traffic disruption .An operator at Main Control Centre can manually request take over control from another operator at Station Control Centre Automatic /Manual Hand Over Over Similarly Handover can be done between control centre and Operator. Modern Day Automatic Train Supervision Refer Figure 2 for Modern Automatic Train Supervision System block diagram .Interconnection to other subsystem is out of scope for this article which will be covered in Architecture of a CBTC Figure 2 Typical Automatic Train Supervision Interface 17. Summary Automatic Train Supervision System paved way for a highly capable automatic “vital “ control centre from conventional “Non Vital “Entry-Exit Control Panel or a One Control Switch Graphical User Interface Control Terminal .In a modern train control system ,especially Automatic Train Control system used in Communication Based Train Control System ,ATS has a significant role .
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CH18 | Automatic Train Control (ATC)
Automatic Train Control is the CBTC system which automatically controlling the train movement while enforcing safety. ATC provide command to motoring the train, coasting, braking, regulating the speed with accurate station stopping. This system automatically protects the train (ATP) while ensuring safe separation between trains and protection from over speeding. Refer Fig 1 for the functionality of Automatic Train Control. Through out this document “Trackside” means equipemnt installed on track or along the geometry of the track , “Wayside” Means equipment associated with Signalling Equipment Room /Control Room and Trainborne means equipment installed on the train. Fig 1 Functionality of Automatic Train Control Based on Functional Elements, ATC has wayside equipment and Train borne equipment to perform this functionality. a) Train Borne (Signalling) Train-Borne Signalling Cabinet ( ATO , ATP ,TDM ) Radio Antennas Under Train Sensors (Balise Scanner) Tachometer Video Display Unit (VDU) b) Train Borne (Train Supplier) Speedo Meter and Or Odometer Brake Relay Encoder Cabinet Train Integration Management Cabinet c) Trackside Balise DCS Access Point (AP) for Wifi d) Wayside DCS Equipment Cabinet Wayside Computers for ATP and ATO (Zone Controller ) Interlocking Subsystem Automatic Train Supervision Subsystem Functionalilty of Element 1.1 Train-Borne Signalling Cabinet – Train Borne ATP System Automatic Train Protection processing unit belong to the train borne computer system. This subsystem is in charge of the continuous control of the train speed according to the safety profile and applying the brake if it is necessary. It is also in charge of the communication with the wayside ATP subsystem in order to exchange the information needed for a safe operation sending speed and braking distance, and receiving the limit of movement authority for a safe operation. Refer Section 1.15 for wayside system details 1.2 Train-Borne Signalling Cabinet – Train Borne ATO System Automatic Train Operating Processing unit belong to the train borne computer system, It is responsible for the automatic control of the traction and braking effort in order to keep the train under the threshold established by the ATP subsystem. Its main task is either to facilitate the driver or attendant functions, or even to operate the train in a fully automatic mode while maintaining the traffic regulation targets and passenger comfort. It also allows the selection of different automatic driving strategies to adapt the runtime or even reduce the power consumption. Refer section 1.14 for wayside ATO system details 1.3 Train-Borne Signalling Cabinet-TDM Train Data Management Processing System is providing the interfacing functionality with Trains Integrated Management. This is the processor-based System offer health monitoring, train event recording and control of train initialisation. It is also the interface for train door control, command train to wake up and sleep. Processor process the data received from various train borne subsystem and send to the ATS. 1.4 Radio Antennas Antennas are generally installed on exterior roof of the train, part of the Data communication system Transmit and Receives (Bi -Directional ) processed data between Train-Borne Radio Unit and Trackside Data Radio Unit . Refer Access Point section 1.12 & DCS 1.13 1.5 Under Train Sensors (Balise Antenna ) Part of Geographical Position (Location Reference ) functionality of a CBTC solution ,train borne balise transmission module radiates energy wave to activate the ground balise to uplink geographical position transmission and these sensors are mean for collecting the spot transmission data . Spot transmission wayside devices (Fixed Balise) provide the train with information allowing the train to check and to calibrate its odometer, and to identify the actual train location. In general on CBTC solution rest of the bi directional data transfer is happening through radio. Refer Balise section 1.11 1.6 Tachometer This is the speed sensing device otherwise known as wheel impulse generators or speed probe .It can be opto isolator slotted or Hall Effect sensors .It detects the speed of the train and passes on to Train-Borne controllers(ATP and ATO) and speed measured b Tachometer is also used to ensure train at stand still before TDM inform TIM to open the door. Speed will be displayed on the speedometer 1.7 Video Display Unit (VDU) Optional unit for GoA4 (Grade of Automation 4), however most of the CBTC has a VDU , accessible only when needed ,especially during the degraded mode of Train Operation. Its kept covered in a box and accessible for Train Operator when needed. GoA1 and GoA2 will necessarily have a unit displaying speed ,brake enforcement needs ,warning etc (Train Protection ,Train Control Information ) ,mode of operation selector switch , driving ontrol handle etc where as GoA4 VDU could be displaying equipment operating staus ,failure conditions rest are defined based on operator needs. Refer Fig 2 for a ruggedized VDU designed per EN 50155 / EN50121-3-2 / EN61373. Fig 2 Video Display Unit 1.8 Speedo Meter Unit which display of the speed of the train 1.9 Brake/ Relay Encoder The Trainset unit automatically control the speed and regulate the speed based on the information received from ATP and ATO processors. 1.10 Train Integration Management (TIM) The system which interface between Signalling and Trainset ,which provides health monitoring status to Train Data Management(TDM) upon request .When ATS issue Train readiness command via TDM to TIM and ATC for their preparedeness for readiness .Train(TIM) and ATC set ready and send the readiness status back to the ATS via TDM .If readiness is not available a fault code will be send back to ATS .TIM use the clock along with TDM to ensure synchronisation.TIM also communicate with the Passenger Information System with the same “clock” 1.11 Balise The Track Installed Transmission System performing a safe spot transmission, conveying safety related information between the track and the train. Information transmitted from an Up-link Balise to the On-board Transmission Equipment is fixed Spot transmission, when a transmission path exists between the wayside equipment and the On-board Transmission Equipment at discrete locations. The information is provided to the train only as the Antenna Unit passes or stands over the corresponding Balise. The length of track on which the information is passed is limited to approximately one meter per Balise.For CBTC application fixed balises are widely used to provide the train with information allowing the train to check and to calibrate its odometer, and to identify the actual train location. In nutshell in a CBTC solution rest of the vital bi directional data transfer is happening through radio. Refer Balise Antenna section 1.5. 1.12 Access Point (AP) of the Data Communication System ( WiFi ) An access point is a wireless network device that acts as a portal for devices to connect between Wayside and Train borne Equipment installed along the Trackside. Access points are used for extending the wireless coverage of a wired DCS network so that the train passes by the area covered by an access point can establish seamless bidirectional data transfer. A high-speed Fibre Cable a Data Communication Cabinet with router from the equipment room to an access point, which transforms the wired signal into a wireless one. Refer Radio Antennas section 1.4 & Data Communication System 1.13 which work hand in hand. 1.13 Data Communication System (DCS ) The Communication network formed by redundant fibre optic cables based on geographical layout of the railway. This is making use of cable route diversity to ensure no single point failure Occurs. DCS network is normally formed in a ring topology so that any components fails availability is ensured by re routing the communication within the ring. Dual switches are provided at each location for the availability of the local area network. 1.14 Wayside Computers -Wayside ATO System. The system in charge of controlling the destination and regulation targets of every train. The wayside ATO functionality provides all the trains in the system with their destination as well as with other data such as the dwell time in the stations. Additionally, it may also perform auxiliary and non-safety related tasks including for instance alarm/event communication and management, or handling skip/hold station commands. Refer section 1.2 for Train borne ATO system details. 1.15 Wayside Computers -Wayside ATP System This subsystem undertakes the management of all the communications with the trains in its area. Additionally, it calculates the limits of movement authority that every train must respect while operating in the mentioned area. This task is therefore critical for the operation safety. There are bidirectional communication established between wayside and Train borne ATP system Refer section 1.1 for Train Borne ATP system. 1.16 Interlocking system The system which control the wayside equipment such as point machine ,signals (for fall back mode) , and gathers the secondary train detection system such as track circuits /axle counters (train recovery ) for locking the wayside equipment in front of a running train .Depends on utilisation of the non-core functionality of CBTC trackside equipment vary .Interlocking will become master during degraded mode ,during a complete ATO failure ,for recovering the train ,again depends on operator definition. 1.17 Automatic Train Supervision System (ATS) The system responsible for controlling and monitoring the unmanned train operation according to time table or headway .There is another article detailing Automatic Train Supervision in RailFactor . Feel free to refer to that for more details . Architecture of ATC (CBTC ) Refer Fig 3 for System Architecture of an Automatic Train Control System(Distributed) Fig 3 CBTC System Architecture ATC constitute of the complete subsystem as shown in the figure 3. It varies from supplier to supplier. Some of them have centralized architecture, and some have distributed architecture. In the figure 3, above shows a distributed architecture. For the explanation purpose consider .Three (3) trains stabilized in the depot are to be operated with 3 min headway. 2. CBTC Operation For the sake of explanation, Automatic Operation (Un manned Train Operation) has been taken into consideration and degraded mode of operation or manual route setting ,or operator intervention sequence are not included .Sequence of route call will be same as automatic ,only difference is that these process of route call is made manual from control centre ,or back up control centre or station control centre. 2.1. Role of ATS Primary objective of Automatic train Supervision is to control and monitor the train operation and manage the train operation according to timetable or specific headway. Trains stabilized in the depot has a profile that can be uniquely identified by the ATS for tracking. ATS maintain a record of each train consist profile with a train identification. Note : The train identification is a static field uniquely sourced from train supplier with physical car numbers ,ATC number of the train. Refer section 1 .17 and another Article Automatic Train Supervision in RailFactor 2.2. Role of ATC (ATO/ATP/TDM) ATC receive the wake up command from ATS through the DCS network transmitted to the trackside via Access point .Train antenna mounted on roof of the train captures this signal and pass on to the train-borne ATC equipment .ATC has three functionality which includes ATO ,ATP and TDM .Refer section 1 for details of individual system within Train borne ,Trackside and Wayside (ATO, ATP ,TDM ,Antenna ,DCS ,Balise ) 2.3. Role of Interlocking Interlocking is an arrangement of signalling appliances in which operation of one appliance will depend on the status of other in a proper sequence to ensure safe conditions . In other words Interlocking is a failsafe system responsible for controlling ,gathering vital equipment status , locking and releasing trackside equipments such as Signals ,Points and other equipment .There could be various other trackside equipment such as stop Plungers , track circuits ,and wayside equipments such as control panel for degraded operation .These are part of the non-core functionality of the CBTC which varies from operator to operator for train recovery ,maintainers protection, diagnostics ,fall back mode etc .There will be a dedicated interlocking chapter published in RailFactor in future. 