Electrical Protection
What the exam tests
This domain tests fuses, MCBs, MCCBs, RCCBs, RCBOs, overload and short-circuit protection, ratings, trip curves and discrimination.
Core rules
- A fuse melts on overcurrent. An MCB combines thermal protection for overload and magnetic protection for short circuit. Its rating In must be above normal load current and not above cable capacity.
- MCB curves: B trips magnetically at roughly 3–5 times In, C at 5–10 times, D at 10–20 times. Use the curve suited to the load.
- An RCCB detects residual current to earth and protects people against shock; it does not detect overload. An RCBO combines overcurrent and residual current protection.
- Breaking capacity in kA must exceed the prospective fault current at the point of installation.
- Discrimination means the downstream device clears a fault before the upstream device.
Worked example
A 2 kW heater draws 8.7 A. A 10 A MCB is suitable if the cable current rating is at least 10 A and voltage drop is acceptable. Protect the socket circuit with a 30 mA RCCB. A 6 A MCB would nuisance-trip; a 16 A MCB may not protect a 1.5 mm² cable if its capacity is lower.
Traps
Do not use an RCCB as a substitute for earthing. Do not fit a larger fuse to stop tripping. Do not ignore trip curve coordination. Do not assume an MCB protects against electric shock; it protects the circuit, while an RCD protects people.