CH2 | BASIC SIGNALLING PRINCIPLES | PART 2
Signalling
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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