APPENDIX III

CODE OF PRACTICE

FOR EARTHING OF POWER SUPPLY INSTALLATIONS FOR 25kV, ac, 50Hz, SINGLE PHASE TRACTION SYSTEM

(This is a reproduction of RDSO's CODE No. ET1/PSI/120 (2/91) )


1. Scope

1.1 This code of practice caters for general arrangements of system and equipment earthing at 220/ 25 kV or 132/25 kV or 110/25 kV or 66/25 kV traction substations, 25 kV switching stations, booster transformer stations and auxiliary transformer stations. Low voltage (LT) electrical power distribution system, 25 kV overhead equipment system as well as signal and tele-communication equipment do not come within the purview of this code.

2. Terminology

2.0 The following terms wherever occurring in this code shall, unless excluded or repugnant to the context, have the meaning attributed thereto as follows:-

2.1 Combined Earth Resistance : The resistance of an earth electrode (s) with respect to earth, with the earth electrode (s) connected to the metal work of electrical equipment other than parts which are normally live or carry current and the masts/structures but without connection with the traction rail (s).

2.2 Earth: The conductive mass of the earth, whose electrical potential at any point is conventionally taken as zero.

2.3 Earth electrode: A conductor mild steel (MS) pipe, or group of conductors in intimate contact with and providing an electrical connection to earth.

2.4 Earthing grid: A system of a number of interconnected, horizontal bare conductors buried in the earth, providing a common ground for electrical devices and metallic structures, usually in one specific location.

2.5 Equipment earthing: Earthing of all metal work of electrical equipment other than parts which are normally live or current carrying. This is done to ensure effective operation of the protective gear in the event of leakage through such metal work, the potential of which with respect to neighbouring objects may attain a value which would cause danger to life or risk of fire.

2.6 Mesh Voltage (E mesh): The maximum touch voltage to be found within a mesh of an earthing grid.

2.7 Power supply installation: The electrical equipments and associated structures provided at a Railway Traction Substation or Switching Station, or Booster/ Auxiliary transformer Station on the 25 kV over head equipment.

2.8 System earthing: Earthing done to limit the potential of live conductors with respect to earth to values which the insulation of the system is designed to withstand and thus to ensure the security of the system.

2.9 Step Voltage (E step): The potential difference between two points on the earth's surface separated by a distance of one pace, that will be assumed to be one metre in the direction of maximum potential gradient.

2.10 Traction Rail means a non track circuited rail of a wired track, not required for signalling purposes and which may be earthed. In non-track circuited sections, both the rails of a wired track are traction rails and in single rail circuited section, the traction rail is the non-track circuited rail.

2.11 Touch Voltage (E touch): The potential difference between a grounded metallic structure and a point on the earth's surface separated by a distance equal to the normal maximum horizontal reach of a person, approximately one metre.

3. Object of Earthing

The object of an earthing system is to provide as nearly as possible a surface under and around a station which shall be at a uniform potential and as nearly zero or absolute earth potential as possible. The purpose is to ensure that generally all parts of the equipment, other than live parts are at earth potential and that attending personnel are at earth potential at all times. Also by providing such an earth surface of uniform potential under and surrounding the station, there can exist no difference of potential in a short distance big enough to shock or injure an attendant when short circuits or other abnormal occurrences take place. The primary requirements of a good earthing system are:

i) It should stabilize circuit potentials with respect to ground and limit the overall potential rise.

ii) It should protect men and materials from injury or damage due to over voltage.

iii) It should provide low impedance path to fault current to ensure prompt and consistent operation of protective devices during ground faults.

iv) It should keep the maximum voltage gradient along the surface inside and around the substation within safe limits during earth faults.

