The earth leakage circuit breaker, or ELCB, is a device designed to isolate the supply of electricity to a circuit should an earth fault occur. It consists essentially of a tripping coil which is energised by an earth fault current. As it trips it opens a circuit breaker, isolating the faulty circuit. There are two types of circuit beaker, one being voltage operated, the other current operated. However, in most countries the voltage operated device is not allowed under modern electrical regulations.
The current operated device is known as the RCD, or residual current device. This operates by detecting an out of balance current caused by earth leakage. The current in a circuit flows from the live wire, through the appliance and back through the neutral. Therefore the current in the live wire is the same as that in the neutral wire. When there is a leakage of current to earth, the current in the neutral is reduced, causing an imbalance. It is this imbalance which causes the ELCB to trip.
A single RCD can be used to protect all of the circuits in the house, and is either fitted in the consumer unit or in a separate enclosure between the meter and the consumer unit. Alternatively a single circuit supplying socket outlets can be protected, or even just one socket outlet. Often a socket in a garage or greenhouse would be fitted with an RCD, as would a socket into which a lawnmower or similar outside appliance is often plugged. For the above an RCD with a tripping current of 30mA (30 thousandths of an amp) would normally be used, as opposed to the 100mA unit often used to protect a whole house.
