RCDs are often known by other names such as earth leakage circuit breakers (ELCB) or safety switches.
An RCD is an electrical safety device specially designed to immediately switch the electricity off when electricity leaking to earth is detected at a level harmful to a person using electrical equipment.
An RCD offers a high level of personal protection from electric shock.
Fuses or over current circuit breakers do not offer the same level of personal protection against faults involving current flow to earth.
RCDs have another important advantage they reduce the risk of fire by detecting electrical leakage to earth in electrical wiring and accessories.
RCDs work on the principal what goes in must come out. They operate by continuously comparing the current flow in both the Active (supply) and Neutral (return) conductors of an electrical circuit.
RCDs are designed to operate within 10 50 milliseconds and to disconnect the electricity supply when they sense harmful leakage.