A residual-current device (RCD), residual-current circuit breaker (RCCB) or ground fault circuit interrupter (GFCI) is an electrical safety device that interrupts an electrical circuit when the current passing through a conductor is not equal and opposite in both directions, therefore indicating an improper flow of current such as leakage current to ground or current flowing to another powered conductor. The device's purpose is to reduce the severity of injury caused by an electric shock. Injury from shock is limited to the time before the electrical circuit is interrupted, but the victim may also sustain further injury, e.g. by falling after receiving a shock. This type of circuit interrupter can not distinguish between current flowing though power carrying conductors that passes through a person from current that passes through electrical equipment and offer no protection when a person touches both conductors at the same time.
If the RCD device has additional overcurrent protection integrated in the same device, it is referred to as RCBO. An earth leakage circuit breaker may be an RCD, although an older type of voltage-operated earth leakage circuit breaker (ELCB) also exists.
These electrical wiring devices are designed to quickly interrupt the protected circuit when it detects that the electric current is unbalanced between the supply and return conductors of the protected circuit. Any difference between the currents in these conductors indicates leakage current, which presents a shock hazard. Alternating 60 Hz 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.
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