2.1.1 Launching of Train from Depot to Mainline Based on time table , ATS automatically set a route for the train consists 1 assigned with a head code ,entered by the operator so as to allow the trains to reach its destination without having the operator to set route manually.ATS will send the route request in advance to the interlocking to set the route before the departure time .Once the route is set ATS will remotely send the wake up command to switch Train 1 On .Upon receipt of this command on ATC computer (ATP/ATO) of Train 1 perform a wake -up test automatically. This wake-up test will test the operational and safety capabilities of the train-borne ATC(ATO /ATP/TDM) system and its interfaces with the train and other subsystems .TDM will provide the front end processing for interface between ATC and Train Integrated Management system of the train. This is to allow data transmission between TIM of Train and ATC. As part of the wake-up test ,ATP system confirm that the train 1 has not moved during sleep and train position is known. Wake up sequence commence with door closed. In case door is open, then TDM (Train Data Management) will request door closed and check doors are closed prior to commencing the wake up sequence .In case door failed to close an Alarm will be send to the ATS .Train will verify its geographical position through the balises and its transferred to the Train Borne equipment. When train is ready to start his ride ,ATP will send a signal to the relay /brake encoder panel of the train to release brake and train 1 takes the route assigned to it and commences the journey from depot to the mainline reception track .Train 2 follow the same process following the initiation from ATS based on Time table and enters into the main line reception track from depot following Train 1 (maintaining a headway of 3 min) . Train 3 will follow Train 2 maintaining headway of 3 min with Train 2. As shown in Fig 4, Train 2 will maintain a safety margin with train 1. This safety margin is based on various parameters such as Train 2’s braking characteristics ,gradient of the track etc. In nutshell when Train 1 stops on platform 1, Train 2 will apply service brake to maintain safety margin by either reducing speed or a complete halt, until he can maintain safety margin with Train 1. All the trains are updating the geographical position and speed to the wayside ATC system and receives a movement Authority continuously .Thanks to the bi-directional ,radio system making continuous communication between wayside to trackside(wired) and trackside to the train (wireless).When Train passes over the balises ,it update the position and send to the wayside ATC Fig 4 ATC Train Operation 3. Summary This article covers the general operation using the core functionality of a CBTC system ,non core functionality such as monitoring ,maintenance ,train recovery ,cut over etc are subject to operators requirement and are out of scope for this article.This will be covered in another article
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CH14 | Power Supplies
SIGNALLING BOOK | CHAPTER 14 1. Introduction As Signalling system and its elements seeks 100% availability in almost all units and subsystems. Seamless power supplies are required for all electrical products to drive and achieve highest level of availability .It is possible to achieve various levels of redundancy based on end operators requirement and capacity to invest to make it 100% available at all time ! We can categorize Signalling power supply into centralized and distributed .Centralized are the one which Signalling Equipment Room has its own feeders with alternate sources of upstream supply with UPS back up (Number of Hours for back up needed are decided by the operator) and feeding into wayside location cases along the line ,where as distributed option has individual Main and alternate feeders available at each location cases /huts along the line with relevant UPS back up. Both has its own pros and cons .For a centralized power supply ,it requires higher capacity feeders and bigger dia conductors to reticulate into trackside location cases. Alternatively can step up the source at SER and step down at location cases to reduce cable cost.Where as distributed system requires main source feeder and alternate feeder at each location .Selection shall be based on budget and trade off between pros & cons for both system. 2. Steps to design the power supply There are many practices which could be implemented .Below steps are the one I apply for designing a power supply system if something specific is not asked for .It depends on individual designers /employers/operators practice Familiarize end operators requirements from contract or detailed specification. Understand country based standards required to be applied for the system. Gather power supply requirements of each drives. Assess the supply requirement for grouping based on voltages. Identify Static and Dynamic Load. Prepare the load requirement excel with possible grouping. Calculate Reactive Power for Transformers Calculate feed Circuit Breaker rating ,wire sizing Detail Design of the sub system . Procurement and Manufacturing of one Power Distribution Cabinet (PDC) Type approval to comply EMC requirements ,IP requirements and other needs to be approved for the railways. Mass production of power distribution cabinets Factory Acceptance Test. Installation of the System Power On and Integrated Test . 2.1 Familiarize end operators requirements from contract or detailed specification Contract or Technical Specification or System Requirement Specification define the requirements to be followed for the power supply design .It could include the local rules ,standards and practices to be applied ,redundancy requirements (Eg:-N+1 ,N+ N ) ,Spare Capacity Requirement ,Rated load for transformer (Eg: Transformer shall not be loaded not more than 75 % of rated load ),local and remote monitoring features ,UPS back up requirements ,Battery Types ,Hours of back up needed ,pre-emptive warning of components needed (advance warning for the product before its going to be faulty ) ,Insulation Monitoring Requirements ,Earth Leakage Protection Requirements ,Lightning Protection ,Type Test (For EMC /IP) ,How system shall behave when earth leakage happens (Eg:IT Earth Systems per IEC 60364-Clause 4.3.3.1),require to warn for maintainer to rectify the fault at secondary side of a transformer ) ,detailed calculation and other requirements (Eg:- detailed calculation for each feeder ,each fuse/wire size ,calculation for identification of maximum short circuit current ,Voltage drop ,maximum load to validate the wire are considered for protector and wire/cable selection.) 2.2 Understand country based standards required to be applied for the system Each country have either its own standard or follow reputed standards ,which shall be applied while designing a signaling power system .Again this is based on technical spec ,some time some requirement may be over ruled in technical specification.As an Example ,Australia seeks MEN (Multiple Earth Neutral) ,where as Signalling system require IT earthing system per IEC 60364.Direction shall be sought from customer unless otherwise specified.Country standard/practice includes earthing systems requirements ,wire colour coding ,EHS requirements (RCD fitted MCB) and code of practices. 2.3 Gather power supply requirements of each drives. Designer shall gather the approved system architecture with power needs ,product data sheets (Detailing reactive Power /Active Power ,Frequency /Voltage Tolerances,start up current ),any tools for validation of the calculation comes under this step. 2.4 Assess the supply requirement for grouping. In this step ,we identify the possibility of power grouping .Systems which require two separate sources of supply (Two UPS supply or Two Main Supply or One UPS and One Main Supply) ,subsystem which require one source ,but from a fail proof Auto Transfer Switch (ATSw),Types of Voltages (AC/DC with rating) ,Reticulated Supply to other destinations are the factors to be considered. Note :While preparing a load calculation excel ,these grouping will prove useful 2.5 Identify Static and Dynamic Load. As you know some of the loads to the power system are permanent (static) ,where as some loads are required while the drive operates(dynamic) .As an example ,Point Machine draws current when system calls the machine to operate ,similarly only one signal operate out of a two or three aspect signal at any time, and all relay don't operate at same time to mention some examples (Some relays are normally up but some are picked as per control logic .Designer shall identify these factors to save cost and to avoid over designing . There could be numerous point machines in the layout which don't operate at same time .Designer shall either identify himself /Herself or get identified from Interlocking expert how many points can be operated at same time to decide the total load for the system .Its waste of money to provide feeder capacity for 60 point machines when maximum 20 can be operated at any time ) .Designer shall also identify any point sequencing are considered in the route table .This means if there are 10 point machines to be called for a particular route setting and if all are lying in unfavorable position from previous train move or have self normalizing feature in the interlocking and route requires opposite move ,interlocking can call five of them first and next five with a time delay .This will be defined in signaling interlocking control table Another factor is electro mechanical components .High current is only required in few seconds at start up to gain initial torque for a point motor and reduce gradually which will be defined in the product manual.This factor shall also be considered .I like to consider starting current as worst case instead of operating current when other factors are optimized to the maximum. 2.6 Prepare the load requirement excel with grouping. Now its time to record all these in an excel sheet as per the grouping ,some employers /operators might have predefined tools ,if so please use the validated tool. Creating new tool /programmed excel require authentic review ,verification to make it error free.This table can group as per the reactive power(Apparent Power) or Active Power to identify the rating of feeder and transformers. If the data sheet provide active power ,designer can consider to convert to reactive power based on power factor of the product .Electronic products have close to unity power factor theoretically ,where as heating elements might have power factor of 0.8 or less .This tool contains all the different voltage requirements with its group to arrive at total load to the Power System . 