FAQ: What does the earth do in a cable?

It may also be uninsulated in the case of British standard BS6004 or BS7211  twin and earth. When uninsulated, there is normally a requirement to sleeve with green/yellow for identification.

The earth wire is there in case a fault causes the external facing of an appliance to become live – that is, if it comes into contact with the live wire. Without the earth wire, if someone were to touch that appliance they would complete the circuit and so receive an electric shock. The earth wire instead causes the current to flow down into the ground (another metal pole inserted into the ground and connected to another part of circuit means that earth wire still makes a complete circuit).

Because the earth wire is much more conductive than a human hand, the person can now touch the appliance safely without fear of electrical shock. In most cases the earth offers a very low resistance path, allowing a temporary high fault current, which will rupture any fuse and prevent further current flow.

It should be noted that if an appliance is double insulated, then it is impossible for the casing to become live and so an earth wire is not necessary; however, for all other appliances the earth wire is vital for the safety of the user. It also means that it’s essential that the cables connecting the appliance contain an earth wire so that the earth circuit will work correctly. This includes cabling from an appliance to a power socket, and the cabling between the socket and the main circuit board of the house or workplace. 

A special case to consider is armoured cable, which must itself be earthed in order to be safe. This is because armoured cable includes aluminium or steel under the sheath, which could become live if the cable is damaged (for instance if a nail went through the cable, connecting the live wire to the armour). Without earthing, the cable would be potentially lethal to anyone who tried to handle it.