Residual current devices, also known as each leakage circuit breakers, are rapidly increasing in popularity largely because of their high sensitivity, ease of application and the deficiencies of voltage operated devices.

These devices operate on the principle that in a healthy circuit, the currents in the live and neutral conductors are equal i.e. the current that does into a healthy appliance must come out of it. The live and neutral conductors in the device are wound around an iron ring so that their currents are in opposite directions. This results in no magnetic field in the ring under normal load conditions. Should an earth fault develop, some of the current will return to the supply through the case of the appliance and the earth circuit and some through the neutral conductor. The live and neutral currents therefore will no longer be equal. This current imbalance produces a magnetic field in the ring which produces a voltage in a small trip coil would on it. This causes a current to flow in the trip coil and the main switch to be opened automatically at a predetermined value directly related to the current leaking to earth. Normal earthing arrangements are not altered under this scheme. Residual current devices do not operate on overloads because the same high current flows in both live and neutral conductors and there is thus no imbalance. Overload protection must be ensured by the use of fuses.

Residual current devices can be obtained which are sensitive enough to trip the supply in hundredths of a second. It should be noted that the device does not prevent the person getting a shock nor does it limit the magnitude of the current but it may limit the duration of the shock.

Each device has a test button that should be used routinely to check the operation of it and to free the mechanism of any friction.