BS 1363 Plug
British Standard BS 1363 specifies the type of mains power plug and socket most commonly used in the United Kingdom, Ireland, and many former British colonies. It also known as the 13 amp plug/socket. The BS 1363 is considered to be one of the safest domestic power-connector system in the world. Other plug types, such as IEC 309 and BS 546, are only seen in old installations and specialised applications, where either the BS 1363 plug is unsuitable or where mateability with the standard variety is not desired.
A BS 1363 plug has two horizontal, rectangular pins for live and neutral, and above these pins, a larger, vertical pin for an earth connection. Unlike with most other types of sockets, the earth pin is mandatory in the BS 1363 plug, as it is needed to open a shutter in the socket. It also polarises the plug. Moulded plugs for unearthed, double-inslated appliances can substitute this contact with a plastic pin.
The fuse is required to protect the flex, as British wiring allows high-current circuits; without the fuse the flex is not considered to be sufficiently protected. Accepted practice is to choose from the commonly available ratings (3 A, 5 A, and 13 A) the smallest rating that will allow for the appliance's peak power consumption. Using a 13 A fuse on an appliance with thin flex is considered extremely bad practice but is still better protection than the flexes of those appliances would get in most other countries.
The live and neutral pins on modern plugs have insulated bases to prevent finger contact with pins and also to stop metal sheets (for example, fallen blind slats) from becoming live if lodged between the wall and a partly pulled out plug. A downside to this prong insulation is that it may contribute to damaged sockets not making good contact with the prongs, which may even melt the latter. No such problems exist with healthy sockets.
BS 1363 Features
- The plug base is broadened near the pins to help keep fingers away from the pins (this was a very early modification to the standard, which was later improved by the use of the insulated pin bases described above).
- The plug sides are shaped to improve grip and make it easier to remove the plug from a socket without placing fingers behind the plug where the pins are.
- The plug is polarised, so it should always be clear which lead or prong is live. This feature is also important to make sure the fuse is in the live side of the supply.
- Wall sockets usually incorporate switches to turn off the power. Some interpret the regs as requiring an adjacent switch where this is not incorporated into the socket, however this interpretation is not widely agreed on by electricians.
- The cable always enters the plug from the bottom, thus making it difficult for people to unplug the (quite firmly fitting) plug by tugging on the cable (this practice — possible with many other plug designs — can be hazardous as it can damage the cable insulation causing shorts or tear the conductors, leading to overheating or even internal arcing across the resulting break in the conductor). On the other hand this tight fitting and bottom entry of cable means that the stresses on the cable when it is pulled hard (by for example tripping over it) are even higher.
- The plug is firmly fitting and therefore difficult to dislodge by accidental knocks or strains on the cord
- The design of the earth pin ensures that the earth path is connected before the live, and remains connected after the live is removed.
- Should the cable be tugged from the plug, the designed lengths of the internal wires should mean that the live wire is disconnected first, followed by the neutral and finally the earth. This ensures that an abused plug will fail safely.
Other devices covered by BS 1363
As described above, BS 1363 specifies 13 A plugs and sockets. It also specifies the following devices:
Adaptors, which permit two or more plugs to share one socket.
Switched and unswitched fused connection units, which take the same BS 1362 fuses as the plugs. These are the standard means of connecting permanently wired appliances to a socket circuit (most often but not always a 30 A ring circuit). They are also used in other situations where a fuse and/or switch is required. Such as when feeding lighting off a socket circuit, to protect spurs off a ring circuit with more than one socket and sometimes to switch feeds to sockets for kitchen appliances (20 A DP switches are also sometimes used for this but doing so can easily violate the rule of no more non-fused spurs than sockets on the ring).