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:
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.