Friday, April 15, 2011

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
(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.

Standard requirements for Insulating Oil

Equipment Voltage
Permissible Limits
Dielectric Strength
145 kV and above
30 kV (Min)
Break down voltage
Below 72.5 kV
40 kV (Min)
72.5kV and <145kV
50 kV (Min)
Water content
Below 145 kV
35 ppm (Max)
145 kV and above
25 ppm (Max)
Specific resistance
(Ohm-cm) at 90 deg C
All voltages
0.1 x1012 ohm-cm (Min)
Dielectric dissipation factor (tan d) at 90°C
All voltages
0.005 (Max:)
Neutralisation value (Total acidity)
All voltages
0.4 mg KOH/g (Max)
Interfacial tension at 27 deg C
All voltages
0.018 N/M (Min)
Flash point
All voltages
Max -15 °C
decrease of initial value
Sediment or precipitable sludge
All voltages
No sediment or precipitable sludge should be detectable
Dissolved gas analysis
145 kV and above
Refer IS 10593-1993 & DGA study chart

Friday, April 1, 2011

Requirements of Transformer Installations in India


1. Minimum clearance between supplier's and consumer's 11 kV structures shall be 3 m. If it is 5 m or less they should be braced together both at top and middle.

2. Minimum size of RSJ poles shall be 175 x 85 mm. Channels shall be of minimum 75 x 50 mm.

3. Phase to phase and phase to earth clearances at AB switch shall be minimum 915 mm and 610 mm respectively.

4. Minimum size of 11 kV jumpers shall be Number '0' SWG copper.

5. Maximum length of unsupported 11 kV jumpers shall be 2.44 m for solid conductors and 1.5 m for stranded conductors.

6. Minimum safety working clearance of 2.6 m shall be maintained for bare conductors and live parts.

7. Lightning arrestors shall be provided between AB switch and D.O. / H.G. fuse.

8. Group control AB switch shall be provided for multiple transformer installations.

9. The minimum clearance from ground to the 11 kV bushings of the transformers shall be 2.6 m

10. AB switch operating handle shall be fixed at 1 m to 1.25 m from ground level.

11. 11 kV AB switch shall be three insulator type.

12. Fencing with a height of 2.5 m shall be provided, keeping a minimum clearance of 1.5 m from the transformer body.

13. Minimum clearance between 11 kV up and down jumpers shall be 75 cm.

14. Outdoor transformer is not permitted in corporation areas and at the premises of high-rise apartment buildings. However outdoor transformers with cable end boxes at both sides and indoor breaker/SFU at primary side may be permitted judiciously.

15. Indoor transformer is recommended for rating above 630 kVA

16. If more than one transformer, having more than 2000 litres of oil, are installed side by side, baffle walls of minimum 4 hr fire rating shall be provided in between transformers.


1. Only dry type transformers shall be installed inside residential/commercial buildings (Rule 64(2)(e)(iv)).

Oil filled transformers are permitted outside residential and commercial buildings subject to the following conditions.

(a) Only indoor type transformers are permitted in Corporation areas and at the premises of residential buildings.

(b) There shall be free access for fire engines to the transformer stations.

(c) The building in which transformer is installed shall have minimum clear distance of 1.5 m from the residential/commercial buildings.

(d) The substation building shall be of weatherproof and fire proof construction. Walls shall be of 355 mm thick brick or 230 mm thick RCC. If there is any window/door opening towards the residential/commercial building, there shall be a fire resistant wall between the substation building and residential/commercial building keeping a minimum clear distance of 1.5 m from both the buildings.

(e) Transformers may be installed outdoor, keeping a minimum distance of 6 m from commercial buildings in areas other than corporation areas.

(f) If the transformer is installed outdoor with in 6m from commercial buildings, there shall be a fire resistant wall in between the transformer and the building keeping a minimum distance of 1.5m both from the building and the transformer.

2. Only dry type transformer shall be installed in cellars and above ground floor. Indian Electricity Rules, 1956 (Rule 64(2) (f) (iii))

3. ABSF unit/VCB shall be provided at primary side. The 11 kV ABSF unit shall have CPRI test certificate and brand certification. At hospital premises breaker shall be recommended.

4. Minimum size of cable at 11 kV side shall be 150 XLPE (Aluminium). But size may be 70 XLPE (Aluminium) if back up fuse is provided.

5. Fire separation walls shall be provided in between oil filled transformers. Thickness of the wall shall be minimum 30 cm (Brick) or 20 cm (RCC)

6. Oil filled transformers shall be segregated from HT and MV switchgears using fire separation wall

7. Oil drainage facility/soak pits shall be insisted for oil filled transformers if quantity of oil is more than 2000 litres.

8. All-round clearance shall be provided for indoor transformers. It shall be minimum 1.25m if walls are provided at 4 sides.

9. Transformers shall be installed at the periphery of the building for getting easy access and adequate ventilation.

10. Doors and windows of transformer room shall be of non-inflammable materials.

11. Minimum 50 cm headroom clearance shall be provided for the transformer.

a). If dry type transformers are installed above ground level, emergency switching off facility shall be provided at ground floor.

b). Structural fitness certificate from a competent structural engineer shall also be furnished.

13. Minimum phase to phase and phase to earth clearance of bus bars in 11 kV panel shall be 130 mm and 80 mm respectively.

14. Minimum 1.0 metre clear space shall be available in front of HT panel in breaker drawn out position.