BS 1363 Plug
BS 1363 is a British Standard which specifies the common single-phase AC power plugs and sockets that are used in the United Kingdom. Distinctive characteristics of the system are shutters on the line and neutral socket holes, and a fuse in the plug. It has been adopted in many former British overseas territories. BS 1363 was introduced in 1947 as one of the new standards for electrical wiring in the United Kingdom used for post-war reconstruction. This plug corresponds to Type G in the IEC table. BS 1363 replaced the BS 546 plug and socket (which are still found in old installations or in special applications such as remotely switched lighting). Other exceptions to the use of BS 1363 plugs and sockets include equipment requiring more than 13 A, low-power portable equipment (such as shavers and toothbrushes) and mains-operated clocks.
UK plugs are no doubt among the safest in the world, but also among the most hulking and cumbersome. That’s why people often make fun of them saying that a British plug is mostly bigger than the appliance it is connected to… Moreover, the bottom-heavy design of the plug makes it a perfect caltrop.
BS 1363-1 specifies the dimensions of plug pins and their disposition with respect to each other in precise, absolute terms. The line and neutral pins have a rectangular cross section 6.4 mm by 4.0 mm, 17.7 mm long and with centres 22.2 mm apart. The protective-earth pin is a rectangular cross section 8.0 mm by 4.0 mm, 22.3 mm long and with a centre line 22.2 mm from the line/neutral pin centre line. The dimensions were originally specified in decimal inches with asymmetric tolerances and redefined as minimum and maximum metric dimensions in BS (hidden):(hidden).
Dimensions are chosen to provide safe clearance to live parts. The distance from any part of the line and neutral pins to the periphery of the plug base must be not less than 9.5 mm. This ensures that nothing can be inserted alongside a pin when the plug is in use, and helps keep fingers away from the pins. The longer earth pin ensures that the earth path is connected before the live pins, and remains connected after the live pins are removed. The earth pin is too large to be inserted into the line or neutral sockets by mistake.