4. Governing Specifications

Assistance has been taken from the following standards/ codes of practices in the preparation of this code of practice:

i) IS:3043-1987                       - Code of Practice for Earthing (first revision)

ii) Indian Electricity Rules           - 1956 (latest edition)

iii) National Electrical Code         - 1985 of Bureau of Indian Standards

iv) IEEE Guide for safety in a. c. substation grounding. No. ANSI/IEEE Standard 80-1986.

5. Earth Resistance

At each power supply installation, an earthing system as specified in this code shall be provided. The combined resistance of the earthing system shall be not more than the following values: -

S. No.  Name of the station The limit of combined earth  resistance in ohms
1.  Traction substation 0.5
2. Switching station 2.0
3. Booster transformer station 10.0
4. Auxiliary transformer station 10.0

6. Earth Electrodes

6.1 The earth electrode shall normally be of mild steel galvanised perforated pipe of not less than 40 mm nominal bore, of about 4 m length provided with a spike at one end and welded lug suitable for taking directly MS flat of required size at the other end. The pipe shall be embedded as far as possible vertically into the ground, except when hard rock is encountered, where it may be burled inclined to the vertical, the inclination being limited to 30 degree from the vertical. The connection of MS flat to each electrode shall be made through MS links by bolted joints to enable isolation of the electrode for testing purposes. A sketch for typical arrangement of an earth electrode is shown in Figure 3.01.

6.2 Earth electrodes shall be embedded as far apart as possible from each other. Mutual separation between them shall usually be not less than 6.0 m ( which is twice the length of the electrode).

6.3 If the value of earth resistance specified in clause 5 can not be achieved with a reasonable number of electrodes connected in parallel such as In rocky soil or soil of high resistivity, the earth surrounding he electrodes shall be chemically treated. The earth electrode shall be surrounded in an earth-pit by alternate layers of finely divided coke, crushed coal or charcoal and salt at least 150 mm all-round. Though substantial reduction in earth resistance can be achieved by coke treated electrode, yet as this method results in rapid corrosion not only of electrode, but also of steel frame work to which it is bonded, stoke treatment shall be used only where absolutely necessary and such electrodes shall not be situated within 6.0 m of other metal work.

6.4 In high embankments, it may be difficult to achieve earth resistance specified in clause 5 even after chemical treatment of electrodes. In those locations, use of electrodes longer than 4 m so as to reach she parent soil is recommended.

6.5 As far as possible, earth electrodes for traction sub-stations/ switching stations shall be installed within and adjacent to perimeter fence. At large sites, apart from securing a sufficiently low resistance and adequate current carrying capacity a reasonable distribution of electrodes is also necessary.

7. Earthing Arrangement at Traction Substation

7.1 Earthing grid.

7.1.1 An earthing grid is formed by means of bare mild steel rod of appropriate size as indicated in clause 7.1.2 buried at a depth of about 600 mm below the ground level and connected to earth electrodes. The connection between the earth electrode and the grid shall be by means of two separate and distinct connections made with 75 mm x 8 mm MS flat. The connection between the MS flat and the MS rod shall be made by welding, while that between the earth electrode and the MS flats through MS links by bolted joints. The earth electrodes shall be provided at the outer periphery of the grid as shown in Fig.2. As far as possible the earthing grid conductors shall not pass through the foundation block of the equipments. All crossings between longitudinal conductors and transverse conductors shall be jointed by welding. The transverse and longitudinal conductors of the earthing grid shall be suitably spaced so as to keep-the step and touch potentials within acceptable limits; the. overall length of the earthing grid conductors shall not be less than the calculated length (refer Annexure- I).