2.7 Calculate MCCB Circuit Breaker rating ,Wire and Cable sizing(Downstream -Power Distribution Cabinet) Excel spread sheet is prepared with load requirement for each group ,including AC voltages and DC voltages (with converters) . Adding all the load will give the total power for the power supply system . As an Example :- If Point Machine and Signals requires 110V AC ,which can be clubbed into one group or as separate group to feed from separate isolation transformers with Automatic Transfer Switch.Interlocking cabinet ,AutoMatic Train Super Vision /Control Cabinet ,Communication Cabinet ,Secondary Train Detection Cabinet (Track Circuit /Axle Counter ) might need 230V with two independent supply (One from UPS supply Source and One from Rail Supply Source ) to feed the Redundant hardware of the respective cabinets .Similar feeder requirement can be clubbed together as per operator practice .In order to read the dry contact (for Point Machine Detection,other field status ) into I/O card file or to Pick a relay we might need 24V DC or 48V DC .All DC requirement of same voltage can be grouped together .These DC voltages are generated with the help of AC/DC converters (N+1 OR N+N *arrangement as per end operator requirement) which requires a fail proof 230V AC source which we might use a static Auto Transfer Switch (ATSw) Note* : Assume Total DC load requirement is 500 Watts ,If one of the AC/DC converter capacity is 500 Watts .One number of AC/DC converter is required (ie . we need 1 No of AC/DC converter rated for 500Watts to feed the load which is considered as ‘N’ unit ) .To make an N+N arrangement we use 2 nos of AC/DC converter and the output is paralleled to same bus bar .There by if one converter(N) is failed other will be continuously feeding the bus bar .There by making 100% redundancy .By using independent ATSw ,to feed both N will make the availability even better. Sample Calculation : we get total reactive power requirement from the excel table =60KVA (while adding all the total static and dynamic load for one single feed at any instance of the operation) and end operator specification requires an additional 20% capacity with feeders to be fully wired for future expansion and downstream isolation transformers shall not be loaded not more than 75% rated load . So the total load requirement = Actual Load X 20% Spare Capacity /75% rated load =60KVA x 1.2 /0.75 =96 KVA In order to achieve 100% redundancy we need two independent MCCB and Transformer with 96KVA load for the Power Distribution System .Both sources can be UPS or One UPS and One Normal Supply .There by achieving redundancy from single point failure(Refer the figure) .Both these sources are fed into two independent MCCB which is feeding 100KVA transformer (Next higher size of total load 96KVA) .Let the downstream cabinet transformer be 3 phase 4 Wire transformer and we can segregate and balance the load on each phases at secondary of the transformer for each group (230VAC , 230V/110VAC ,48/24V DC (AC/DC converter with input supply 230V AC ) making use of different phases by balancing the load on 1 :1 Isolation Transformer (400V ,3 Phase ,4 Wire Transformer) 2.8 Downstream Power Distribution Cabinet MCCB Rating Calculation Total Load is =96KVA (100KVA Transformer) Total Current on each MCCB =Reactive Power /1.73 (Root 3 ) x Voltage =96 /1.73 x 400V =138.72 Amps We shall consider the inrush current of the transformer ,which is 1.5 times of total current (worst case) So total current will be =138.72 x 1.5 =208 Amps We can select the next higher available rating for MCCB =250 Amps This means we need two sources to feed these two MCCB in the Power Distribution Cabinet(Downstream Incomer) Cable and wire size shall be selected based on this load or voltage drop from feeder or Short Circuit Current which ever is the highest . Upstream UPS and Battery to feed these sources shall be designed for Online operation .Battery banks are designed based on the back up hours needed as per end operator requirement (say 8 hours or 24 hours or 48 Hours ).As UPS design itself is a long topic will include UPS design in another article . 2.9 Detail Design the sub system . We have selected the downstream transformer ,MCCB rating which are fed from Two UPS or 1 UPS and 1 Non UPS Main source Supply . Upstream sources (UPS or Normal ) shall be connected to MCCB which is fed to Primary of Transformer .Out put of the Transformer is fed into a 3 phase bus bar (4 wire ) .Power shall be distributed from this bus bar into each drives /cabinet protected with MCB's From the excel spread sheet, for each group MCB has to be designed as per Reactive Power value . Say for Example if Cabinet 1 require two sources of supply and each load is 1000VA .MCB rating is calculated as below MCB 1 =1000/230=4.34 Amps and the next available size is 6Amps selected for MCB 1 .Redundant supply also have same 6Amps protection(MCB 2) As per the grouping in the excel ,other distribution to be made to feed all the drives and cabinets. A sample single line diagram(Figure 1) is shown below for better understanding Detailed drawings to be produced further from block diagram /Single Line diagram for each feed with correct MCB rating and wire sizes. This shall be selected based on the current carrying capacity. Figure 1. Sample Single Line Diagram 2.10 Procurement and Manufacturing. Equipemnts and components shall be procured as per the designed parameters and built into cabinets (earth metallic enclosures) with correct IP requirement defined by the end operator .An IP 42 cabinet shall have a roof with fan .Filters to be designed and installed ,if EMC test is failed as per relevant standards requirements (EMC standards and requirements will be covered in another topic) and further tested to clear EMC test 2.11 Factory Acceptance Test. A detailed procedure to be prepared to perform factory acceptance test ,to test Insulation monitoring , Earth Leakage ,No load test ,Full load test to cover all the functionality and the readings shall be recorded in the corresponding recording templates and duly signed by relevant parties. 2.12 Installation of the System Factory tested cabinet shall be transported to project site for installation and completion of UPS and other sub system integration wiring. 2.13 Power On and Integrated Test A professional Engineer shall witness the power on after his inspection and Testing and commisioning team will further perform the integration test to all end drives 3. Earthing Systems Functional Earthing and Protective earthing(PE) are carried out at the electrical installations .A functional earth connection serve the purpose other than electrical safety and might carry electrical current as part of normal operation .For the functional earthing cases a special terminal is provided for the installer to connect external earth usually for the purpose of noise reduction .Screen earthing of a cable is a functional earth . A protective earth is used to protect the operator by means of reliable ground connection to make sure the touch current wont exceed certain values .In nutshell Protective earth is intended to protect personals from electric shock during an earth fault .This earthing is performed for any metallic exposed part to the Main earth terminal in the Signalling Room. Fault current will flow through this conductor to earth which in turn on to the protective devices such as RCD (Eg residual Current Device Fitted MCB) to safely open the circuit within 0.4 seconds,where as functional earth is used to reduce radio frequency noise .Functional Earthing and Protective Earthing must be connected to separate earthing system and can be tied together through Potential Equalisation Clamp.(PEC) . Example :- Cabinet case earthing is a type of Protection Earthing ,Shield /screen earthing of a cable is a type of functional earthing in a signalling installations. 4. Type of Protective Earthing System. There are few types of earthing system (Protective Earthing ) implemented as per country practices. These can be broadly classified as TNS Earthing System ,TNC Earthing System ,TNC-S Earthing System ,TT Earthing System and IT Earthing System. This can be applied on to the primary side of the isolation transformer of a downstream Power Distribution cabinet. The supply directly feeding the signalling equipment shall be isolated from earth(floating) as per IEC 60364 IT system (e.g. 600 V a.c and 110 V a.c supply). Refer to Clause 4.3.3.1.All the supply fed to the signalling element will have floating neutral (IT earthing system ) with neutral cut through Insulation Monitoring Devise or Earth Leakage Device . The ELD/IMD device shall be used for first fault condition monitoring only – the ELD device shall not be used to trip circuit protection during either a first or second fault condition. The first fault in the IT systems should be identified and fixed immediately to avoid a second fault from occurring.Refer Figure 1 to identify IMD connected to the secondary side of the Isolation Transformers and Earth Leakage Relay connected at the Primary side of the transformer in which the relay contact is used to trip the MCB and there by isolating the group .This is to avoid nuisance tripping of upstream supply when earth fault occurs in a power group in the downstream. T(Terre or earth) Denotes that the Power Distribution System at SER is solidly earthed independently of the source earthing method. N(Neutral) Denotes that a low impedence conductor is taken from earth connection at the source and directly routed to the Power Distribution System at SER (Signalling Equipment Room Signalling Power Supply) for the specific purpose of earthing of the PDC system S (Separate) Denotes that the neutral conductor routed from the source is separate from the protective earthing conductor ,which is also routed from the source (Upstream council/Rail supply ) C(Common ) Denotes that the neutral conductor and the protective earthing conductor are one and the same conductor used 4.1 TNS (Terre ,Neutral, Separate ) Earthing System. Here Terre stands for Earth .In this type of earthing system neutral conductor routed from the source is separate from protective earthing conductor ,which is also routed from the source .Upstream supply normally tapped from Council (Government Electricity Authority ) or Railway exclusive supply which is the source for power distribution system .This is used as supply source in the Power Distribution Cabinet or UPS depends on the power supply distribution requirement at the downstream on a signalling power distribution cabinet .For a three phase source ,will have five wires (Phase 1 ,Phase 2 ,Phase 3 , Neutral and Exclusive earth ) and a single phase source have three wires (Phase ,Neutral and Earth ) leading into your Signalling Power Distribution Cabinet or UPS .Here Neutral and Earth are separate conductors and earthed at source.No separate dedicated earth used at Power Distribution Cabinet End in the Signalling Equipment Room. Refer Figure 2 for TNS earthing system details Figure 2 TNS Earthing System 4.2 TNC (Terre Neutral Common ) Earthing System . Here C denotes that the neutral conductor and the protective earthing conductor are ONE and same conductor is used .For a three phase source will have four wires (Phase 1 ,Phase 2 ,Phase 3 ,Combined Neutral and Earth ) and two wires for a single phase system (Phase and combined Neutral and Earth) In this type of system joined Neutral and Earth are earthed at source(upstream) end and destination end (downstream ) on separate earth electrodes.There could be additional electrodes at source end as shown in the Figure 3 Figure 3 TNC Earthing System 4.3 TNC-S System This earthing system is an enhanced version of TNC system .In this type of system a three phase source(Upstream) will have four wires (Phase 1 ,Phase 2 ,Phase 3 ,Combined Neutral and Earth ) and two wires for a single phase system (Phase and combined Neutral and Earth) .That is,Joined Neutral and Earth at source(Upstream) end but separate Neutral and Earth conductor at downstream(PDC) end .In nutshell the difference for TNC-S from TNC system is that there are five wires used at downstream end(PDC ) joined into four wires towards source(Joined Neutal and Earth toward Source) for a three phase system and three wires (Phase ,Neutral and Earth ) joined earth and Neutral towards source and joined Neutral and Earth in a single phase system .Refer the Figure 4 below for TNCS-S earthing system details . Figure 4 TNC-S Earthing System 4.4 TT System In this type of earthing system ,there are four wires (Phase 1 ,Phase 2,Phase 3 and Neutral ) for a three phase system and two wires for a single phase system .Both source (Upstream ) and load (Downstream ) have exclusive earth and neutral is tied to earth at source end not at load end (downstream /PDC). Refer Figure 5 below for TT Earthing System details Figure 5 TT Earthing System 4.5 IT Earthing System This is similar to TT earthing system .However neutral is floating compared to other types of earthing system .Neutral is not earthed at source (upstream ) or Load (Downstream ) .That is your Power Supply Cabinet . Both upstream and Downstream has exclusive earth ,however source earth is routed via Insulation Monitoring Device which can use for monitoring the insulation fault.Accepted earthing practice for Railway control systems are as defined in IEC 60364 -4-41 .This means the earthing system of the Low Voltage supply directly feeding the signalling equipment shall be IT earthed (Isolated from Earth) through an Insulation Monitoring device or an earth leakage detector .An earth leakage in the secondary side of the transformer feeding the signalling gears shall not trip ,but it shall be warned to the operator when first leakage detected and maintainer to be send to identify the cause .This will not cut the feed to the system at same time at the primary side of the isolation transformer ,eearth leakge realys are implemented to prevent nuisance tripping to the upstream UPS or the alternate normal supply.Refer Fig 6 for TT earthing system Figure 6 IT Earthing System 5. Power Earth ,Signalling Earth ,and Communication Earth It is always advisable to have separate dedicated earthing system for Power supply subsystem ,Signalling System and Communication System . Communication and Signalling system exclusively requires functional earthing for EMC purposes where as Power Distribution System requires Protective earthing .Signalling Earthing System can be used to earth all the exposed metal part for the protective earthing of the signalling system and communication earthing system can be used for functional earthing and protective earthing of the communication cabinets respectively . All these earthing systems can be tied together with a potential equalization clamp which is an open circuit between earthing systems and which conducts when there is an earth fault . This practice is adopted in Sydney Practice 6. Earth Leakage Detection An earth Leakage System and Insulation Monitoring system shall be implemented in the signalling system at primary of the isolation transformer for the Power Distribution System and Secondary side of the isolation transformer .Earth Leakage Relay contact can be wired on to the trip coil arm of the transformer to isolate the downstream from source when leakage exceed a certain set value .This set value shall be based on the maximum leakage current possible from the end equipment and the current that can electrocute the person comes in contact .For an example .If leakage current from a signalling cabinet is 40mA and above this current can electrocute a person ,ELR shall be set for 40mA and if it exceed this value the incoming MCB will isolate the source by tripping the system . 