8.12 - Fuses & Earthing

How the earth wire and fuse protect an appliance

• Appliances with a metal case are usually earthed - the case is connected to the earth wire, which has a very low resistance.
• A fuse is a component containing a thin strip of metal that melts if the current through it becomes too high.
• The fuse is connected in the live wire, so when it melts it breaks the circuit and cuts off the live supply.

How they work together in a fault

1. A fault develops so that the live wire touches the metal case.
2. Because the case is earthed through a low-resistance path, a very large current surges through the live wire, the case and the earth wire.
3. This surge melts the fuse, breaking the circuit and cutting off the live supply.
4. The appliance is now isolated, preventing an electric shock from the case and stopping the wires from overheating and causing a fire.

Fuse rating

• The fuse rating is the current at which the fuse melts.
• A fuse should be rated as close as possible to, but just above, the appliance's normal operating current:
  • Too low - The fuse melts during normal use.
  • Too high - The fuse won't melt when a dangerous surge occurs.

What a fuse can and can't do

• A fuse on its own protects the appliance and its wiring, not the person - it can take a second or two to melt, long enough for someone touching a live case to get a shock.
• The earth wire and fuse together protect the person: the earth wire offers a low-resistance route to earth, so fault current flows away through the wire rather than through anyone touching the case, and this large current melts the fuse.

Earthing and double insulation

• Earthing means connecting the metal case of an appliance to the earth wire.
• An earthed conductor can never stay live: if a fault makes a live wire touch the case, a large current flows away through the low-resistance earth wire, blowing the fuse and cutting off the supply.

Double insulation

• An appliance is double insulated if its case is made of a non-conducting material such as plastic, with no exposed metal parts.
• The plastic is an insulator, so the case can't become live and give the user a shock.
• A double-insulated appliance doesn't need an earth wire, so it connects with just a live and a neutral wire (two-core cable).
• Double-insulated appliances still contain a fuse to protect their circuit and cabling from excessive current.