The ground on which we stand (the earth) is a more effective conductor than the copper wire used in electrical circuits, and electricity will always travel to earth by the shortest means whenever possible. Under normal circumstances, electricity flows a predetermined course (A circuit); feeding all the households appliances as required before returning to its source via the neutral conductor. If a situation were to arise where the 'live' conductor was to come in contact with a metal component, such as the casing of an electrical appliance, a 'short circuit' would occur.
A 'short circuit' allows the electrical current to by-pass its normal route resulting in the metal casing that it has come in contact with to become electrically charged. Anyone unfortunate enough to touch the metal casing would become the electrical current's new route to earth, a situation in some circumstances that could prove fatal.
Though our electrical systems are designed in such a way that it would be difficult to touch any part of the 'live' or unshielded conductor, faults can occur. This is why a 'protective conductor' (earth) is part of electrical circuits. Insulated by the green and yellow cover, the earth conductor is attached to all metal components in the circuit and appliances, which are in turn connected to one heavy earth cable in the consumer unit. This earth cable is usually clamped onto the metal sheathing on the mains service cable, which travels to the Electricity Board's neutral pole, which is connected to earth.
In the event of a short circuit, the fault current flows safely to earth through the earth conductor, and the resulting large current flow disconnects the supply from the appliance, by breaking the circuit or fusing within the consumer unit.