The purpose of the fuse within a BS1363 plug is to protect the cable from the plug to the appliance. The size of the fuse is dependant on the current rating of the cable fitted. However in most cases we can assume that the manufacturer has fitted the correct size of cable and corresponding fuse.
If it has a moulded (non-rewirable) plug fitted, the easiest way is to check the markings on the plug. All UK moulded plugs (BS1363) must have the fuse rating marked on them.
If it’s a re-wirable type plug you can use the 700W rule:
For an appliance rated as 700W or less, a 3A fuse should be fitted. For appliances above 700W, a 13A fuse should be fitted.
One complication of using the 700W rule is that some appliances can draw a higher current when they are first switched on (inrush current). Electric motors for example can draw several times their normal load. So for some appliances, the simple 700W rule may not work and they must be fitted with a fuse higher than the power rating would suggest.
We recently had a call from someone inspecting a fridge. They found it had a 0.75mm2 cable fitted with a 13A fuse and asked if they should change it 3A? This raises a few good points. Firstly, although the fridge is likely to be less than 700W, the compressor on the fridge draws a higher current when first switched on, so a 3A fuse may be too small. But if we look at the table in the back of the IET Code of Practice, a 0.75mm2 cable is only rated at 6A so it seems like the 13A fuse fitted by the manufacturer may be incorrect!
However on page 133 of the IET CoP, there is a note:
"Where manufacturer’s flex is less than 2m in length, has a csa of 0.75mm2 and is fitted with a non-rewirable plug, it may be rated at 10A and be fused to 13A, in accordance with BS1363 Table 2 note C." IET Code of Practice.
Note a - The figure in brackets indicates the fuse rating when a non rewirable plug is used with certain types of equipment where the use of a 5A fuse link is necessary because of the high instantaneous inrush current.
Note c - The figure in brackets indicates the flexible cord size which may be used for cord sets where non-rewirable plugs are used with a maximum flexible cord length of 2m.
So surprisingly, although a 0.75mm2 cable is rated at 6A, a 13A fuse can be used provided the supply cord is less than 2m in length!
Another point worth mentioning is that although BS1363 has standardised on just two preferred fuse ratings, 3A and 13A, any fuse rating up to 13A can still be used. The non-standard fuses are coloured black and are available in 1A, 2A, 5A, 7A & 10A sizes. 5A and 10A are often found in IEC leads connected to IT equipment and do not need to be changed provided they are appropriate for the size of cable.