Electrical Installations and Safety in Buildings
What this domain tests
Electricity questions ask for supply voltages, conductor sizes, protective devices, earthing, colour codes and standard mounting heights. Safety interlocks in wet areas such as bathrooms and kitchens are a frequent theme.
Core rules and concepts
- Supply is 230 V single phase, 415 V three phase, 50 Hz, in India; use ISI-marked copper conductors in conduit.
- Lighting circuits use 5 A outlets with 1.0-1.5 mm² copper; power circuits use 15/16 A outlets with 2.5-4 mm² copper.
- MCB protects against overload and short circuit; RCCB (30 mA sensitivity) protects people against earth leakage shock; both are required in wet zones.
- Earthing: pipe or plate earthing, earth wire of adequate size, earth resistance kept low (typically 1-5 ohm) and measured periodically.
- Mounting heights: switchboard about 1.2 m, socket about 1.3 m, light point 2.1-2.5 m above floor.
- Conduit runs must be continuous, with draw boxes at bends; keep at least 300 mm clearance from water pipes.
Worked example
Load of one room: eight lamps of 40 W and four fans of 70 W. Total load = (8 × 40) + (4 × 70) = 320 + 280 = 600 W. Current at 230 V = 600 ÷ 230 = 2.6 A. A 5 A lighting circuit with 1.5 mm² copper conductor is adequate; if a 1.5 kW geyser is added later, it needs a separate 15 A circuit.
Traps that cost marks
Calling an MCB a shock-protection device (it is not; the RCCB is); using the earth wire as a neutral; undersizing the earth conductor; placing a socket inside a bathroom shower zone without the required protection.