Fuses
Because typical British ring circuits can deliver more current than appliance flexible power cords can handle, BS 1363 plugs are required to carry a cartridge fuse. The fuse is manufactured to BS 1362. The maximum load that can be placed on a socket is 13 A; triple and larger sockets are fitted with a 13 A fuse of the same type used in the plugs. The former BS546 standard relied on different sized sockets, and fuses installed at the consumer unit to protect both the permanent wiring and the appliance flexible cord.
When the BS1363 plug was first introduced, there were 5 fuses in the official BS1362 range which were (with their specified colour): 2 (blue), 5 (grey), 7 (black) 10 (yellow), and 13 (brown) amps. The 7 amp fuse was deleted from the official range fairly early on, though remained available for many years along with many other sizes from 250 mA upwards. The current version, BS1362:1973, allows any fuse rating up to 13A, with 3 amp (coloured red) and 13 amp (coloured brown) as the preferred (but not mandated) values when used in a plug. All other ratings to be coloured black (this is why 5 amp fuses are now black instead of grey). Plugs when supplied separately from any appliance should be available with either a 3 or 13 amp fuse fitted, but some are supplied with both (and a few also include the 5 amp size). The fuses are mechanically interchangeable; it is up to the end user or appliance manufacturer to install the appropriate rating. Some suppliers only supply plugs fitted with 13 amp fuses to save duplicating stock
BS 1362 specifies sand-filled ceramic-bodied cylindrical fuses, 1" (25.4 mm) in length, with two metallic end caps of 1/4" (6.3 mm) diameter and roughly 1/5" long. The standards specifies breaking time versus current characteristics only for 3 A or 13 A fuses.
- For 3 A fuses: 0.02–80 s at 9 A, < 0.1 s at 20 A and < 0.03 s at 30 A.
- For 13 A fuses: 1–400 s at 30 A, 0.1–20 s at 50 A and 0.01–0.2 s at 100 A.
Read more about this topic: BS 1363