6.3 Fuses

Why do wires get hot when a current passes through them?

6.3 Fuses• Fuses and circuit breakers cut off the

electrical circuit when the current gets too high.

• Circuit breakers have advantages over fuses.

• The correct size fuse is important.• Double insulated devices have a shell

of insulating material and so do not need to be earthed.

April 19, 2023

Fuses

• Fuses contain a thin piece of wire which melts when the current is higher than the value shown on it. This prevents fires.

Symbol

Fuse size = Power in Watts230V

E.g. a toaster has a power of 920W. Fuses are available in 3A, 5A and 13A sizes. Which one should be used?

Fuse size = 920W / 230V

= 4A. Use 5A the next highest value.

Testing a fuse

• Set up this circuit with 3cm of fuse wire on a heat proof mat.

• Switch on the power supply and move the variable resistor. What current does the fuse melt at?

AFuse wire

1

Connect a lead from point 1 to the red terminal and switch on. What happens?

The Earth Wire• Electrical objects with a metal case

must be earthed – the case connected to the earth with a wire. This provides a path for electricity to flow if there is a fault “blowing” the fuse.

Earth

Live

Neutral

Fuse

Metal Case

Safe

Earth

Live

Neutral

Fuse

Metal Case

Fault

Faulty

Circuit Breakers• Circuit breakers are electromagnetic

switches which cut off the power when the current gets too high.

Advantages• Faster than fuses• Can be reset

Key points

• A fuse contains a thin piece of wire which melts when too much current passes through it, cutting off the power.

• A circuit breaker is an electromagnetic switch that opens and cuts off the power if too much current passes through it.