Electrical Wiring, Installation and Safety
What this domain tests
This domain tests domestic and industrial wiring practice, protective devices, earthing, conductor selection and safe working. Questions often ask you to size a circuit or identify a wiring fault.
Core rules
- Conductor size is chosen from current-carrying capacity, voltage drop and mechanical strength; a thicker conductor has lower resistance and less heating.
- The fuse or MCB goes in the phase conductor, never the neutral, and must be rated above normal load current but below the cable's capacity.
- Earthing provides a low-resistance path so fault current trips protection and exposed metal stays near earth potential. Common systems include pipe and plate earthing.
- An MCB protects against overload and short circuit; an RCCB detects residual current to earth and protects people, but does not replace an MCB.
- Insulation resistance is measured with a megger, typically at 500 V for low-voltage installations; good readings are in megohms, not ohms.
Worked example
A room has six 60 W lamps and two 80 W fans on 230 V. Total load = 360 + 160 = 520 W; current = 520/230 = 2.26 A. A 6 A MCB with 1.5 mm² copper cable is adequate, while a 20 A breaker on the same cable would allow overload heating before tripping. The earth conductor must be sized and connected to every metal enclosure.
Traps that cost marks
Putting a fuse in the neutral; assuming an RCCB protects against overload; using a larger breaker to stop nuisance tripping instead of finding the fault; and confusing earth wire with neutral.