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6.3 Fuses
Why do wires
get hot when a
current passes
through them?
6.3 Fuses
23 May 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.
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.
Testing a fuse
A
1
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?
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
Live
Faulty
Fuse
Live
Fault
Neutral
Neutral
Earth
Fuse
Metal
Case
Earth
Metal
Case
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.