7. Insulation Monitoring Device (IMD) This is another protective mechanism ,especially in IT earthing system which is used to monitor the leakage on the insulation of the conductor and can be locally and remotely monitored 8. Lightning Protection Its advisable to implement lightning protection for the conductors from the source for a distributed power system when conductors are exposed to lightning and thunder .There are protective devises available to connect across the conductors which are available in the market .It shall be implemented based on your operator practice 9. Local and Remote Monitoring Its also vital to read the status of various group of supply, incoming power availability ,UPS availability ,UPS on battery , Earth Leakage /Insulation Monitoring Status ,Current ,Voltage ,Frequency ,Synchronization ,ATSw status to be locally monitored (through local indication on the cabinet ) and remotely monitored (for maintainer to rush ) through Automatic Train Supervision system .Telemetry could be used to transmit these status remotely. 10. Summary This article cover the downstream power supply cabinet requirements and its out of scope for Upstream Feeder and UPS design which is considered as Electrical scope .However will cover in another article for those who are keen on it
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CH12B | Standards for Signalling Cables
Railway Signalling, Communication and Power distribution for signalling equipment falls under Signalling Cables category. Signalling Cables are categorized as high voltage, low voltage ,extra low voltage and fibre cables. According to IEEE ,high voltage cables are cables handling voltages greater than 1kV up to 35kV for AC Vrms and 1.5kV to 50kV for DC respectively .There are difference in this range from country to country and can go up to 50kVrms . Low voltage cables carry voltage between 50V to 1kV for AC Vrms and 120V to 1.5kV for DC and extra low voltage cables carry voltages less than 50v for AC Vrms and 120V for DC . Note!: Standards and requirements vary from country to country and user of this article shall check with the operator for the standards with in the country of application. Below table list the standards used in major railway countries and alternate standards may be proposed where no standard exist for certain requirement. List of Standards and Application Below are a list of requirements , standards along with title for various cable requirements .Requirements are classified as Mechanical and Electrical . 1.1. Mechanical Properties Requirements for the protection of the cables against potential threats 1.1.1. Water Protection Stringent water protection requirements are applied based on the intendent environment and cables properties are defined from negligible water threat to immersion or complete submersion. Water penetration can be Radial due to sheath damage and water travels longitudinally. Water blocking tapes are considered as moisture protection.Below are various standards which are applied depend on intended application. 1.1.2. LS0H (Low Smoke, Zero Halogen) Especially tunnel and enclosed environment its important to ensure cable properties are free from emitting hazardous gas and smoke during fire. 1.1.3. Fire& Flame Retardant ,Fire& Flame Resistant Please note that retardant and resistant have different meaning ,Fire /Flame resistant is more stringent and cable will be expensive .Fire/Flame retardant cables resist the spread of fire into a new area, whereas fire-resistive cables maintain circuit integrity and continue to work for a specific time under defined conditions. Resistant in this context is defined as a material that is inherently resistant to catching fire (self-extinguishing) and does not melt or drip when exposed directly to extreme heat. Retardant is defined as a material that has been chemically treated to self-extinguish. 1.1.4. Ultra Violet Protection Cable which are directly exposed to sunlight shall be resistant to Ultra Violet radiation. Test shall be conducted as per requirement on the intended terrain of cable application for Low(AN1) ,Medium(An2) or High(AN3) 1.1.5 Abrasion & Crush Abrasion and crush load resistance test to be performed to ensure the cable can have longer life span for the signalling system life. 1.1.6 Longer Service Life Crush Thermal aging tests are performed as defined in EN 60811-401 to ensure cables can long last at least life span of the signalling system 1.2. Electrical Properties Below list cover some of the standards for electrical properties of the cable 1.2.1. Ohmical Resistance 1.2.2. Mutual Capacitance 1.2.3. Insulation Resistance 1.2.4. Di-Electric Strength 1.2.5. Conductor 1.3. Australian Standards
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CH5 | CONTROL TABLES
SIGNALLING BOOK | CHAPTER 5 CONTROL TABLES CONTENTS 1.INRODUCTION 2. IDENTIFICATION OF ROUTES 3. MAIN ROUTES 4. SHUNT ROUTES 5. APPROACH LOCKING 6.POINT CONTROLS 7.ROUTE LOCKING 8.OVERLAPS 1.INRODUCTION Control Tables express the conditions for the setting of routes, clearing of signals and the setting and locking of points and ground-frames. They are a vital part of the specification of any signalling installation (in conjunction with the signalling plan). They specify precisely the controls applicable to all signals, points and other signalling functions. They are the starting point for detailed design and the main reference documents for testing. Control tables may be presented in a variety of different formats. Although a railway administration may at one time standardise on a particular format, requirements change over a period of time. It is quite normal to find several different formats in current use (although for different installations). Although control tables are essentially written documents, they are frequently produced as drawings on a large page containing several routes or points one above the other. Alternatively there are many advantages in using a small page size with only one route or one set of points per page. This section of the course will often show several sets of controls on a page within the notes to assist understanding and comparison. For practice work, separate pages for each function will be used. Two main types of control tables will be used. The first giving the conditions for the operation of signals and the second giving the conditions for operation of points. There may also be a need for various other specialised types of control table, e.g. for automatic signals, ground frames, level crossings etc. These will not generally be covered here, although reference may be made to some of them in other section,s. Note :Detailed explanations are made with respect to the real track layout below (Bakaburke Junction -Figure 1 ) LHCT_Left Hand Caution Turnout RHMT-Right Hand Medium Turnout Table 1 -Route Table for Signal 24 Details of entries will be at the bottom of this article. 2. IDENTIFICATION OF ROUTES 2.1 Types of Route Each route may be one of two types:- a) Main Routes (M) A main route extends from one running signal to the next running signal or to the buffer stops at a terminal station. A route Leading up to another running signal will include a further margin beyond that signal, known as the overlap. b) Shunt Routes (S) A shunt route is one which allows for a shunting or other low speed movement which is not already provided for by running signals, or where the route may possibly be occupied by another train. This includes entry to and exit from sidings, and wrong-line movements. Where passenger trains are required to enter occupied sections of track, some railway administrations classify such routes separately as calling-on routes. TfNSW (Transport for New South Wales ) does not make such a distinction. 2.2 Naming of Routes Routes are named after the signal from which they apply (the entrance or start signal for the route). If a signal has more than one route, each must be uniquely identified. This will normally be by a letter or number suffix. In these notes, letters will be used with the left-band route labelled A and the remainder consecutively B,C,D etc. in geographical order, according to destination. Alternative routes to the same destination are identified by adding -1, -2, etc. to the route letter. The exit (or finish) of the route will be shown on the control table. · If there is more than one type of route from a . signal, then the type is indicated by a distinguishing letter in brackets after the number, i.e. (M) or (S). These routes should already be defined on the signalling plan. EXAMPLES (Refer with respect to Figure 1) 3. MAIN ROUTES A main route proves that the section from one running signal to the next is clear and in addition proves that the overlap beyond is also clear. The overlap will be marked on the plan. Determination of the length of the overlap has already been dealt with in the "Signalling a Layout" section. The overlap, and the controls associated with it will vary according to whether or not trainstops are fitted. Signal controls are required at two distinct levels. The interlocking controls are those conditions which must be present to allow a route to be set. The aspect controls are those which permit the signal to show a proceed indication after the route has been set and locked. 3.1 Setting of Points Assuming a route setting system of interlocking, we must list all the points which will be set and locked when the route is selected. These are interlocking controls. If any of the required points are locked in the position opposite to that specified, the route will fail to set. To set a main route,the points must be lying in the correct position or free to move. The detection of the same points will generally appear in the aspect controls. Trailing points in an overlap are also set, locked and detected in the same way as points in the route. However, facing points in the overlap which give a choice of possible overlaps are not usually locked. This allows the overlap to be "swung", for example when setting the route on ahead from the exit signal. 3.2 Locking of Opposing Routes The interlocking controls require that no opposing routes have been set. Where a signal has more than one type of route, the setting of one route will lock out any other to the same destination. All these routes will be listed under "routes normal" in the signal control table. An opposing route may pass through all or any part of the route to be set, or its overlap. 3.3 Track Circuits All track circuits in the route (and to the end of the overlap) are proved clear in the signal aspect controls. The first track in the route places the signal to danger and prevents it from reclearing automatically. It is designated as the Lever Stick track. Track circuits are NOT proved clear in the route setting controls, in order to allow the signalman to reoperate the route for a following train without having to wait for the first train to clear the section. 