What is a circuit breaker (MCB)?

Glossary · Consumer units and protection

How a circuit breaker works

A circuit breaker sits in the consumer unit and guards one final circuit. When the current in that circuit goes too high, it switches off by itself.

The IET explains the job. Fuses give protection against overcurrent and short-circuit faults, and the circuit breaker is the modern equivalent. The devices are rated in amps, and there is one per final circuit, for example the cooker, the sockets and the lights.^{[1]IET, Consumer units: safe fuse-box upgrades for modern homes} NICEIC adds that breakers switch off the current faster than traditional fuses, which helps limit damage to wiring and appliances.^{[2]NICEIC, Consumer units and fuse boxes: installation and replacement}

Two faults trip a breaker. An overload is too much demand on a circuit for too long. A short circuit is a fault where current finds a path it should not. Both are overcurrent faults.

Circuit breaker and fuse compared

NICEIC says consumer units with circuit breakers took over from fuse boxes from the mid-20th century. A breaker can be safely reset after it trips. A fuse has to be replaced, or in the rewireable kind have its wire replaced, to restore the circuit.^{[2]NICEIC, Consumer units and fuse boxes: installation and replacement} See fuses for how those work.

What a circuit breaker does not do

A circuit breaker does not protect a person from electric shock. That is the job of an RCD. The IET says RCDs are rated in milliamps, so they are thousands of times more sensitive than circuit breakers or fuses.^{[1]IET, Consumer units: safe fuse-box upgrades for modern homes} The IET also notes that an RCCB, the plain RCD, gives no overcurrent protection unless it is an RCBO, the combined device.^{[3]IET, BS 7671 FAQs: Consumer Units and Protective Devices} The page on MCB vs RCBO sets out the difference.

Example

Sam in Roundhay plugs a heater, a kettle and a toaster into one kitchen socket circuit. The circuit breaker for that circuit trips and the sockets go off.

Sam unplugs the heater, switches the breaker back on, and the circuit holds. The breaker did its job. It cut the circuit before the cable could overheat. If it had tripped again with everything unplugged, that would point to a fault in the circuit, and a registered electrician should look at it.

What people get wrong

Many people think a circuit breaker is what keeps them safe from shock. It is not. A circuit breaker protects the cable and the circuit from overheating. Protection from shock comes from an RCD or RCBO.

A second mistake is to treat a breaker that keeps tripping as faulty. The IET says that if a device trips, the likelier cause is an appliance connected to the circuit, and that resetting repeatedly on a fault could damage the device.^{[1]IET, Consumer units: safe fuse-box upgrades for modern homes} If one trips often, see why does my RCD keep tripping, which covers the same steps.

Replacing a consumer unit is notifiable work under Part P of the Building Regulations in England and Wales, so it needs a registered electrician.^{[2]NICEIC, Consumer units and fuse boxes: installation and replacement} For that work, see consumer unit upgrades.

The bottom line

A circuit breaker, or MCB, is the resettable switch that protects each circuit from overload and short circuit. It has replaced the fuse in modern homes. It does not protect people from shock, which is the work of an RCD or RCBO. A breaker that keeps tripping is a sign to look for the cause, not to keep switching it back on.