Circuits and Protection Devices
What this domain tests
Circuits and Protection Devices tests fuses, MCBs, RCCBs/ELCBs, earthing and coordination, so that a fault is cleared without fire or shock.
Core rules
- A fuse melts on overcurrent. The fuse wire must match the carrier rating; a thicker wire defeats protection.
- An MCB protects against overload and short circuit. Its curve matters: B trips at 3–5 times rated current, C at 5–10 times, D at 10–20 times. B is common for domestic lighting and sockets; C or D for motor loads.
- An RCCB (or ELCB) detects earth leakage and trips at its rated residual current, commonly 30 mA for personnel protection. It does not replace an MCB for overload.
- An RCBO combines overcurrent and residual-current protection in one device.
- Earthing provides a low-impedance fault path so that the protective device operates quickly and touch voltage is limited.
- Selectivity means the upstream device should not trip before the downstream device for a fault in the final circuit.
Worked example
A 2.5 mm² copper circuit is protected by a 16 A MCB and a 30 mA RCCB. A live-to-earth fault occurs with a loop resistance of 1 Ω. Fault current is about 230 ÷ 1 = 230 A, far above the instantaneous trip level, so the MCB trips quickly. The RCCB also sees leakage and adds backup protection.
Traps that cost marks
Using an RCCB as the only protection against overload; fitting a fuse wire thicker than the rating; bypassing the earth; and choosing a D-curve MCB for a domestic lighting circuit without reason.