3.4 Displayed Aspect All the aspect controls described so far will permit the signal to display its most restrictive main running aspect for that route (including Low speed where applicable). Clearance to other less restrictive aspects will generally depend on the signal ahead, as shown in the "further aspects" line of the signal controls. Examples (Refer with respect to Figure 1 ) 3.5 Foul Track Circuits When a track circuit is not in the direct line of a signal route (or overlap), but one of its extremities is within the required clearance point for that route, then the track circuit is said to be foul of the route. It is unsafe to allow a train to proceed if any foul track circuit is occupied. Most foul track circuits can be included in the point controls to prevent the points aligning for the foul route. If this is not possible, the foul track circuit should be proved clear in the signal aspect controls. If the foul track is conditional on the position of other points, it may be possible to set those points to the position which excludes the foul track circuit. This should only be done if it does not restrict the setting of other routes. The two ends of a crossover are an example of excluding a foul track by the setting of points. Examples (Refer with respect to Figure 1 ) 3.6 Flank Protection Flank protection is the proving of additional track circuits and/or setting of points to protect the route from irregular converging movements, i.e. to protect the flanks of the route. Diversionary flank protection by points not in the route should only be provided if it can be achieved simply, and without inhibiting other valid routes. The proof of track circuits clear from the line of route back along each converging route as far as the first protecting signal provides protection against overruns. It also increases the impact of track circuit failures and can be restrictive on some other legitimate moves. It also provides no protection at the time it is most needed - after the train has passed the signal. An example would be the inclusion of 10A track in No. 8 signal aspect. This type of flank protection is not now generally favoured. Setting and locking the points in the converging line to a position which will divert an unauthorised converging movement away from the legitimate route is simpler and more effective. This protection should be provided where it does not restrict other permissible traffic movements. The most important use of this protection is for the setting of trap (catch) points protecting siding or lines which are unsignalled or where vehicles may be stabled unattended. Examples (Refer with respect to Figure 1 ) 3.7 Junction Signalling Where a purely speed signalling system is used junction signalling is handled simply by the use of appropriate aspect corresponding to the required reduction in speed. The driver will be given advance warning that he is to take a slower speed route by the display of an appropriate aspect at the signal in rear of the junction signal. Information on the precise route is not essential. TfNSW (Transport For New South Wales) signal aspects do not imply a specific speed but the usual practice is to display a medium caution at the signal in rear of a junction signal showing any turnout aspect. This would be a pulsating yellow for single light signals and green over yellow for double light signals. The junction signal itself will display a caution turnout aspect if the signal ahead (the next signal after the junction) is at stop or a medium turnout if the next signal ahead shows a caution or other less restrictive aspect. Examples (Refer with respect to Figure 1 ) Other route signalling systems may not provide any indication of the turnout route at the previous signal. In this case, the application of approach control to the junction signal is used to regulate trains to the appropriate speed for the turnout route. The approach release may be either from stop or from caution, depending on·the turnout speed. There are two potential problems with this method of junction signalling. a) If the sighting of the junction signal is restricted, trains speed may be reduced to a lower speed than the turnout speed. b) If trains of widely differing speed and braking characteristics use the line, approach release from stop may be too restrictive to slower trains but approach release from caution may permit faster trains to take the turnout at excessive speed. The BR system of junction signalling now employs additional aspects to give an advance indication of the junction at the signal in rear, especially where the line is used by trains with widely differing braking characteristics. The junction signal will release to a less restrictive aspect when the train is within sighting distance of the junction indicator on the junction signal. This control may use an existing conveniently placed track circuit or, if no suitable joint exists, timed occupation of the track circuit. Additional tracks are not provided for the specific purpose of approach control - the provision of a timer is considerably cheaper than a set of insulated joints. This also avoids drivers regularly anticipating the signal clearing when the train reaches a particular block-joint. It will also provide an earlier release for a slower train - this could be of benefit to heavy freight trains where it may take several seconds for brakes to release. 3.8 Trainstops TfNSW started rolling out ATP with ETCS levels ,Trainstops are decommisinoned ,but for the sake of learning its mentioned here Where trainstops are provided, the signal control tables must show their operation in association with the signal. The trainstop associated with a main signal will be lowered when all aspect controls are present and proved down before the signal shows a proceed aspect. On replacement of the signal to danger, the trainstop is proved to have returned to the raised position before another route can be set up to the signal. On reversible lines, trainstops within the route intended for the opposite direction of traffic must be lowered (trainstop suppression) until the rear of the train has passed. Controls must also be shown for intermediate trainstops, not located at a signal. The lowering of such trainstops will normally be permitted by the signal ahead showing a proceed aspect or the timed occupation of the approach track circuit. 4. SHUNT ROUTES Shunt routes are used for shunting and other slow-speed movements, e.g. into or out of sidings or for movements when the route ahead is not necessarily clear. Any route to or from a shunt signal (including limit of Shunt signals) is classed as a shunt route. A shunt route from a signal which can also display main route is set with an alternative entrance button on the panel. A dwarf colour light or position light signal displaying a yellow aspect is given for shunt routes. Stencil route indicators are generally provided for shunt routes if there is more than one route from the signal. A route indicator is always provided for a wrong road movement. Where a route indicator is provided, it is not usually proved alight as all routes must be indicated. Where a shunt route is set from a main signal, a subsidiary yellow light is provided below the main signal aspect. This may also have a route indicator as above. Shunt routes prove all points in the route, but do not necessarily prove track circuits clear. The first track circuit in the route is normally included as this provides the lever stick feature. Shunt routes are normally provided with an overlap of 100 metres clear of other fouling moves. Unlike main routes, however, track circuits are not proved clear. Wherever possible, this overlap is achieved by setting and locking trailing points. TfNSW does not provide shunting or subsidiary signals with "last wheel" replacement. Signal replacement is always by occupation of the lever stick track(Birth Track) . Examples (Refer with respect to Figure 1 ) When signalling trains into partially occupied station platforms, TfNSW does not require the train length to be measured before clearance of the signal. Where a preset or facing shunt is situated in a main route reference should be made to this in both control tables. The main route should be shown as setting the shunt signal and proving it cleared while the shunt route should be shown as being preset by the main route. 5. APPROACH LOCKING Once a signal has been cleared, the route must not be released if the signal is restored to danger in front of the approaching train, unless certain safeguards are taken. This is enforced by applying Approach locking to the route. In simple cases, a signal will become approach locked as soon as it has been cleared. However, in most cases this arrangement would cause unacceptable delay, so it is arranged that the interlocking looks back for an approaching train and the signal will only become approach locked when it is cleared AND a train is detected occupying an approach track. If no train is detected approaching, the route will release immediately the signal is replaced to danger. This is called "comprehensive approach locking". Once a signal has become approach locked, the route can only be released by either: a) The train entering the route (it then locks the remainder of the route by its presence.) b) A suitable time delay sufficient to ensure that an approaching train has either come to a stand or entered the route (in which case the remainder of the route will be held by route locking). 5.1. Release by Train Entering Route The passage of a train into the route is proven by the simultaneous occupation of the two track circuits immediately beyond the signal in conjunction with proving of the power off timer. The power off timer proving provides a delay in operation of approach locking releases following a power supply failure which would cause track circuit repeat relays to drop. This safeguards against a false release being given when power is restored. Other systems often enhance this approach lock release by proving sequential operation of track circuits usually the 1st and 2nd tracks past the signal occupied together followed by the clearing of the 1st track. This is done to ensure that a block joint failure between the two tracks concerned, causing both tracks to drop, does not give a premature release of the approach locking. A further safeguard is often included by providing a stick feature on the approach tracks such that once the approach locking tracks have been occupied by the approaching train and the approach locking has become effective, then the locking cannot be released by clearance of the approach tracks. This ensures that the approach locking is not released by a track circuit picking under a train due to bad rail conditions or poorly adjusted track circuits etc. 5.2. Release After Time Delay The delay applied before a route is freed if the train does not proceed through the route is intended to give time for the approaching train to be brought under control. For a main aspect the release time is generally 120 seconds and 60 seconds for a shunt signal. However each case should be considered carefully as in some cases a different time may be required. For example, 120 seconds for terminal platform starting signals may be unnecessarily restrictive and where the distance back to ' the previous signal is long 120 seconds may be insufficient to prove that the train has been brought under control. The same approach locking is usually applied to all routes from a signal, so a subsidiary signal will have the same time delay as its associated main aspect. This is a purely practical consideration to avoid the expense of providing more than one approach lock timer on any signal. This need not be the case with Computer Based lnterlockings as no separate hardware is necessary. 5.3. Operation of Comprehensive Approach Locking This becomes effective when the driver of an approaching train is likely to have seen a proceed aspect on the signal being replaced, or has seen aspects of previous signals which would lead him to expect to see a proceed aspect on that signal. Comprehensive approach locking looks back at the approach track circuits. For shunt signals it is usually only necessary to look back at the berth track circuit, provided it is at least 200m long. However, for running signals it is necessary to look back at least as far as the sighting point of the earliest signal whose aspect is affected by the replacement. For signals with only a caution aspect in rear this will involve looking back at the section in rear of the previous signal, but where a signal has a medium caution and caution aspects in rear this will involve looking back at the sections in rear of the previous two signals. Often these approach tracks will be conditional on one or both of the following factors:- a) A train being routed up to the replaced signal. This will depend on the lie of points. Facing points will exclude the approach if set for another route. b) The signals approaching the replaced signal displaying a proceed aspect. If the previous signal is at red (and free of approach locking) then it is not necessary to look back at the section in rear of it. c) The existence of the appropriate track circuits. Signals reading out of non-track circuited sidings will not have any approach track circuits. These signals will therefore be approach locked when cleared. As there is no means of detecting an approaching train it must be assumed, for safe operation, that a train is always approaching. Route Normalisation Route normalisation is provided at certain interlockings to release the route automatically. It operates once the train bas passed the signal and the approach locking bas released, provided that no other train is approaching the signal. Such a facility reduces the signalman's workload by removing the need to cancel most routes after use. If any form of automatic route setting is introduced, automatic normalisation must be provided on all associated routes. As the normalisation of the route will release the interlocking on other routes and points, it is important that safeguards are provided against the momentary irregular operation of a track circuit releasing a route ahead of an approaching train. It is therefore usual to use the same "look back" controls as the comprehensive approach locking to prove, in addition to the train passing the signal by operation of the approach locking tracks, that no train is approaching the signal at the time normalisation is initiated. This condition may need to be relaxed at terminal and other platforms where trains may divide, if Route Normalisation is required to operate for the first half of the train to depart. One option is to include additional sequential track circuit operation beyond the normal approach lock release tracks as an alternative to the "no train approaching" condition. Where there is an automatic signal in rear, then the "look back" controls may detect a second train approaching behind the automatic signal, and so inhibit the route from releasing behind the first train. In this case the look back controls behind the first automatic signal are suppressed, once the automatic signal has been replaced by the passage of each train. POINT CONTROLS 6.1 Point Setting Point Control Tables contain the conditions which allow points to be operated and locked normal or reverse. The "Set and Locked by Routes" entry area contains all routes that require the setting and locking of the points, whether they be in the route, in the overlap, or providing flank protection. An important check in the preparation of control tables is that the routes appearing in this portion of the table include the setting of the points in their own route control tables. 