Just because it says 13A on the plug doesn’t mean it has to have a 13A fuse in!
The plugs that we use on a day-to-day basis are BS1363 – that is they conform to the British Standard 1363, which first came into being in 1947 and has been updated a few times since. Most people refer to the them as ‘square pin plugs’ or simply ’13A plugs’
Usually, you’ll see 13A stamped onto the plug – because this is maximum current the plug is allowed to carry.
However, this does NOT always mean that a 13A fuse should be fitted, or even that it will be fitted.
The purpose of the plug fuse is to protect the cable. Therefore the fuse in the plug should be rated in such a way that, in the event of a fault, the fuse will blow before the cable melts. Fuse sizes do not always correspond exactly to cable sizes, but as a general rule a 0.5mm2 cable (3A) cable should be protected by a 3A fuse, a 0.75mm2 (6A) cable should be protected by a fuse of 5A or 3A, and a 1.25mm2 (13A) or 1.5mm2 (15A) cable should be protected by a fuse of 13A or less. There are one or two exceptions, but as a general rule the fuse rating should be the same or less than the rating of the cable.
A common mistake made by people starting out in PAT Testing is to read the 13A rating on the plug and assume that means the plug is fitted with a 13A fuse, but in fact the correct procedure would be to open the plug and have a look. You could fit a 13A plug onto a washing machine or tumble dryer, fitted with heavy cable, in which case a 13A would probably be the correct value. But, the same plug could be removed and fitted to a small appliance, such as a table lamp, with thinner cable, in which case a 3A fuse should be fitted. It’s still a 13A plug, but the smaller fuse inside is providing protection to the cable.
For the purposes of being complete I will also mention that some moulded plugs (especially those found on computer leads) might be rated at 5A, 7A or 10A. In this case, obviously, a fuse of that value or less would be required.
Don’t assume that a 13A fuse will be fitted with a 13A cable. Open the plug and have a look! (moulded plugs can’t be opened but you can, and should, still check the correct fuse is fitted.)