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A residual-current device (RCD), residual-current circuit breaker (RCCB) or ground fault circuit interrupter (GFCI) [a] 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 leakage current to ground or current flowing to another powered conductor.
This is a category of devices, which are used to protect instruments, circuits and operators, while Earth leakage. Early ELCBs were voltage operated devices (VO-ELCB), detecting a voltage rise between installation metalwork, and an external electrode. These have now been replaced by current sensing devices (RCD/RCCB).
Breakers for protections against earth faults too small to trip an over-current device: Residual-current device (RCD), or residual-current circuit breaker (RCCB) — detects current imbalance, but does not provide over-current protection. In the United States and Canada, these are called ground fault circuit interrupters (GFCI).
They can be used to test residual-current devices because the current drawn trips most RCDs when the solenoid voltmeter is connected between the live and earth conductors. Some manufacturers include a continuity test lamp function in a solenoid meter; these use the same probes as the voltage test function.
In the case of a TT system where the impedance is high due to the lack of direct connection to the transformer neutral, a residual-current device (RCD) must be used to provide disconnection. RCDs are also used in other situations where rapid disconnection of small earth faults (including a human touching a live wire by accident, or damage) is ...
A residual-current device (RCD), or residual-current circuit breaker (RCCB), is a device that instantly breaks an electric circuit to prevent serious harm from an ongoing electric shock. A fuse or circuit breaker also does that, 'instantly' of course, is subject to qualification.
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When current flows from a line conductor to an earth wire, as is the case when a line conductor makes contact with an earthed surface in a Class I appliance, an automatic disconnection of supply (ADS) device such as a circuit breaker or a residual-current device (RCD) will automatically open the circuit to clear the fault.