FIG. A 3.01

7.1.2 The size of the earthing grid conductor shall be decided based on the incoming system voltage and fault level (refer Annexure I). The fault level considered shall take into: account the anticipated increase in fault current during the life span of the station. The size shall be as given below:

S.No System voltage (kV) Fault level (MVA) Diameter of the grid conductor (MS rod) in mm
1. 66 upto 4000  32
above 4000 upto 5000  36
above 5000 upto 6000 40
2. 110 upto 6000 32
above 6000 upto 8000 36 
above 8000 upto 10000 40
3. 132 upto 7000  32
above 7000 upto 10000 36
4. 220 upto 12000  32
above 12000 upto 16000 36
above 16000 upto 20000 40

7.2 Buried rail.

7.2.1 A steel rail of section 52 kg/m (the one used for the railway track) and length about 13 m shall be buried near the track at the traction substation at a depth of about one metre to form part of the earthing system. Two separate and distinct connections shall be made by means of 75 mm X 8 mm MS flat between the earthing grid and the buried rail. The buried rail shall also be connected by means of two separate and distinct connections made with 75 mm X 8 mm MS flat to the traction rail(s) in a single-rail track circuited section and to the neutral point(s) of the impedance bond(s) in a double-rail track circuited section.

7.2.2 In cases where the feeding post is located separately away from the traction substation, the buried rail shall be provided at the feeding post (where one terminal of the secondary winding of the traction power transformer of the substation is grounded).

7.3 System earthing.

7.3.1 One terminal of the secondary winding (25 kV winding) of each traction power transformer shall be earthed directly by connecting it to the earthing grid by means of one 75 mm x 8 mm MS flat, and to the buried rail by means of another 75 mm x 8 mm MS flat.

7.3.2 One designated terminal of the secondary of each potential, current and auxiliary transformer shall be connected to the earthing grid by means of two separate and distinct earth connections made with 50 mm x 6mm MS flat.

7.4 Equipment earthing.

The metallic frame work of all outdoor equipments such as transformers, circuit breakers, interrupters and isolators, as well as steel structures shall be connected to the earthing grid by means of two separate and distinct connections made with MS flat of size as indicated below; one connection shall be

FIG. A 3.02

made with the nearest longitudinal conductor, while the other shall be made to the nearest transverse conductor of the grid:



7.5 Earthing inside control room.

An earthing ring shall be provided inside the control room by means of 50 mm x 6 mm MS flat which shall be run along the wall on teak wood blocks fixed to the wall at a height of about 300 mm from the floor level. The earthing ring shall be connected to the main earthing grid by means of two separate and distinct connections made with 50 mm x 6 mm MS flat. The earthing ring shall also be connected to an independent earth electrode by means of two separate and distinct connections made with 50 mm x 6 mm MS flat. The metallic framework of control and relay panels, LT a.c. and d.c. distribution boards, battery chargers, remote control equipment cabinets and such other equipments shall be connected to the earthing ring by means of two separate and distinct connections made with 8 SWG galvanised steel wire. The connections shall be taken along the wall and in recesses in the floor. All recesses shall be covered with cement plaster after finishing the work. Connections between the MS flats shall be made by welding.

7.6 Earthing of lightning arrester.

In addition to the earth electrodes provided for the main earthing grid, an independent earth electrode shall be provided for each lightning arrester. This earth electrode shall be connected to the ground terminal of the lightning arrester as well as to the main earthing grid by means of two separate and distinct connections made with 50 mm x 6 mm MS flat for the 25 kV side lightning arresters, and with 75 mm x 8 mm MS flat for the primary side lightning arresters. The earth electrode shall be provided as close as possible to the lightning arrester and the connections shall be as short and straight as possible avoiding unnecessary bends. For lightning arresters provided for the traction power transformers, there shall also be a connection as direct as possible from the ground terminal of the lightning arrester to the frame of the transformer being protected; this connection shall also be made by means of two separate and distinct connections made with 50 mm x 6 mm MS flat for 25 kV side arresters, and with 75 mm x 8 mm MS flat for primary side lightning arrester.

7.7 Earth screen

The area covered by outdoor substation equipments shall be shielded against direct strokes of lightning by an overhead earth screen comprising 19/2.5 mm galvanised steel stranded wire strung across the pinnacles of the metallic structures. The earth screen wires shall be strung at a height as indicated in the approved traction substation layouts (not less than 2.5 m above the live conductors) and shall be solidly connected to the traction substation earthing grid at each termination by means of 50 mm x 6 mm MS flat.

