The external metal casing of electrical apparatus must be earthed. There are three reasons for this level requirement (1) to prevent the casing rising to a dangerous voltage if there is a short circuit between the conductor and the casing; (2) to conduct any current away by a safe route; (3) to make sure that a faulty circuit is disconnected from the supply by blowing the fuse when a high current occurs in the lower resistance path to earth.

In most cases where the electrical supply is taken from the public system, the Electricity Board can provide a satisfactory connection to earth although they are not obliged to do so. Because of the increased use of plastic materials for water mains, the traditional method of earthing to the incoming water pipe is no longer acceptable. Under no circumstances should any attempt be made to earth an electrical installation to an incoming mains gas pipe.

Where it is difficult to obtain an earth with a resistance low enough to operate fuses or circuit breakers in the event of an earth fault occurring, it is possible to fit a fault voltage operated device. This will disconnect the supply if a fault causes any metal work to become live at more than 35V. There are, however, some disadvantages and technical difficulties in the application of these devices and they should only be used after careful consideration. The present trend is to use residual current devices instead.