AQA GCSE Fuses

Fuses

Fuse is a thin piece of wire between two metal end caps, they are found in plugs.

A fuse acts as a safety device, if too large a current flows along the wire, then the wire becomes too hot, melts and breaks the circuit.

Fuses have different ratings e.g. 3A, 5A or 13 A. This is the maximum current that can flow through the fuse wire.

If a hairdryer uses 4 amps, then a 5 Amp fuse needs to be fitted to the plug.

A 3A fuse would automatically blow as 4A current is too high.

If the hairdryer were to become faulty, then it would draw a current much greater than 4A, causing the fuse to blow, once current exceeds 5A.

What can cause a large current to flow?

In the diagram below, the live wire has broken off, it is now in contact with the metal case of the oven.

This will cause a large current to flow, as charge moves onto the metal case. The large current would cause the fuse wire to become hot, melt and break, therefore breaking the circuit.

At the same time, as the Earth wire connects the metal case to Earth, charge on the metal case would flow to Earth, preventing an electric shock.

Another way for a large current to flow is if the live wire makes contact with the neutral wire in a circuit. This would also cause the fuse wire to become hot, melt and break the circuit.

Example calculations

1. A mains kettle uses 2300W and 230V.

(a)Calculate the current that kettle uses for normal use.

P = V x I

I = P/V

I = 2300/230 = 10A

(b) Suggest the correct fuse rating for the kettle.

Fuses typically are 3A, 5A or 13A.

If the kettle uses 10A for normal operation, then a 13A fuse should be used.

Practice Questions

1.Explain how a fuse works as a safety device

2. A hair dryer uses 1100W and 230V whilst switched on.

(a) Calculate the normal current that the hairdryer would use

(b) Fuses have ratings of 3A, 5A or 13A. Which fuse should be placed into the plug of the hair dryer?

3.Ben is using his radio to listen to music, the fuse blows in the radio. Ben doesn’t have a 3A fuse to replace the one that is broken. However, he does have a 13A fuse. Explain why it would be unsafe to use the 13A fuse to replace the broken 3A fuse.