Guidelines for Earthing

1. Minimum fault level at 11 kV side shall be assumed 150 MVA for all earthing designs.

2. Number of earth electrodes required shall be calculated for fault current at 11 kV side.

3. Duration of fault current shall be taken as 3 sec for earthing design at HT installations.

4. Maximum earth resistance at HT premises shall be limited to 1 ohm.

5 Earth mat shall be provided at EHT premises for limiting step voltage and touch voltage to tolerable values. Conductor size and mesh size may be designed for fault duration of 1.0 second and 0.5 second respectively.

6. All earth electrodes shall be interconnected using the conductors of largest size in the earthing system.

7. All non-current carrying metal parts of equipments shall be double earthed using conductors of adequate size.

8. Transformer and generator neutral shall be double earthed. One independent earth electrode shall be provided for neutral earthing. Links shall also be provided in neutral earthing conductor. Generator neutral shall be earthed at control panel.

9. Lightning arrestor shall be connected to a separate earth electrode using No. 6 SWG Cu. The L.A. earth electrode may be interconnected with other earth electrodes.

10. If fault current exceeds 6 kA plate earth electrodes shall be used.

11. If soil resistivity is < 60 ohm-m, only copper shall be used as earthing conductor.

12. Distance between plate earth electrodes shall be 8 m minimum.

13. Distance between pipe earth electrodes shall be 5 m minimum.

14. Minimum distance between earth electrode and adjacent civil structure shall be 1.5 m.

15. In location of SBs, DBs etc. sub earth buses of 25 x 3 mm² copper may be provided.

16. All joints in earthing conductors shall be properly tinned.

17. Following guidelines shall be adhered to when GI earthing is adopted.

(a) GI strips used for earthing shall be minimum 6 mm thick and hot dip galvanised.

(b) If round GI conductors are used it shall have double the calculated area of cross-section.

(c) The minimum cross-sectional area of the GI should be 50 sq.mm.

(d) Where a Copper conductor is to be joined to GI, the joints should be tinned to prevent electrolytic action.

(e) If atmosphere is corrosive, GI conductors shall not be used for earthing.

(f) Joints shall be invariably done by means of welding except in such locations where intermittent disconnection of joints is necessitated for testing purpose, or for the earthing connection to the body of equipments etc.

(g) Barium Chromate or Zinc dichromate treatment should be done at the welds. Joints should be provided with coating alternative layers of red oxide and aluminium. Joints are to be covered with hot bitumen. Also jute band covering of adequate thickness is to be made on the portion of earthing susceptible to higher corrosion and environmental influence.

(h) The area of strip electrode thus covered with bitumen and jute should not be reckoned for the calculation of area of dissipation for the fault current.

(i) In places where the resistivity is very high and atmospheric corrosion is not significant even ordinary steel conductors shall be permitted.

(j) Earthing system with GI conductors should be subjected to a thorough physical examination after a period of 10 years and modifications made if found necessary on such examination.

(k) Joints using GI conductors should be welded as far as possible and kept separated from air by a thick coating of tar or similar non-hygroscopic materials. In case bolted joints can not be avoided, there should be a minimum of 2 bolts for sizes up to 25 mm x 6 mm, 3 bolts for sizes up to 31 mm x 6 mm and zig-zag bolting for large sizes.

18. Following guidelines shall be adhered to when covered aluminium is used for earthing:-

(a)Aluminium should not be used for final underground connections to earth electrode.

(b)Aluminium shall be used below ground only if efficiently protected or sheathed against contact with soil and moisture. PVC covering shall be allowed.

(c)Where a copper conductor is to be joined to Aluminium conductor, the joints should be tinned to prevent electrolytic action.

(d)Sharp bends required in Aluminium strip should be formed by the use of a bending machine.

(e)Aluminium to Aluminium joints on strip conductor shall be arc welded using either the Tungsten inert gas arc or Metal inert gas arc techniques. Oxy-acetylene gas welding or bracing shall also be allowed.

(f)Round and rectangular conductors can be joined with bolted clamps. Rectangular conductors can be joined or terminated by drilling and bolting.

(g)When making a bolted type joint the surface of the Aluminium strip should be cleaned thoroughly by wire brushing and greased or an approved jointing compound applied immediately to both mating surfaces. Bolts should then be tightened and all excess grease or compound wiped off and discarded.

(h)Aluminium conductor connections to equipment, whereas possible, should be in the vertical plane. Surface preparation of Aluminium and making of the joint should be as previously described. The finished joint should be protected by bitumastic paint.

19. Earthing shall conform to IS 3043-1987.

20. Earthing grid should be run at a minimum depth of 50 cm below the ground.