Selecting the correct fuse

The current flowing through an appliance can be calculated by rearranging the equation:

Electric power = voltage x current

P = VI

or

I = \(\frac{P}{V}\)

Once the current is known the next highest fuse rating is chosen.

A fuse rated below the normal operating current would “blow”.

A fuse rated too much above the normal operating current would allow dangerously high current to flow without “blowing”.

This could cause the wiring and the appliance to overheat and start a fire.

Example

A toaster has a power rating of 750 W, 230 V.

I = \(\frac{P}{V}\)

P = 750 W

V = 230 V

I = \(\frac{\text{750}}{\text{230}}\)

I = 3.26 A

The normal current for the toaster is 3.26 A. Hence a 5 A fuse would be selected.

A 3 A fuse would “blow” when the normal operating current flowed.

A 13 A fuse would allow dangerously high current to flow and still not blow.

This could cause the toaster to overheat and start a fire.

Question

A bed side lamp is rated 60 W, 240 V.

Calculate the size of fuse that should be fitted to the lamp for it to operate safely.

The fuse available are 3 A, 5 A and 13 A.

Key points

A fuse is a safety feature.
The wire inside a fuse melts if something goes wrong and the current is too large - this protects the wiring and the device from overheating, and possibly starting a fire.
Once the fuse has melted, the circuit is broken and no more current flows.
The most common fuses are 3 A, 5 A and 13 A.
The fuse selected should be rated slightly higher than the current the device needs to operate normally.