Earthing Metal Cases
Physics: Cambridge International GCSE Supplement, topic 4.10.3
Electrical Safety: Earthing Metal Cases
Metals conduct electricity, so metal cases may need earthing to stop them giving us electric shocks.
Metal cases
- Lots of electrical appliances, such as toasters and microwave ovens, have metal casings.
- If there is a fault in the appliance, one of the wires may touch the metal case. Metals conduct electricity, so anyone touching the case could then receive an electric shock.
Casing
- The user cannot receive an electric shock if the casing is non-conducting (double-insulated) because no live part can touch the outside surface.
- If the casing is metal and earthed, any fault causing the live wire to touch the casing makes current flow safely to earth, causing the fuse/circuit breaker to disconnect the supply.
Earth wire
- An earth wire is a low resistance wire that connects the metal case to the ground.
- If the casing is touched by a wire inside the appliance, the current will flow through the earth wire.
- Because the resistance of the earth wire is low, a very high current will flow through it, breaking the fuse in the device. This will stop the flow of current and make the appliance safe.
Test yourself on Earthing Metal Cases
What is the purpose of an earth wire?
Earth wires are needed for appliances with what?