Earthing

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

Without the earth wire, if a fault occurs and the live wire becomes loose, there is a danger that it will touch the metal case.

The next person who uses the appliance could get electrocuted.

The earth pin on the three pin plug is connected by a wire to the metal body of the appliance.

This wire is connected to earth via the plug socket 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.

As the earth wire has virtually no resistance, a large current flows.

This causes the fuse to blow, preventing any further current flow.

Key points

  • the earth wire is a safety feature;
  • it provides a low resistance path to earth.

Fuses

A fuse provides a built-in fail-safe to the electrical circuit for a device.

The fuse contains a thin wire that will melt if the current gets too high.

If there is a fault that causes the casing of the device to become live, a large current will flow through the live wire and low-resistance earth wire.

This high current will cause the fuse to melt.

Once the fuse has melted, the circuit is broken and no more current flows through the device.

This means the case of the device is no longer live and there is no more risk of electrocution.

A circuit breaker can serve the same function as a fuse but can be reset without the need for replacement if it trips.

The fuse or circuit breaker must be connected in the live wire side of a domestic circuit to ensure that it keeps high voltage from reaching the user, or surroundings, if a fault develops.

A fuse protects the appliance but does not protect the person using it.

It can take up to 2 s for a fuse to melt which is long enough for the user to be electrocuted.

Currents as small as 50 mA (milliamp) can cause electrocution and a fuse would not prevent this from flowing along the live wire.

Key point

  • a fuse is a safety feature for the appliance;
  • a fuse is connected in the live wire;
  • the wire inside it melts if the current is too large;
  • once the fuse has melted, the circuit is broken and no more current flows.