Circuit Breaker
A circuit breaker is a mechanical switching device that makes, carries and breaks current under normal conditions and automatically interrupts abnormal currents such as overloads and short circuits. A trip unit, whether thermal-magnetic, electronic or relay-controlled, detects the fault and releases the operating mechanism, and the arc formed between the separating contacts is extinguished in air, vacuum, SF6 or oil. Unlike a fuse, a circuit breaker can be reset and reused after operation. Low-voltage types include miniature (MCB), moulded-case (MCCB) and air circuit breakers (ACB), covered by IEC 60898-1 and IEC 60947-2; medium- and high-voltage AC circuit breakers fall under IEC 62271-100. Selection parameters include rated voltage and current, breaking capacity (Icu, Ics or rated short-circuit breaking current), trip curve or protection settings, number of poles and selectivity with upstream and downstream devices.
Examples
- Moulded-case circuit breaker protecting a motor feeder
- Vacuum circuit breaker in 11 kV switchgear
- Miniature circuit breaker on a lighting circuit