MCB
/ ɛm siː biː /
Also known as: miniature circuit breaker, circuit breaker, Type B MCB, breaker
Definition
An MCB (Miniature Circuit Breaker) is an automatic protective device in a consumer unit that trips to disconnect a circuit when current exceeds its rating (overload) or spikes sharply (short circuit). Unlike old fuses, MCBs reset by pressing the toggle after the fault is cleared. Rated by current (6A lighting, 32A ring main, 40A+ cooker) and trip characteristic: Type B (trips at 3-5x rated current, standard for domestic circuits); Type C (trips at 5-10x, for motor or high inrush loads). MCBs protect wiring only from overcurrent - they do not detect earth faults or shock risk, which is the function of the RCD.
In practice
In modern consumer units, MCBs are almost always used in combination with RCDs - either as separate MCBs under an RCD (split-load board) or as combined RCBOs (Residual Current Breaker with Overcurrent protection) that provide both functions in one device. An MCB on its own provides no protection against electric shock - it will not trip in response to a 30mA earth fault current (the threshold for cardiac fibrillation risk) because 30mA is far too small to trip even a 6A MCB (which carries 6000mA continuously). The RCD is the device that provides shock protection; the MCB is purely for cable and equipment protection against overcurrent.
When an MCB trips and cannot be reset, do not force it back on. Investigate the cause: disconnect all loads from the circuit, reset the MCB, then reconnect loads one at a time to identify which load is causing the fault. If the MCB trips with nothing connected, the fault is in the wiring itself and a qualified electrician must investigate. An MCB that trips repeatedly on a lightly loaded circuit (well below its rated current) may itself be faulty and require replacement - MCBs can develop thermal trip faults over time, particularly old or poor-quality devices.
See also