7. 8 Earthing of fencing uprights and panels.

Each metallic fencing upright shall be connected to the traction substation main earthing grid by means of two separate and distinct connections made with 50 mm x 6 mm MS flat. In addition, all the metallic fencing panels shall be connected to the uprights by means of two separate and distinct connections made with 6 SWG G.I. wire. All the metallic door panels shall also be connected to the supporting uprights by means of two separate and distinct connections made with 6 SWG G.I.wire.

7.9 Earthing at the point of 240 V ac 50 Hz supply for oil filtration plant.

The 240 V ac 50 Hz distribution board for power supply to oil filtration plant shall be connected to the main earthing grid by means of two separate and distinct connections made with 50 mm x 6 mm MS flat.

8. Earthing Arrangement at Switching Station

8.1 A minimum number of three earth electrodes (excluding the one to be provided separately for the remote control cubicle earthing- refer clause 8.4) shall be provided at each switching station, and they shall be interconnected by means of 50 mm x 6 mm MS flat forming a closed loop main earthing ring. This ring shall be connected by two separate and distinct connections made with 50 mm x 6 mm MS flat, to the traction rail in a single-rail track circuited section and to the neutral point of the impedance bond in a double-rail track circuited section of the nearest track, so as to limit the potential gradient developing in the vicinity of the switching station in the event of a fault.

8.2 System earthing.

One designated terminal of the secondary of each potential, current and auxiliary transformer shall be connected to the main earthing ring by means of two separate and distinct connections made with 50 mm x 6mm MS flat.

8.3 Equipment earthing.

8.3.1 All masts, structures, fencing uprights, and all outdoor equipment pedestals including auxiliary transformer tank shall be connected to the earthing ring by means of two separate and distinct connections made with 50 mm x 6 mm MS flat. All fencing panels shall be connected to the supporting uprights by means of two separate and distinct connections made with 6 SWG G.I. wire All the metallic door panels shall be connected to the supporting uprights by means of two separate and distinct connections made with 6 SWG G.I. wire.

8.3.2 The metal casing of potential and current transformers shall be connected to the mast/ structures by means of two separate and distinct connections made with 50 mm x 6 mm MS flat.

8.3.3 The ground terminal of lightning arrester shall be connected directly to the earth electrode by means of two separate and distinct connections made with 50 mm x 6 mm MS flat The earth electrode shall be so placed that the earthing leads from the lightning arrester may be brought to the earth electrode by as short and straight a path as possible.

8.4 Earthing inside remote control cubicle.

An earthing ring shall be provided inside the remote control cubicle by means of 50 mm x 6 mm MS flat the earthing ring shall be run along the wall on teak wood blocks fixed to the wall at a height of 300 mm from the floor level. The earthing ring shall be connected to the main earthing ring as well as to an independent earth electrode by means of two separate and distinct connections made with 50 mm x 6 mm MS flat. The metal casing of LT a.c. and d.c. distribution board, battery chargers, terminal board, remote control equipment cabinets and other such equipments shall be connected to the earthing ring by means of two separate and distinct connections made with 8 SWG G.I. wire. The connections shall be taken along the wall and in recesses in the floor. All recesses shall be covered with cement plaster after finishing the work. Connections of earth strips to each other shall be made by welding.

9. Earthing of Neutral of Local Power Supply System

At traction substations and switching stations where power supply at 415 V/ 240 V, ac, 50 Hz, is taken from the local supply authority and having neutral earth at some distant point in the premises of the supply authority, the neutral of such supply shall also be earthed by means of two separate and distinct connections made with 6 SWG G.I. wire by connecting to an independent earth electrode.

