RCCB

Residual current circuit breakers (RCCB), also known as residual current devices (RCD) in some regions, are protective devices designed to quickly disconnect electrical circuits when they detect an imbalance between the live and neutral conductors. This imbalance indicates a leakage current, typically through a person or into earth, which could cause electric shock or fire.

RCCBs operate by measuring the current in the live and neutral wires with a built-in current transformer. Under normal operation, currents are equal, so the transformer net flux is zero and the trip mechanism remains unenergized. If a leakage occurs, the difference (residual current) exceeds the device’s sensitivity, typically 30 mA for personal protection, and an instantaneous trip opens the circuit.

An RCCB does not provide overcurrent protection. For combined protection, many installations use RCBO (residual current breaker with overcurrent) or pair an RCCB with an MCB.

Common sensitivity ratings include 30, 100, and 300 mA, with 30 mA widely used for living spaces. Types AC, A, and B refer to the residual current waveform they can detect: AC detects alternating residual current, Type A detects pulsating DC as well, Type B detects smooth DC as well, suitable for modern power supplies and EV charging.

RCCBs should be installed in a distribution board protecting circuits that could present shock risk. They require a test function that simulates an earth fault; many units have a TEST button. They do not protect against faults that do not create earth leakage, and improper wiring or nuisance tripping can occur if sensitive devices such as RCDs on shared neutrals are miswired.