Circuit breaker (MCB)
Automatically disconnects a circuit on overload or short circuit, and can be reset rather than replaced.
What it is
A circuit breaker is an automatic protective switch. It carries normal current but disconnects quickly on a fault — and unlike a fuse, it can be reset.
How it works
It has two trip mechanisms. A thermal element (bimetal) handles sustained overloads with a time delay. A magnetic element (a coil) handles short circuits, tripping almost instantly on the large fault current.
That's why an instant trip on reset usually means a short, while a trip after running a while means an overload — the two mechanisms tell you which.
Where it's used
Protecting final circuits and sub-mains in switchboards. Breakers come in 'curves' (characteristics) suited to different loads — some ride through motor inrush, others trip more readily.
Never fit a larger breaker to stop nuisance tripping — that defeats the protection. Find the overload or fault instead.
Safety first
A tripping breaker is protecting against a real fault. Don't repeatedly reset into a short or upsize the device — find the cause.
Isolate, lock out / tag out, and prove dead before working unless a live test is specifically required, authorised, and carried out under proper supervision. Always follow local regulations, your site procedures, and the equipment manufacturer's documentation.
Related faults
MCB (circuit breaker) keeps tripping
A circuit breaker trips repeatedly — instantly on reset, or after a load runs for a while — and you need to tell a short from an overload from a faulty breaker.
A circuit breaker keeps tripping (domestic)
One circuit breaker keeps tripping — instantly on reset or after a load runs — and you need to tell an overload from a short or a faulty appliance.
Old rewireable fuse blowing repeatedly
An older home with ceramic rewireable fuses keeps blowing a fuse on one circuit — the fuse is doing its job; something is overloading or faulting that circuit.
Related definitions
RCD (safety switch)
Detects earth leakage and disconnects fast to protect people from electric shock.
RCBO
Combines an RCD and a circuit breaker in one device — earth-leakage plus overload/short-circuit protection.
Overload relay
Protects a motor from sustained over-current by tripping the control circuit if it runs too hot for too long.
Earthing & bonding
Connecting exposed metal to earth so a fault blows protection fast and metalwork can't become live.