 A residual current device (RCD) is the generic term for a device which monitors the current in the line conductor and the neutral conductor of a circuit in an earthed system.In a normal healthy circuit, the vector sum of the live and neutral current values added together will be zero. Current flowing to earth, due to a line earth fault, will return via the earth conductor, and regardless of load conditions, will be registered as a fault. This current flow will give rise to a residual current (Ires) which will be detected by the device. If the Ires exceeds the rated sensitivity of the RCD, it will automatically activate a tripping of the faulty circuit, thereby protecting people, equipment and buildings from the risk of electrocution and fire. | 