6.2 Track Controls on Points The "Tracks Locking" entry area is self explanatory for most cases. However, there are examples where more than the obvious track circuits are required. Foul tracks must be included where necessary, to protect moves over the points. By proving the foul track clear in order to move the points away from it, this will prevent a fouling route being set. Examples (Refer with respect to Figure 1 ) 7. ROUTE LOCKING Once a train has passed a signal, its route can be restored but any points, opposing signals etc. ahead of the train must remain locked. This is done by the "route locking", which is indicated by the line of white lights on the signalman's panel. On the control tables, the route locking of opposing routes and the route locking of points is shown separately (in the route and point control tables respectively). Some railways prefer to produce separate control tables for route locking. 7.1 Route Locking of Signals Directly opposing signals with no others intervening will be locked against each other and the releasing of this locking must not occur until the train has Cleared all track circuits between the signals. Where an intervening signal occurs, the opposing signals are again locked against each other, the locking being held until the train has cleared all track circuits in the route. Indirectly locked signals may also require route locking. Where opposing signals do not require direct locking because they require points in opposite positions, route locking may be required if the relevant points are released as a train proceeds through the route. This locking is applied to the route setting (interlocking) controls, regardless of tracks proved dear in the signal controls, to prevent preselection of a route. No example of this situation appears on Fig 1 but if we assume that 7A track is to be divided into separate tracks over the two point ends, indirect route locking would be needed to prevent 9B route from setting after a train had passed 6 signal and cJeared the track locking 103 points. Examples (Refer with respect to Figure 1 ) Opinion varies as to whether it is best to show route locking between opposing routes on the signal controls of the route which applies the route locking or the route which is locked by the route locking. It is more consistent with other controls to show a route as being locked by the route locking of another route and listing the track circuits required for its release. This is current TfNSW practice. From the viewpoint of the tester, however, it is often more convenient to show the route locking which is applied by that route with its signal controls, as the contents of the whole sheet can normally be tested at the same time. 7.2 Route Locking or Shunt Routes Route locking must always be provided where main routes lock each other or a shunt route locks a main route. However, as many shunting movements take place into occupied sections, it is impractical to apply the same conditions to many shunt routes. The options available are:- a) Do not provide route locking between shunt routes where it is possible that a shunt movement into a section from one direction will be followed by another in the opposite direction to attach locomotives or vehicles. b) Provide the route locking as for the main routes but provide an additional release by the timed occupation of one or more track circuits after the first movement has come to a stand. TfNSW practice is normally not to provide route locking between opposing shunt signals. The signalman is responsible for ensuring the first movement has come to a stand before setting the opposing route. If a timed release was provided it would take the form shown in the table below. Examples (Refer with respect to Figure 1 ) 7.3 Route Locking or Points in the Route Points are locked by setting a route through them. However, once a train has passed the signal at the entrance to the route, that signal's approach locking will be released, which potentially would release all locking on the points ahead. Route locking is therefore applied to the points to maintain the locking until the train has passed clear of the points. Points which are situated in the overlap of a route will have there route locking released when the train is proved at a stand at the exit signal in a similar manner to the timed release of shunt route locking described above. Route locking of points is normally shown on the points.control sheets. Examples (Refer with respect to Figure 1 ) 8. OVERLAPS As previously described main signal controls include an overlap beyond the exit signal. The length of this overlap will depend on the type of signalling, the speed of traffic, whether or not trainstops are fitted and whether or not the signal in rear has been conditionally cleared. With overlaps up to 500 metres in length, the controls associated with overlaps create considerable additional complexity in the specification of controls. The general rules for overlap controls for signals are summarised below. a) Facing points in the overlap are generally left as they are lying. Exceptions to this are where there is no route forward from the exit signal in that direction, flank protection from sidings etc. would be unnecessarily removed, or the overlap in that direction is locked by a route already set but another overlap is free. Facing points are not generally locked as they may be required to move to set the next route forward or to change to an alternative overlap (swing the overlap) due to the setting of another route. b) Trailing points must be set to complete the overlap according to the lie of any previous facing points (in the direction of travel). Trailing points will be locked unless the overlap is swung clear of them. c) Track circuits in the selected overlap will be proved clear in the signal aspect controls. Examples (Refer with respect to Figure 1 ) 8.1 Overlap Point Route Locking Trailing Points in overlaps are set, locked and detected and then route locked in all cases. A timed release of the route locking upon the occupation of the protecting signal's berth track allows the points to be moved once the approaching train has c.ome to a stand. Usually facing points may lie either normal or reverse in an overlap. If the facing points in an overlap must be locked in one position only, because the overlap to which they would lead is not permitted, then the locking on those points is the same as for trailing points. The time of track occupation to release the overlap locking will be dependent on the braking performance of trains and the length of the exit signal berth track circuit. Examples (Refer with respect to Figure 1 ) 8.2 Time of Operation Locking Because facing points in an overlap are not locked, there is a danger that a train over-running into the overlap could reach points close to the signal while they are moving. Some railways attempt to reduce this risk by preventing the points from moving if the exit signal berth track is occupied. This locking would release when the train has timed to a stand. This is often called "Time of Operation" locking. The maximum distance between the first block joint and the facing points for which time of operation locking will vary in practice according to the type of point operation. British practice uses a minimum distance of 20 metres, extended where necessary up to approximately 50 metres. 8.3 Swinging Overlaps Where facing points in the overlap can lie in either position creating alternative overlaps, then this is called a swinging overlap. Special controls must be provided to ensure that once a route has been set, it will always have an overlap. The facing points which select the alternative overlaps are generally known as the hinge points. Where the overlap beyond the hinge points contains further points, then the locking of the hinge points is dependant on the state of the other points beyond them in the overlap. Generally if the points beyond the hinge points are lying in the correct position for the overlap or are free to move to the correct position, then they will be called and locked in the required position. If however the points beyond the hinge points are locked in the wrong position for the overlap by some other control then the hinge points will be called to the alternative overlap position. This indicated by an order of preference note in the control tables or by a note indicating the control being dependant on points being locked. The route locking of the points beyond the hinge points is standard for trailing points in overlaps, except that it is conditional on the lie of the hinge points. Examples (Refer with respect to Figure 1 ) 8.4 Overlap Maintenance Once the entrance signal to the route bas cleared overlap maintenance locking is applied to the hinge points to ensure that the points cannot swing to an occupied alternative overlap. This locking is maintained by route locking until the train is timed to a stand on the berth track of the exit signal. During the time that the overlap is in use, the facing points can only be moved provided that the alternative overlap is available. Examples (Refer with respect to Figure 1 ) 8.5 Swinging by conflicting Routes Routes which conflict with an overlap which is already set will set, lock and detect the hinge points to a position giving a non conflicting overlap, even though the conflicting route would not normally set the hinge points. This would also apply for two overlaps which conflict as well as a route conflicting with an overlap. The setting of points is generally done in sequence, because the hinge points must be moved to the new overlap before the points at the point of confliction can be released for the second route. In other words, the order of setting must ensure that before the first overlap can be released (by swinging the hinge points), the alternative overlap bas been established. Examples (Refer with respect to Figure 1 ) In the case of 111 points above, route locking from 19 signal extends only to 28HT because beyond this point overlap maintenance becomes affective. In the case of 103 points above, the locking shown would be a first preference. The second choice would be to swing 112 normal if 103 were locked normal. 8.6 Setting by Track Circuit Conditions Sometimes the hinge points are swung to an alternative overlap position by the occupancy of track circuits within the overlap. This is shown in the 'Set Only by Routes' entry area, with the locking being maintained by the overlap maintenance condition. However care must be taken to ensure that this does not give rise to a restrictive condition, locking out other routes. Each case needs to examined carefully on its operational merits and its affect on other routes. EXAMPLES (Refer with respect to Figure 1) If 121 points are lying reverse and 25Ff is occupied by a previous train then calling 121 points normal when 15(M) is set would successfully swing the overlap up to 31 signal and allow 15 signal to clear. However if 25Ff remains occupied then 121 points will be locked normal by the overlap maintenance until the train has. come to a stand at 25 signal. During this time, the routes from 52 and 24 signal will be locked. It may have been more desirable in this case to hold the train at 15 signal and allow movements from the branch instead. In the case of 7(M)A route swinging 111 points normal if 15CT, 25AT or (25BT w 121N) were occupied would not adversely affect other movements as the new overlap does not conflict with any other routes or overlaps. Route Table Entries Point Table Entries.
