Electricity comes into the house from the mains electricity system via a 2-wire service cable usually laid underground, but in rural areas it often comes via an overhead service cable. The cable terminates in a sealed unit containing the Electricity Board’s service fuse, and mounted on the same board is the electricity meter which registers the electricity consumption in units (KWHs). Fixed close to the meter is the householder’s consumer unit containing a double pole main switch and the circuit fuses, though in some installations miniature circuit breakers (MCBs) are fitted instead of fuses. The consumer unit is linked to the meter by a pair of large size PVC insulated and sheathed cables, one red, the other black in colour. There is also a green, or green yellow PVC insulated earth conductor running from an earth terminal block in the consumer unit to an earth terminal which is usually an earthing clamp secured to the metallic sheath of the Electricity Board’s underground service cable. In some older installations the earthing clamp is on the mains water pipe (which may no longer be used as the sole means of earthing).
Where the Board is unable to provide earthing facilities an earth leakage circuit breaker (ELCB) is fitted in conjunction with an earthing rod driven into the ground outside the house. This circuit breaker is a double pole main switch which automatically trips and cuts off the electricity to all circuits when a line/earth fault (a live conductor coming into contact with earthed metalwork) occurs in the installation. This ELCB is normally wired into the mains between the consumer unit and the meter but some consumer units incorporate this as the main switch. The electricity supply throughout the UK is standardised at 240 volts AC 50 Hz and is a 2-pole service. One pole is termed the live (or line) the other is termed the neutral. As the neutral is solidly connected to earth in the mains distribution system the live pole has a voltage of 240 volts above earth and the neutral is at potential which means zero volts between neutral and earth.
It is because the neutral is earthed that anyone who is standing on a concrete floor or on the earth itself or in contact with earthed metalwork and touches a live wire or contact will receive an electric shock of up to 240 volts which can be fatal. Or if metalwork such as the casing of an electric kettle or heater is not properly earthed and through a fault becomes live with mains electricity, a person touching it will receive a serious electric shock. Normally, should a live conductor come into contact with earthed metalwork a heavy surge of current results and this blows the fuse or trips a circuit breaker, so cutting off the electricity feeding the fault.