The families

MCB — miniature circuit breaker, for final circuits in domestic and light commercial boards.

MCCB — moulded case circuit breaker, for sub-mains and motor circuits with adjustable protection.

ACB — air circuit breaker, for main incoming positions at high current ratings.

VCB — vacuum circuit breaker, for medium-voltage switching.

RCCB — residual current circuit breaker, sensitive to earth leakage but without overcurrent protection.

RCBO — a device combining earth leakage and overcurrent protection in one unit.

Choosing between them

The deciding factors are the current rating, the prospective fault current at the point of installation, whether adjustable protection is needed, and whether earth leakage protection is required at that point.

Above a certain rating and breaking capacity, a moulded case device is no longer available and the choice moves to an air circuit breaker. At medium voltage it moves to vacuum or gas.

Protection is a system, not a device

Breakers are selected together. The upstream device must back up the downstream one, and the settings must be coordinated so that a fault trips only the device nearest to it. Selecting each device in isolation produces installations that trip at the wrong level.

Earth leakage devices

An RCCB protects people, not cables — it has no overload element. Where both are needed at one point, an RCBO does both jobs in one device width.