Electrical Wiring and Fittings in Buildings
This domain tests circuit design, cable and accessory selection, protection, earthing and the safety rules that keep a building safe.
Circuits and cables
Lighting circuits use 1.0 to 1.5 mm2 copper; power sockets 2.5 to 4 mm2; mains and sub-mains larger. Indian colour convention: phase red, yellow or blue, neutral black, earth green or yellow-green. Wiring systems are batten, casing and capping, and concealed PVC conduit. Conduit runs must be straight with draw boxes at bends, and the earth conductor must be separate and continuous.
Protection and earthing
An MCB protects against overload and short circuit; an RCCB or ELCB of 30 mA protects people against earth leakage. Earthing provides a low-resistance path through a plate or pipe electrode and an earth wire to every socket and metal body. Switch height is about 1.2 to 1.3 m, socket about 0.3 m, and no switch should be within reach of a bath.
Two-way switching
Two two-way switches at the ends of a staircase, linked by a three-core strapper, let one lamp be controlled from either floor.
Worked example
A circuit has 6 lamps of 60 W and 2 fans of 75 W = 360 + 150 = 510 W. Current at 230 V = 510 divided by 230 = 2.2 A. A 1.5 mm2 cable rated about 14 A with a 6 A MCB is adequate. A 2.5 mm2 cable with a 16 A MCB would be safe but wasteful.
Traps
Putting the switch in the neutral, using the earth wire as a neutral, omitting the RCCB, and ignoring derating for conduit grouping and ambient temperature.