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Residual-current device

21/06/2017 Comments off
From Wikipedia, the free encyclopedia
 

An RCD does not provide protection against unexpected or dangerously high current (called spikes or surges) when current is flowing in the usual wires in the circuit, therefore it cannot replace a fuse or protect against overheating or fire risk due to overcurrent (overload) or short circuits if the fault does not lead to current leakage. Therefore, RCDs are often used or integrated as a single product along with some kind of circuit breaker, such as a fuse or miniature circuit breaker (MCB), which adds protection in the event of excessive current in the circuit (the resulting RCD with overcurrent protection called an RCBO). RCDs also cannot detect the situation where a human accidentally touches both conductors at the same time, since the flow of current through an expected device, an unexpected route, or a human, are indistinguishable if the current returns through the expected conductor.

RCDs are usually testable and resettable devices. Commonly they include a button that when pressed, safely creates a small leakage condition, and a switch that reconnects the conductors when a fault condition has been cleared. Depending upon their design, some RCDs disconnect both the energized and return conductors upon a fault, while others only disconnect the energized conductor and rely upon the return conductor being at ground (earth) potential. The former are commonly known as « double-pole » designs; the latter as « single-pole » designs. 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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