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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.
To eliminate this confusion, the IEC decided to apply the term residual current device (RCD) to differential-current-operated ELCBs. Residual current refers to any residue when comparing current in the outbound and return currents in the circuit. In a single phase circuit this is simply the live or phase current minus the neutral current.
Since the earth leakage is restricted, leakage protection devices can be set to less than 750 mA. By comparison, in a solidly earthed system, earth fault current can be as much as the available short-circuit current. The neutral earthing resistor is monitored to detect an interrupted ground connection and to shut off power if a fault is detected.
Until the 1950s, isolated ground domestic mains supplies tended to have no Residual-Current Device or Earth Leakage Circuit Breaker , and too high a ground impedance to blow a fuse if a live-to-earth fault occurred. This could leave metalwork in the house live.
Earth fault protection also requires current transformers and senses an imbalance in a three-phase circuit. Normally the three phase currents are in balance, i.e. roughly equal in magnitude. If one or two phases become connected to earth via a low impedance path, their magnitudes will increase dramatically, as will current imbalance.
In the case of TT installations, where there is no metallic earth return to the supply transformer, the earth loop impedance may be too high to trip a circuit breaker, which is why RCDs are used for additional protection, typically with a 100 or 500 mA trip current, as compared to the 30 mA typically used for protection against electric shock.
A distribution board (also known as panelboard, circuit breaker panel, breaker panel, electric panel, fuse box or DB box) is a component of an electricity supply system that divides an electrical power feed into subsidiary circuits while providing a protective fuse or circuit breaker for each circuit in a common enclosure.
Current interruption in a high-voltage circuit breaker is obtained by separating two contacts in a medium, such as sulfur hexafluoride (SF 6), having excellent dielectric and arc-quenching properties. After contact separation, current is carried through an arc and is interrupted when this arc is cooled by a gas blast of sufficient intensity.
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