Electrical Wiring in The United Kingdom - Plug and Accessory Fuses

Plug and Accessory Fuses

Some accessories require protection at a lower current than that provided by the ring circuit overcurrent protection device. The protection device used in such accessories is a 1" (25.4mm) ceramic cartridge to the requirements of BS 1362:1973 fuse, commonly rated at 3 A (red), 5 A (black), or 13 A (brown).

In the case of permanently connected equipment the fuse is contained in a holder mounted in an accessory known as a Fused Connection Unit, which usually includes an isolator switch and often a neon bulb to indicate if the equipment is powered. In this case the fuse protects the spur (equipment supply) cable and any switch contacts.

In the case of non-permanently connected domestic equipment, a socket rated at 13 A is attached to the ring circuit, into which a fused plug may be inserted. The fuse protects the contacts (including any switch contacts) and the equipment flex. There are two benefits to this arrangement. Firstly with low power equipment a flex with a low current rating (and therefore small diameter) can be used. Secondly, if the equipment is moved to a different socket, it will remain protected by the same (hopefully correct) fuse. The disadvantage is that despite warnings to the contrary people often use a fuse rated at too high a current, or even wrap a blown fuse in aluminium foil, meaning that under fault conditions the contacts and flex will be subjected to anything up to the maximum ring circuit current. This is likely to cause a fire.

The appliance itself should have its own protection measures, such as another fuse, if the plug fuse does not provide the required overcurrent protection (this is often the case with electronic items).

Most fixed-wiring double socket outlets to BS1363 are rated 20A total, 13A per socket, and do not include a 13A fuse. In practice, they are only protected by the ring circuit overcurrent protection of 30A. However multiple sockets on extension leads are usually rated at 13A in total simply because the plug of the extension cable is fused at 13A, and this rating applies to the total rating of all the sockets together. Some wall mounted multiple sockets are also 13A rated in total, these types having an inbuilt 13A fuse.