MAJOR PARTS OF E&M WORKS IN RAILWAY BUILDING/METRO STATION OR SUBSTATION CONSTRUCTION

1.    Earthing
2.    Conduiting
3.    Light Point wiring, Power Point
4.    Installation, testing, commissioning of light fixtures
5.    laying of cables, jointing, termination etc.
6.    laying of Cable Tray, HDPE pipe. RCC pipe

Let’s get a short brief about all one by one

EARTHING

The earthing protection is an integral part of any electrical system and is required to 
a.    Protect personnel and equipment from electrical hazards.
b.    Achieve a reduction in potential to the system neutrals.
c.    Reduce or eliminate the effects of electrostatic and electromagnetic interference on the signaling and Telecom equipment arising from auxiliary electrical systems.

The main purpose of earthing in the electrical network is for safety. 

i)    When all metallic parts in electrical equipment are grounded then if the insulation inside the equipment fails there are no dangerous voltages present in the equipment case. 
ii)    To maintain the voltage at any part of an electrical system at a known value to prevent over current or excessive voltage on the appliances or equipment. 
iii)    Lightning, line surges, or unintentional contact with higher voltage lines can cause dangerously high voltages to the electrical distribution system. 

The earthing is broadly divided as

a)    System earthing (Connection between a part of a plant in an operating system like LV neutral of a Power Transformer winding and earth).
b)    Equipment earthing (like motor body, Transformer tank, Switch gearbox, Operating rods of Air brake switches, etc.) to earth.

Earthing provides an alternative path around the electrical system to minimize damages in the system. 
There are several types of earthing systems such as Earth Mat, Plate Earthing & Pipe Earthing which could be used in an elevated station and Substations. 

The selection of earthing depends upon several factors such as:

i)    Availability of Land
ii)    Type of Soil
iii)    Resistivity of Soil

Mainly we follow two Specifications for earthing
I.    IS:3043
II.    IEEE 80

The most commonly used earthing method is Earthmat or Grid 

Earth Mat or Grid

The primary requirement of Earthing is to have a low earth resistance.   Substation involves many Earthlings through individual Electrodes, which will have high resistance.  But if these individual electrodes are interlinked inside the soil, it increases the area in contact with soil and creates a number of parallel paths.  Hence the value of the earth resistance in the inter-linked state which is called combined earth value which will be much lower than the individual value.

The interlink is made through a flat or rod conductor which is called an Earth Mat or Grid.  It keeps the surface of substation equipment as nearly as absolute earth potential as possible.
 

                                                                                          Picture: Earthmat 

To achieve the primary requirement of Earthing system, the Earth Mat should be designed properly by considering the safe limit of Step Potential, Touch Potential, and Transfer Potential.

The factors which influence the Earth Mat design are:

a.    Magnitude of Fault Current
b.    Duration of Fault
c.    Soil Resistivity
d.    The resistivity of Surface Material
e.    Shock Duration
f.    Material of Earth Mat Conductor
g.    Earthing Mat Geometry

Step Potential

It is the potential difference available between the legs while standing on the ground. When a fault occurs at a tower or substation, the current will enter the earth. Based on the distribution of varying resistivity in the soil (typically, a horizontally layered soil is assumed) a corresponding voltage distribution will occur. The voltage drops in the soil surrounding the grounding system can present hazards
for personnel standing in the vicinity of the grounding system. Personnel “stepping”
in the direction of the voltage, gradient could be subjected to hazardous voltages

Touch Potential

Touch potential is the voltage between any two points on a person’s body – hand to hand, shoulder to back, elbow to hip, hand to foot, and so on. The touch potential or touch voltage could be nearly the full voltage across the grounded object if that object is grounded at a point remote from the place where the person is in contact with it .

 

 

 

The earth resistance shall be as low as possible and shall not exceed the following limits:

EHT Substations    -    1.0 Ohms
33KV Stations        -    2.0 Ohms
Metro Stations    -    < 1.0 Ohms

Specification of Earthing

Depending on soil resistivity, the earth conductor (flats) shall be buried at the following depths.

            Soil Resistivity in ohms/metre     Economical depth of Burial in metres

                                                                     1)        50 – 100                                              0.5

                                                                      2)       100 – 400                                            1.0
                                                                      3)        400 – 1000                                         1.5

To keep the earth resistance as low as possible to achieve safe step and touch voltages, an earth mat shall be buried at the above depths below ground and the mat shall be provided with grounding rods at suitable points. All non-current carrying parts at the Substation shall be connected to this grid to ensure that under fault conditions, none of these parts are at a higher potential than the grounding grid.