Electrical Wiring
Wiring questions test your knowledge of installation methods, conductor colour codes, protective devices and the rules governing domestic and industrial circuits.
Systems of wiring
Cleat wiring (temporary, cheap), batten wiring (CTS/PVC on wooden battens), casing-and-capping (surface, in dry places), and conduit wiring, either surface or concealed in the wall. Concealed PVC conduit is standard in modern domestic construction because it protects the cable and looks neat.
Wires and colour code
Domestic cable is usually PVC-insulated copper. In Indian three-phase practice the phases are red, yellow and blue, neutral is black, and the protective earth is green (or green with yellow stripes). The earth wire must never be fused or switched, and the switch must always be placed in the phase conductor, never in the neutral.
Protective devices
- Fuse: melts on overload or short circuit; rated above the load current but below the cable rating.
- MCB: resets after overload or short circuit.
- ELCB / RCCB: detects earth leakage current and protects people; it does not replace an MCB.
- Distribution board: splits the supply into sub-circuits.
Earthing
Pipe or plate earthing connects the metal body of an appliance to earth, so that a fault current flows and the fuse blows instead of the user being shocked.
Worked example
A 2000 W geyser on a 230 V supply draws I = 2000/230 = 8.7 A. Choose a 10 A MCB with 1.5 mm² copper conductor, whose current rating comfortably exceeds 8.7 A; a 6 A fuse would nuisance-trip, and a 20 A fuse would fail to protect the cable.
Traps
Confusing neutral (black) with earth (green); putting the switch in the neutral; believing an ELCB protects against overload; selecting a fuse below the load current.