Read Full Article- Posted 3 years ago
CH27 | Introduction to ERTMS
Chapter 1 1.What is ERTMS ? The European Railway Traffic Management System (ERTMS) is a major Train Control development to enhance cross-border interoperability through Europe by creating a single standard for railway signalling. More than 20 different national signalling and speed control systems exist in the European rail system, with each one incompatible with the other. It creates an obstacle to the free flow of rail traffic across Europe. Imagine the difficulty in operating a train with different signalling rules ,signals , even with two different systems you turn off one onboard computer and turn on another and change driver with route knowledge each time you cross the border! Thalys high-speed train, which connects Paris, Brussels, Cologne and Amsterdam, is equipped with a minimum of seven different systems for its cross-border operation. The ongoing deployment of a single common system, such as the ERTMS, aims at removing this technical barrier to cross-border passenger and freight movement. The ERTMS also saves maintenance costs, improves safety and increases traffic capacity. It reduces the headway between trains and allows for a maximum speed up to 500km/h. It increases the current capacity by up to 40% without the need for any infrastructure upgrades. The project is being implemented by eight UNIFE (the Association of the European Rail Industry) and the GSM-R (Global System for Mobile Communications – Railway) industry. The UNIFE members include Alstom Transport, Ansaldo STS, AZD Praha, Bombardier Transportation, Invensys Rail, Mermec, Siemens Mobility, and Thales. In nutshell The European Rail Traffic Management System defined a standard for safety, signaling and communication. Throughout Europe, ERTMS is getting implemented on all high speed lines, specific transit corridors and eventually on all railways. ERTMS as a definition consists of all aspects considering management, safety and communication in the whole railway system. Subsets of ERTMS are ETCS, which stands for European Train Control System and GSM-R, a radio communication system. Note:GSMR is getting obsolete and Long Term Evolution (LTE),otherwise known as 4G/5G is considered as the alternate communication medium.ETCS includes all systems necessary for both drivers’ cab and track signaling. In some publications ETML (European Track Management Layer) and INESS (Interlocking Safety System) are also counted to belong to ERTMS? 2. ERTMS Principles ERTMS’ concept is based on standardization of data exchange in traffic management. This can best be compared with the operational methods in aviation. Aircraft can always land on every international airport due to exchange of standard data then the use of standard systems and procedures for takeoff and landing. Like in aviation, trains should be able to connect stations, regardless the country they are in, crossing over having their destination. This principle however does not implicate automatically that all trains have exactly the same systems on board. Airplanes don’t have that either by the way. They only things really standardized in aviation are communication procedures and specifications of communication equipment. Communication in this perspective means both communication between the train and equipment alongside the track, as well as between train and traffic control centre. Differences between certain interfaces showing information to the driver are still possible. Track – Train communication This is where ETCS comes in. ETCS states specifications for track – train communication, consisting of several systems: • Eurobalise: standardized beacon, acting as an interface between track and train; Note : Balise is the French word for Beacon /Transponder. • Eurocab: standardized cab equipment for the use of ETCS, replenished with specific transmission modules (STM), facilitating existing network specific safety systems. • Euroradio: GSM-R is gsm for railways. GSM-R is used for communication, data exchange and – on longer term – positioning; • Euroloop: facility of loops, making rail guided data exchange possible. Trains equipped with the Eurocab always have STM available, as this interface makes it possible to operate on non-ETCS lines as well, then using the traditional safety system. ERTMS can be implemented on new railways as well as on existing lines. To accomplish compatibility three different levels were developed. 3.ERTMS Levels ERTMS consists of three different Levels Note : (Intermediate Levels are not considered which will be covered in detail ) •Level 1 :Continuous Track-Train Communications; •Level 2 :Continuous Train-Radio block centre Communications; •Level 3 :Moving Block Technology (Almost like CBTC used in urban lines,but some features like Train regulation missing in Level 3 ) System upgrades are possible by expanding components, meeting specifications of the next level. Level 1 ERTMS level 1 is connected with traditional lines equipped with trackside signals and detections. Communications are ensured by means of beacons (“balise”) located adjacent to signals alongside the tracks. As soon as a movement authority is received trough a beacon, the ETCS on board equipment calculates the permitted speed ,taking the characteristics of the train into account .The Trains speed is under continuous monitoring by the system ERTMS level 1 can relatively easy to make traditional lines interoperable, ensuring automatoic braking when maximum speed is exceeded. Level 2 ERTMS level 2 does not require signals along the tracks. Movement authority is therefore not necessarily communicated by a fixed signal, but by means of a radio block centre (RBC) to the on board ETCS unit which is equipped with GSM-R The inline beacons communicate data like location, limits on speed, gradients on the way etc .The information about data and signal ahead is shown to the driver on a cab display. Level 2 counts with a braking distance factor, to ensure there is an adequate braking path available. The removal of trackside fixed signals and replacement by beacons can raise the capacity of a line significantly. Level 3 Level 3 allows “moving block technology” to come in. Whilst level 1 and 2 sill operate with fixed sections and thus limiting use of a block to one train at the time, level 3 supplies the control centre of continuous information about the train’s position. In this level, the train itself is to be regarded as a “moving block” and is a fully radio based system without any trackside equipment. The Radio Block Centre (RBC) receives positioning of each train continuously and calculates smallest possible train distances at any time. Therefor the track is no longer separated in fixed blocks but split into “moving blocks”. At the same time it is vital that trains guarantee their integrity as there is no trackside equipment available to provide this information. ETCS L3 is not yet matured at this time of preparation 4.ERTMS Technology With ERTMS beacons (“balise”) are used for data transfer. Information transfer In level 1 there is communication from track to train only. The beacons transfer information to the trains, containing: • the beacon’s identification; • distance remaining to next point of speed changing; • target speed of train (in relation to braking path etc.); • validity of the information. In level 2 communication is two way: from track to train and reversed. The beacon’s signals are the same as in level 1, but in level 2 the train communicates about data too. In fact it sends information to the radio block centre (RBC) by means of GSM-R. In L-3, which is still in conceptual stage, is based on moving block technology. It involves use of special equipment within the train to continuously supply data on the train’s position to the control centre, rather than by track based detection equipment. The train thus continuously monitors its own position. Transition When a train moves from a traditional line to ERTMS-covered tracks an entry beacon is used to register with the RBC. Specific information about the train – length, weight, braking path etc) is being sent with the registration.
Read Full Article- Posted 2 years ago
Neutral Zone and Its impact for Signalling and Rollingstock
Neutral Zone and Its impact for Signalling and Rollingstock General AC electrified railway system is one in which single phase electrical energy is supplied to trains by means of an Overhead Line Equipment (OLE) system, comprising of a contact wire supported by droppers from a catenary wire and associated support and registration equipment. Depots commonly incorporate a trolley wire system, consisting of a single auto-tensioned contact wire without catenary. Lets consider 25kV AC traction system for the purpose of this study. The system is energised at a nominal system voltage of 25kV 50Hz AC 132/25kV AC TRACTION POWER SUPPLY SYSTEM. On the 25kV side, each transformer could be connected to a double pole 25kV isolating switch / earth switch, from which one conductor is connected by 25kV cable to the adjacent 25kV Feeder Station and onward to the 25kV AC OLE. The other terminal could be connected via the 25kV Feeder Station return current busbar to the track running rails and return conductors (where installed) to create a multiple earthed system. Under normal operating conditions the voltage on the low voltage side may rise to 27.5kV phase to earth, this being an equivalent voltage to that of a 48kV phase to phase three phase system. To separate the electrical phases at each of the Feeder Stations, neutral sections are installed in the OLE. The pantograph traverses the neutral section in the power off state. Sectioning of the OLE is achieved by Track Sectioning Cabins (TSC), insulated overlaps and section insulators. Current collection by trains is obtained by means of a pantograph mounted on the roof of the rolling stock. The pantograph head runs on the underside of the contact wire to achieve a smooth, arc* free current collection. Note: *- Flow of current through an air gap between a contact strip and a contact wire usually indicated by the emission of intense light and heat. Traction current drawn from the overhead contact/catenary wires is returned to the supply point through the traction rails and return conductors. The following are some of the traction return systems utilised: Booster transformer with return conductors; Return conductor only (booster-less); and Rail/earth wire return. An earth wire electrically connects each overhead line structure. The earth wire is connected to the traction rail at prescribed intervals. Where structures cannot be conveniently connected to the earth wire each structure is connected directly to the traction rail by means of a traction bond. 2. Definition of Neutral Section Section of a contact line provided with a sectioning point at each end, to prevent successive electrical sections with a differing phase, being connected directly together by the passage of current collectors. The neutral section is a dead zone and therefore, the locomotive has to negotiate the section in momentum. In nutshell neutral zones are made to achieve the isolation between different power sources and to minimise the synchronisation task in between individual power system. Figure 1: Neutral Section The locomotive is switched off while negotiating the neutral section to avoid flash over at the time of exit and re-entering the live zone. For this track magnets can be utilized to switch off and ON for unattended operations. Some railways use warning board to enable driver to switch off and on. The locomotive negotiates the neutral section in its own momentum. Therefore, the location is always chosen so that the physical terrain should not cause in convenience in the momentum of the train. OLE designers ensure neutral zones are placed away from stopping signal and level crossing and automated trains ensure trains are not stopped in neutral zone, and train can coast through the neutral section. Design also ensure neutral zones are placed in the up gradient but on a flat or down gradient and away from sharp curve as it will not provide sufficient straight length to accommodate the neutral section. 