10. Earthing Arrangement at Booster Transformer Station

10.1 The combined earth resistancef at a booster transformer station shall be not more than 10 Ohm. Normally one earth electrode shall be sufficient for a booster transformer station. The earth electrode shall be connected to the lower end of each mast of the supporting gantry by means of two separate and distinct connections made with 50 mm x 6 mm MS flat. In addition each mast of the supporting gantry shall be connected by means of a 50 mm x 6 mm MS flat to the nearest tractional rail or to the neutral point of the nearest impedance bond in a double rail track circuited section.

10.2 The booster transformer tank shall be connected to the masts of the supporting gantry by means of two separate and distinct connections made with 50 mm x 6 mm MS flat. These connections shall be as short and as straight as possible without unnecessary bends.

11. Earthing Arrangement at Auxiliary Transformer Station

11.1 The combined earth resistance at an auxiliary transformer station shall be not more than 10 ohm. Normally one earth electrode is sufficient for an auxiliary transformer station. The earth electrode shall be connected to the mast on which the auxiliary transformer is mounted by means of two separate and distinct connections made with 50 mm x 6 ram MS flat. In addition the mast shall be connected to the nearest traction rail or to the neutral point of the nearest impedance bond in a double rail track circuited section by means of a 50 mm x 6 mm MS flat.

11.2 The earthing terminal on the transformer tank shall be connected to the mast on which the transformer is mounted by means of two separate and distinct connections made with 50 mm x 6 mm MS flat One terminal of the secondary winding of the auxiliary transformer shall be connected to the earthing terminal on the transformer tank and as well as to the mast by means of 50 mm x 6 mm MS flat These connections shall be as short and straight as possible and avoiding unnecessary bends.

12. Method of jointing

All the joints between the MS flats, MS rods or between MS flat and MS rod shall be made by welding only No soldering shall be permitted. For protection against corrosion, all the welded joints shall be treated with red lead and afterwards thickly coated with bitumen compound.

13. Painting of MS Flats

For protection against corrosion, all the exposed surfaces of earthing connections (MS flats) above ground level shall be given all around two coats of painting to colour grass green, shade-218 of IS:5.

14. Crushed Rock Surface Layer

At the traction substations and switching stations, a surface layer of crushed rock shall be provided to a thickness of about 100 mm. If considered necessary from the point of view of containing the step and touch voltages within the acceptable limits, higher thicknesses may be provided depending on calculation based on site conditions.

15. Step and Touch Voltages

15.1 The formulae for calculating the tolerable touch and step voltages, estimated mesh and step voltages, earth resistance, earth potential rise, size of earthing grid conductor and length of buried grid conductor are given in Annexure-I.

15.2 The design for earthing grid shall be done separately for each location depending on the conditions obtaining and those foreseen.

16. Drawings

The following drawings (latest versions) issued by RDSO in connection with this code may be used for reference:

i)  Typical earthing layout of traction substation. ET1/PSI/224-1
ii) Typical return current connection of buried rail at traction substation. ETI/PSI/0212-1
iii) Typical earthing layout of subsectioning and paralleling station. ET1/PSI/201-1
iv) Typical earthing layout of sectioning and paralleling station. ETI/PSI/202-1
v) Typical earthing layout of  booster transformer station. ETI/PS:/211-1
vi) Typical arrangement of an earth electrode at a traction substation. ET1/PSI/222-1
vii) Typical earthing arrangement for an auxiliary transformer station ETI/PS1/708

ANNEXURE 1


FORMULAE FOR CALCULATION OF EARTHING GRID BASED ON IEEE GUIDE FOR SAFETY IN AC SUBSTATION GROUNDING, No. ANSI/IEEE Std 80-1986

FIG. A3.03

ANNEXURE 2

SAMPLE CALCULATION FOR THE DESIGN OF EARTHING MAT FOR A 132/25kV

TRACTION SUBSTATION

1.0 Data assumed for calculation: