rcd

RCD stands for residual current device, a safety device used in electrical installations to quickly disconnect a circuit when it detects an imbalance between the currents in the live and neutral conductors. This imbalance indicates current leakage to earth, which can occur if a person becomes a path to ground or if insulation fails.

Most RCDs are designed to trip within milliseconds when the leakage current exceeds a specified sensitivity, commonly 30 milliamperes for personal protection. They may be part of a residual current circuit breaker (RCCB) or combined with overcurrent protection in an RCBO (residual current breaker with overcurrent protection). By detecting faults that do not cause a short circuit, RCDs help reduce the risk of electric shock and fire.

RCDs come in several types based on the kind of residual current they detect. Type AC detects alternating leakage currents; Type A detects alternating current plus pulsating DC; Type B detects smooth DC as well as AC, suitable for certain power electronics and EV charging, and Type F offers a compromise for mixed waveforms. The choice of type depends on the installation and the expected leakage profiles.

In practice, RCDs are installed in consumer units or switchboards to provide protection for specific circuits or for the entire installation. They include a test button to verify functionality and should be tested regularly in accordance with local electrical codes. They do not replace fuses or circuit breakers for overcurrent protection and rely on a functioning earth path for proper operation.

Limitations include the possibility of nuisance tripping and the fact that RCDs only detect leakage to earth, not all electrical hazards. Proper earthing, correct device selection, and adherence to standards are essential for effective protection.