Mains Cables

Plugs

The inside of a plug showing the positioning of the earth, neutral and live wires, as well as the fuse and cable grip.

A plug connects a device to the mains electricity supply. The cable between the device and the three-pin plug contains three copper wires that are coated with plastic.

Each wire in the mains cable has an important function:

  • The wire covered in brown plastic is the live wire. This carries the 230 V alternating potential difference from the power supply.
  • The wire covered in blue plastic is the neutral wire and this completes the circuit from the appliance back to the supply. It is usually at 0 V.
  • The wire covered in yellow and green plastic is the earth wire. This is also at 0 V and does not form part of the normal circuit. It is there for safety.

Earthing

Without the earth wire if a fault occurs and the live wire becomes loose, there is a danger that it will touch the case. The next person who uses the appliance could get electrocuted. The loose live wire will have made the case 230 V, a person touching the case is also touching the ground and is, therefore, at 0 V. The large potential difference between the hand, on the case, and the feet, on the ground, will push a current through the person's body and shock them.

The earth wire is therefore connected to the case and is attached to a metal plate or water pipe underground. As the wire is made of copper, the earth wire provides a low resistance path to the ground. In the event of a fault, the live current passing through the case will follow this path to the ground instead of passing through a person.

An electric cooker with the wiring section magnified to show the live, neutral and earth wires as well as the mains cable. The earth wire is also connected to the cooker's casing