Main earthing terminal

Connecting an installation's exposed metalwork to earth, so that fault current has a deliberate path back to the supply. If the line conductor touches an earthed casing, that current lets a protective device switch the supply off automatically: a fuse or circuit breaker where the current is large, an RCD where it is small, as in a home with its own earth rod. Without earthing, a faulty casing can simply stay live.

The earth wire of a circuit. It links the metal parts of the equipment the circuit feeds back to the main earthing terminal. It is not the normal load-current return path, although equipment can produce protective-conductor current in normal operation. During an earth fault it provides a path for fault current, working with the earthing arrangement and protective device to achieve disconnection. In UK wiring it is green and yellow, or bare inside twin-and-earth cable, with green-and-yellow sleeving wherever it is exposed.

Conductors connecting extraneous-conductive-parts — incoming metal water, gas and oil services, and exposed structural steel — to the main earthing terminal. Bonding does not clear faults; while a fault lasts it keeps the touchable metalwork of a building much closer to the same potential, so a person touching a pipe and an appliance at the same moment bridges a far smaller voltage. It reduces the shock risk; the fault still has to be disconnected.