3.15 - 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
- A fault develops so that the live wire touches the metal case.
- 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.
- This surge melts the fuse, breaking the circuit and cutting off the live supply.
- 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.
Worked example - Choosing a fuse rating
A 1,500 W appliance is connected to the 230 V mains supply. Fuses rated at 3 A, 5 A and 13 A are available. Which fuse should be used?
Step 1 - Rearrange the formula
Step 2 - Substitution and calculation
Step 3 - Choose the fuse
- The operating current is 6.5 A.
- The 3 A and 5 A fuses would melt during normal use, so the 13 A fuse is chosen - the next rating above 6.5 A.
As a quick guide, use a 3 A fuse for appliances up to about 720 W, and a 13 A fuse for appliances above 720 W.
TermDefinition
Earth wireThe green and yellow safety wire that connects the metal case of an appliance to the ground, so a current can flow safely to earth if there is a fault.
FuseA component containing a thin strip of metal that melts and breaks the circuit if the current becomes too high.