1. Why do wires get hot when a current passes through them?
6.3 Fuses
Why do wires get hot when a current passes through them?

2. 6.3 Fuses
20 April 2017
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.

3. Fuses
Symbol
Fuses contain a thin piece of wire which melts when the current is higher than the value shown on it. This prevents fires.
Fuse size = Power in Watts
230V
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.

4. Testing a fuse 1 A
Fuse wire
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?
Connect a lead from point 1 to the red terminal and switch on. What happens?

5. 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.
Safe
Faulty
Fuse
Fault
Earth
Live
Neutral
Fuse
Metal Case
Live
Neutral
Earth
Metal Case

6. 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

7. 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.