The Earth Wire
The voltages used in the domestic supplies of many countries is dangerous to many people. Every care must be taken to ensure that people are not exposed to such high voltages. The most commonly used safety device is the fuse/circuit breaker. The fuse in a plug is placed before the live electrode. Devices which have metal cases have earth wires attached to the case. If a fault develops in an appliance and a live wire inside the appliance touches the case, then the case becomes live and since the earth wie is attached to the case, a large current will flow. This blows the fuse – or trips off the circuit breaker if one is used. No current flows, the device stops and the case, which was at a high voltage, is now at earth voltage (zero).
The reason for the high current which blew the fuse or tripped the circuit breaker should be investigated safely before the fuse is replace/circuit breaker reset.
Some devices do not have earth wires. If a device has a plastic case, the no earth wire is needed because plastic is an insulator. Only devices with metal cases need earth wires.