3. Neutral Section Detection When the train run into neutral section where there is no high voltage power supply is available without any precaution, a sudden disconnection of high voltage power supply may disturb the incoming power system and as well as the equipments of rolling stock. A detection method is required to detect the neutral section before entering it, to smoothly negotiate by managing various loads. So, in order to protect the train from the undesired arc between Pantograph and Over Head Catenary line, it is desired that when the train passes through the neutral section the traction power is automatically cut-off via VCB before entering the neutral section and is automatically connected back after passing the neutral section as stated before. This requirement is achieved by the Neutral Section Detection System. The Neutral Section Detection System can detect the marking of the neutral section and can inform onboard Train Control and Monitoring System (TCMS) to minimise the traction load and to open the vacuum Circuit Breaker(VCB). This system can also control the VCB in case of absence of TCMS. To satisfy the power control requirement when passing over the neutral zone, there are two components required. First component is to be situated on the track side which gives indication about the incoming neutral section to the train. Second component is underframe mounted train borne equipment, which receives the indication signal sent by the track side equipment. The generic location of train borne receiver in TP car and track magnet location on the either side of the track. This system is referred to as “Automatic Power Control (APC)” to identify the neutral section. 4. System Composition The APC system comprises of two components: the track side equipment (Inductors or Track side Magnets) and train borne equipment (APC Receiver or NSD Antenna). The track side equipment is a means of providing south polarity magnetic field of the required strength and pattern at the required height, location. The train borne equipment is a means of detecting the magnetic fields of the track side equipment. 4.1 Inductor (Trackside Magnet) The inductor (Track side Magnet) is installed on the one side of the track, at both end of the neutral section. The South polarity face of the magnet (Inductor) is in sky facing position while North polarity of the magnet (Inductor) is in earth facing position. 4.1.1 Location of APC Trackside Magnet Automatic Power Control (APC) track side magnets shall be situated each side of neutral sections. The distance (D) from the centre of the neutral section to the APC magnet shall be calculated as follows. 4.2 Receiver (Train Mounted APC receiver) The APC receiver is mounted below the underframe of the train, there is one receiver per each car with pantograph on the same side of the trackside magnet & train. The train mounted APC receiver is fixed to the under frame. Whenever train passes through the desired magnetic field produced by the track magnet which are mounted on the railway track, the APC receiver detects the magnetic field and sends feedback to TCMS to control the VCB. Refer the Figure 2 you can see APC receiver mounted under the car where pantograph is installed and Track magnets installed on the track Figure 2: APC Receiver and Track magnet 4.3 Method of Operation Refer to Figure 2. Red dots are the two APC receiver mounter under the car frame. APC receivers are mounted on the same car where Pantograph & auxiliary power equipment are installed. Two red rectangular boxes are the track magnet installed on track at same side as the APC receiver. One of them will be at the entry of the neutral section and other is on the exit of the neutral section. While the train reaching the neutral section either driver will put the master handle in coasting position, or the system will put in coasting position for an unattended train. APC receiver is positioned in original status with South contact opened and neutral section relay in the train will be on dropped position. When APC receiver detect the first track magnet, a signal will be sent to the activate neutral section relay, Train Control Management System detect the APC receivers high signal and send a signal to the Train Control Unit to ramp down the tractive effect. Same thing happens when it detects the second magnet. Once the train passes the neutral section area after a pre set time Train Control Management system resets APC receiver. 4.4 Interface between Rollingstock and Overhead Electrification Team Horizontal distance between the centre of pantograph and the APC receiver is known as L AR which shall be obtained from the Rollingstock supplier to do the assessment with various speeds and scenarios to ensure train can coast through the neutral zone. 4.5 Interface between Rollingstock, Overhead Electrification& Signalling Team All three discipline shall gather necessary information for their discipline to ensure that no train will be stuck on the neutral section with all scenarios.
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one year ago
Introduction to CBTC
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one year ago
CRRC, short for China Railway Rolling Stock Corporation, is one of the largest and most influential rolling stock manufacturers in the world. It is a state-owned enterprise based in Beijing, China. The company was formed in 2015 through the merger of China CNR Corporation and CSR Corporation Limited, two major rolling stock manufacturers in China.
CRRC specializes in the design, development, production, and maintenance of a wide range of railway transportation equipment, including high-speed trains, locomotives, metro trains, light rail vehicles, electric multiple units, and other related components. The company has a comprehensive product portfolio that caters to both domestic and international markets.
With its extensive capabilities and expertise, CRRC has become a major player in the global rail transportation industry. It has established a strong presence in both the domestic and international markets, supplying railway equipment to over 100 countries and regions around the world. CRRC's products are known for their advanced technology, high reliability, and energy efficiency.
The company has achieved significant milestones in the development of high-speed rail technology. It has been at the forefront of China's ambitious high-speed rail network expansion, which is the largest and most extensive in the world. CRRC's high-speed trains, such as the Fuxing series, have set world records in terms of speed and have become symbols of China's technological prowess in the rail sector.
In addition to its focus on high-speed rail, CRRC also produces various types of rolling stock for urban transit systems. It has supplied metro trains and light rail vehicles to cities worldwide, contributing to the development of efficient and sustainable urban transportation systems.
CRRC places great emphasis on research and development (R&D) activities. The company invests a significant portion of its revenue in R&D to continuously enhance its product offerings and technological capabilities. It has established several research institutes and collaboration platforms to promote innovation and technological advancement in the rail industry.
While CRRC has experienced remarkable success, it has also faced some challenges and controversies. One major concern is its dominance in the global rolling stock market, which has raised issues related to competition and fair market practices. Some countries have expressed concerns about security risks associated with using CRRC's technology in their rail infrastructure.
In conclusion, CRRC is a prominent player in the global rail transportation industry. It has established itself as a leading manufacturer of rolling stock, with a strong presence in both domestic and international markets. The company's focus on technological innovation and its contribution to the development of high-speed rail and urban transit systems have positioned it as a key player in shaping the future of rail transportation.
2 years ago
We would like to share some exciting upcoming rail projects in the Middle East that are set to make a significant impact on transportation in the region:
Project 1: Etihad Railway Project Phase-3
Location: Abu Dhabi, United Arab Emirates
Description: This project involves the construction of a railway network to enhance connectivity in the region.
Project 2: GCC Railway Project
Location: Kingdom of Saudi Arabia
Description: The GCC Railway Project aims to construct a railway transportation system spanning 117km to connect the Gulf Cooperation Council (GCC) countries.
Project 3: The High-Speed Railway Project - The Line (NEOM)
Location: Kingdom of Saudi Arabia
Description: This project focuses on the construction of a high-speed rail network, providing efficient transportation within the region.
Project 4: Bahrain Metro
Location: Bahrain
Description: The Bahrain Metro project involves the construction of a 105km light rail system to improve public transportation in the country.
Project 5: Oman-UAE Railway Link
Location: Oman
Description: This project aims to construct a rail link connecting Abu Dhabi and Sohar, promoting trade and connectivity between the two locations.
These projects showcase the commitment of Middle Eastern countries to developing advanced transportation infrastructure. Stay tuned for more updates on these transformative initiatives.
2 years ago
There are several major types of manufacturing technologies used in the production of rolling stock wheels. Here are a few prominent ones:
- Cast Iron Wheels: Cast iron wheels are manufactured through the process of casting molten iron into a mold. This method has been widely used for many years and is known for its durability and cost-effectiveness. Cast iron wheels are commonly used in freight cars and locomotives.
- Steel Disc Wheels: Steel disc wheels are made by cutting or stamping steel plates into the desired shape and then welding them together. This manufacturing process provides a strong and reliable wheel. Steel disc wheels are commonly used in passenger trains and high-speed rail applications.
- Monobloc Wheels: Monobloc wheels are manufactured from a single piece of material, usually steel or aluminum alloy. This type of wheel offers improved strength and performance compared to cast iron or steel disc wheels. Monobloc wheels are commonly used in high-speed trains and light rail systems.
- Composite Wheels: Composite wheels are made by combining different materials, such as steel and composite materials, to create a lightweight and durable wheel. These wheels offer reduced weight and improved energy efficiency. Composite wheels are commonly used in modern light rail systems and some high-speed trains.
- Forged Wheels: Forged wheels are produced by applying high heat and pressure to shape and form metal into the desired wheel structure. This manufacturing process results in a strong and reliable wheel with excellent fatigue resistance. Forged wheels are commonly used in heavy-duty applications, such as freight cars and locomotives.
- Machined Wheels: Machined wheels are produced by machining a solid block of material, typically steel, to the desired wheel shape. This method provides precise dimensions and surface finishes. Machined wheels are often used in specialized applications that require tight tolerances, such as high-precision measurement or metrology systems.
- Composite-Metal Hybrid Wheels: These wheels combine the advantages of composite materials, such as reduced weight and corrosion resistance, with the strength and durability of metal. Typically, the wheel rim is made of a composite material, while the hub and spokes are made of metal. This hybrid construction is often used in lightweight and high-performance applications, such as racing trains or specialized rolling stock.
- Hollow Spoked Wheels: Hollow spoked wheels are characterized by a central hub connected to the wheel rim by hollow spokes. The hollow design helps reduce weight while maintaining strength and structural integrity. These wheels are commonly used in light rail systems and other applications where weight reduction is a priority.
- Polymer Wheels: Polymer wheels are made from various types of polymers, such as reinforced plastics or elastomers. These wheels offer advantages such as lightweight, low noise, and improved ride comfort. Polymer wheels are commonly used in urban transit systems and other applications that prioritize noise reduction and passenger comfort.
- 3D Printed Wheels: With advancements in additive manufacturing technology, 3D printed wheels have emerged as a viable option. These wheels are produced by layer-by-layer deposition of materials, allowing for complex designs and customization. 3D printed wheels are still in the experimental and developmental stages but show promise for future applications in rolling stock.
Each manufacturing technology has its own advantages and considerations, and the choice of wheel type depends on various factors, including the specific application, load requirements, operating conditions, and cost considerations.
It's important to note that the availability and suitability of specific wheel manufacturing technologies may vary depending on the industry standards, regulations, and specific requirements of the rolling stock application.
2 years ago
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