Miniature circuit breaker (MCB)
Plain English. Practical detail. UK context.
An MCB is a miniature circuit breaker: a resettable protective device intended to disconnect a circuit under specified overcurrent conditions. Overcurrent includes overload and short circuit. A conventional MCB does not provide the residual-current protection of an RCD, although both functions can be combined in an RCBO.
Safety: use a registered electrician, for example one registered with a competent person scheme such as NICEIC or NAPIT, for electrical installation work. Do not remove covers or open electrical equipment yourself.
Last reviewed: (editorial review against the public sources listed below).
What it protects and what it cannot do
The circuit designer coordinates the MCB with the cable and the expected load. It is not simply a limit chosen to match how many sockets are wanted. Cable installation conditions, fault levels and the ability to disconnect a fault all influence selection. An MCB may operate for a sustained overload or a high fault current, but it should not be described as detecting every dangerous leakage current. Shock protection depends on the complete protective arrangement, not the breaker label alone.
When it comes up in domestic work
Electricians encounter MCBs when investigating trips, modifying circuits and reviewing older consumer units. A new appliance can expose a capacity issue, but the remedy is not automatically to increase the breaker rating. The existing cable, terminations and installation method need assessment. Where additional residual-current protection is needed, an RCBO or another properly designed arrangement may be considered. The practical question is what protection the circuit needs, rather than whether one device looks newer than another.
For the person living with the circuit
Keep the consumer unit labels legible and ask which device serves which area. If a breaker operates repeatedly, note the appliances in use and tell the electrician about recent alterations. A warm smell, scorching or unusual noise needs prompt attention; do not remove the board cover to investigate. Resetting is not a repair. Never substitute a larger MCB, tape the handle in place or use a mismatched replacement purchased because it fits the available slot.
Trade specification and records
The IET's protective-device guidance explains why residual-current and overcurrent functions must be distinguished. Apply the current BS 7671 edition and the equipment manufacturer's assembly instructions when designing or changing protection. A replacement may require investigation of the reason for the original failure and tests of the affected circuit. Record the work actually completed and any limitations. For quotations, separate a device replacement from circuit repair or a consumer unit upgrade: otherwise the customer cannot understand why apparently similar jobs have very different scopes.
Frequently asked questions
Does an MCB protect against electric shock like an RCD?
Not in the same way. Its overcurrent function is different from residual-current detection. The installation needs a coordinated protective design.
Can a tripping MCB be replaced with a higher rating?
Not as a user fix. An electrician must establish the cause and whether the circuit can safely support any proposed change.
Sources and further reading
Public sources and further reading. Sources checked: 8 November 2026. Consult the original for details and later changes.