What is an RCD

An RCD, or residual current device, is a life-saving device which is designed to prevent you from getting a fatal electric shock if you touch something live, such as a bare wire. It can also provide some protection against electrical fires. RCDs offer a level of personal protection that ordinary fuses and circuit-breakers cannot provide.

An RCD measures the current in the circuits that it controls. It measures the current leaving and returning. If there is an imbalance, it assumes that some of the current has leaked out and is causing a danger. The current is immediately (or, at least within 300 milliseconds) switched off and there is no longer any power.