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Mains Electricity - What is a Fuse?

A fuse is a safety device that switches off an appliance
if a current flowing through the live wire gets too big.

A replaceable fuse is inserted in a three-pin plug.
It connects the live pin of the plug to the live wire.

How does a Fuse Work?

A fuse has a rating in amps printed on the outside.
If the current flowing through a fuse goes above
its rated value, then the fuse "blows" (it melts). This
breaks the circuit and disconnects the appliance.
For example, if the fuse rating is 3 amps,
then a current greater than 3 amps will blow the fuse.

We must look at the heating effect of current
to understand how a fuse works in more detail.

Which Fuse is Correct for an Appliance?

If we know the voltage and the power of an appliance
then we can calculate the current flowing through it.
The current tells us which fuse to use for the appliance.

For example, if a lawn mower has a power of 900 watts
and uses the UK mains voltage of 230 volts,
then we can use the equation for current  I = P ÷ V

I = 900 ÷ 230
      = 3·913 amps.

A current of 3·913 amps would blow a 3 amp fuse
(see above) and so we should use the
next highest fuse rating available which is 5 amps.

More about Fuses.

A fuse has its own circuit symbol. Fuses have different
colours for different ratings. This is called colour coding.

A 3 amp fuse is red,
a 5 amp fuse is black (or very dark blue),
a 13 amp fuse is brown.

A fuse does the same job as a circuit breaker.
A RCCB is much quicker to act than a fuse.

See also how a fuse acts with earthing.

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