Wiring and Installation
What this domain tests
Questions cover Indian wiring colours, IS 732 practice, choice of conductor and protective device, earthing, and the tests done before a domestic installation is energised.
Core rules
- Current IS 732 colours: brown = phase, light blue = neutral, green with yellow stripe = earth. Older wiring used red, yellow and blue for phases, black for neutral and green for earth.
- The earth pin of a 3-pin socket is longer and thicker so it makes contact first and breaks last. Standard domestic sockets are 6 A and 16 A.
- A fuse or switch must be in the phase, never in the neutral.
- Insulation resistance is measured with a 500 V megger and should exceed 1 MΩ for a healthy domestic circuit.
- Earth continuity, polarity and insulation resistance are checked before commissioning.
Worked example
A room has four 40 W lamps, two 75 W fans, one 200 W refrigerator and one 100 W television. Find the connected load and the main switch rating. Load = (4 × 40) + (2 × 75) + 200 + 100 = 160 + 150 + 200 + 100 = 610 W. Current at 230 V = 610/230 = 2.65 A. Because not everything runs at once, a 16 A main MCB with 6 A lighting circuits is sensible; a 6 A main would nuisance-trip when the refrigerator starts.
Traps that cost marks
Interchanging neutral and earth; fitting the fuse in the neutral; using an earth wire thinner than the phase conductor; assuming a megger can be used on a live circuit; forgetting that the earth pin is longer, not shorter.