Any material through which electricity can flow is known as a conductor. Most metals conduct electricity well which is why metal (most often copper, as it is properly the most efficient conductor of electricity) is used for electrical wiring.

However, the earth itself, the ground on which we stand, is also an extremely good conductor. In fact, it is an even better conductor than the wiring that’s used for electrical circuits which is why electricity will always flow in to the earth, if it has an opportunity to do so, by the shortest available route through your body to the earth perhaps with fatal results.

A similar thing can occur if live wire comes accidentally in to contact with any exposed metal component of an appliance, including its casing. To prevent this, a third wire is included in the wiring system and connected to the earth, usually via the outer casing of the electricity company’s main service cable called the earth wire is attached to the metal casing of some appliances and to special earth terminals in others, providing direct route to the ground should a fault occur.

The sudden change of route by the electricity known as an earth fault causes a fuse to blow or circuit breaker to operate, cutting off the current. Appliances that are double insulated which usually means they have a non conductive plastic casing that insulates the user from metal parts that could become live must not be earthed with a third wire.

The earth wire either has a green a yellow covering or is bare copper wire sandwiched between the insulated live and neutral wires in an electrical cable. Whenever a bare earth wire is exposed for linking to socket outlets or lighting fittings, it should be covered with a green and yellow sleeve. Metal pipes must also be connected to the earthing system by a separate cable to ensure they do not precipitate an accident during the time it would take for a fault to blow a fuse.