A residual-current device (RCD), or residual-current circuit breaker (RCCB), is a safety device that quickly breaks an electrical circuit to protect equipment and to reduce the risk of serious harm from an ongoing electric shock.
Injury may still occur in some cases, for example if a human receives a brief shock before the electrical circuit is isolated, falls after receiving a shock, or if the person touches both conductors at the same time.
RCD is the name used in the United Kingdom. In the United States and Canada, the terms ground fault circuit interrupter (GFCI), ground fault interrupter (GFI) or appliance leakage current interrupter (ALCI) (also known as a Leakage Current Detection Interrupter (LCDI)) are used.
If the RCD device has additional overcurrent protection integrated in the same device, it is referred to as RCBO.
These electrical wiring devices are designed to quickly and automatically isolate a circuit when it detects that the electric current is unbalanced between the supply and return conductors of a circuit.
Any difference between the currents in these conductors indicates leakage current, which presents a shock hazard.
Alternating 110 volt current above 20 mA (0.020 amperes) through the human body is potentially sufficient to cause cardiac arrest or serious harm if it persists for more than a small fraction of a second.
RCDs are designed to disconnect the conducting wires (“trip”) quickly enough to potentially prevent serious injury to humans, and to prevent damage to electrical devices.
RCDs are testable and resettable devices—a test button safely creates a small leakage condition, and another button resets the conductors after a fault condition has been cleared.
Some RCDs disconnect both the energized and return conductors upon a fault (double pole), while a single pole RCD only disconnects the energized conductor. If the fault has left the return wire “floating” or not at its expected ground potential for any reason, then a single-pole RCD will leave this conductor still connected to the circuit when it detects the fault.