Requirements of Transformer Installations in India

Outdoor

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.

Indoor

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 sq.mm XLPE (Aluminium). But size may be 70 sq.mm 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.

12.

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.