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.
I = \(\frac{P}{V}\)
P = 60 W
V = 240 V
I = \(\frac{\text{60}}{\text{240}}\)
I = 0.25 A
The normal current for the lamp is 0.25 A.
Hence a 3 A fuse would be selected.
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.