BS 1363

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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. In these countries, the system is colloquially also known as the 13 amp plug/socket. Some praise BS 1363 to be one of the safest domestic power-connector system in the world, although it is difficult to make objective comparisons. 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 phase 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-insulated appliances can substitute this contact with a plastic pin. Virtually all non fixed domestic equipment is connected using the BS1363 plugs, the main exception being electric cookers.

Because typical British circuits (especially ring circuits) can deliver much more power than an appliance flex (power cord) can safely handle, these plugs are required to carry a cartridge fuse. The fuse is manufactured to BS 1362 and can be rated at 3, 5, 10 or 13 amperes. 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 plugs and sockets are designed to carry up to 250 volts AC, 50 hertz. For information on the safe installation of BS 1363 sockets, see the article on Electrical wiring (UK).

This plug is often described as the safest in the world and to many outsiders it often seems excessively safety conscious and somewhat cumbersome. The high extraction force can be inconvenient, particularly for people with weak hands, such as the elderly. To counter this, plugs with handles, and straps to provide existing plugs with a handle, have been produced but have not been popular. In addition, most wall sockets have built in switches, reducing the need for plug removal when power is not required. The large size can make the plugs inconvenient when there are many plugs in a small space, as on power strips. On the other hand the side entry flex (cable) gives the combination of plug and flush socket a low profile to the wall (which is useful if plugging something in behind a cupboard).

Safety measures

Fusing

All plugs with the exception of some plug top power supply units are fused. This is in addition to the fuses or circuit breakers in consumer units/distribution boards.

Prior to the introduction of the 13 amp plug the BS546 2 A, 5 A and 15 A plugs were used. With this system the only fuses were in the consumer unit. 15 A sockets were generally given a dedicated 15 A circuit. 5 A sockets might be on a 15 A circuit with multiple sockets or on a dedicated 5 A circuit. 2 A sockets were generally connected to the lighting circuit which was fused at 5 A. Adaptors were available from 15 A down to 5 A and from 5 A down to 2 A so in practice it was quite possible for an appliance with the smallest size of flex to be protected only by a 15 A fuse.

When the practice of using 30 A ring circuits was introduced (to save copper whilst also allowing more flexibility) it was considered unsafe to allow appliance flexes to be connected to such a high rated circuit with no further protection. Due to this the plug/socket combination had to provide the fuse. It was decided to place the fuse in the plug and in order to make sure people only used fused plugs to connect to the new circuits a new plug type was required: the BS 1363 13 A plug.

Putting the fuse in the plug also allows a range of ratings to be fitted providing superior protection for smaller flexes. This is especially useful for long small flexes or for extension leads that are not rated at the full 13A. Fuses for fittings to BS 1363 must conform to BS 1362. This specification describes a sand-filled ceramic bodied fuse, 1" (25.4 mm) in length and 1/4" (6.35 mm) in diameter.

• 3 A fuses (colour-coded red) are intended mainly for small load (750 W max.) appliances such as radios and desk lamps.
• 5 A fuses (black, but so are other less common ratings) are for medium load (1250 W max.) appliances such as desktop computers and TV sets.
• 13 A fuses (colour-coded brown) are for heavy load (3250 W max.) appliances such as irons and electrical heaters.
• Ratings of 1, 2, 7 and 10 amperes are available and are all coloured black, but are rare (1 A is extremely rare).

The fuses are mechanically compatible, but inserting a fuse too weak for the appliance will likely cause the fuse to blow prematurely while using too strong a fuse will degrade safety as the fuse will blow less quickly or in extreme cases not at all in the event of a fault. Having said that, most other systems do not bother with using smaller fuses to protect the smaller flex sizes and it doesn't seem to be a problem in practice.

Rewirable BS 1363 plugs are always sold fitted with a fuse, usually 13 A. This fuse must be changed if fitting the plug to an appliance which requires a fuse with a smaller rating. Plugs with smaller fuse sizes already fitted are sold but are not as common. As a result, before factory-fitted plugs became the norm it was common to have a lot of spare 13 A fuses around that had been removed from newly purchased plugs.

Shuttering

British power outlets incorporate shutters on the phase and neutral contacts to prevent someone from pushing a foreign object into the socket. On most sockets these are opened by the earth pin which is longer than the others and must always be present (though on Class 2 (double insulated) equipment with moulded-on plugs or euro converter plugs (see later) and on class 2 power supply units it may be plastic). A notable exception to this method of opening shutters are sockets made by MK. These use a proprietary system which seems to depend on the shape of the phase and neutral pins and does not depend on the earth pin. Some older sockets simply require equal pressure to be placed upon the phase and neutral shutters (towards the outer edges so as to stop insertion of 4-mm plugs). On sockets which use the earth pin to open the shutters, said shutters can be opened by inserting an object into the Earth hole (a screwdriver works well for this). Electricians do this frequently to allow them to insert test probes into sockets. The 4-mm pin European plug types (CEE 7/16, GOST 7396) will fit in after doing this, and 4.8-mm ones (CEE 7/4, 7/7 or 7/17) will go in if forced. However the lack of earth connection (if the plug has it) and the fact that BS 1363 sockets will often only be protected by a 30 A or 32 A fuse or breaker (which is much higher than the wiring regulations permit for other non industrial socket types) means this is not recommended.

Pin insulation

The phase 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.

Replacement

Plugs for hand-wiring are commonly available, and are usually attached with instruction cards displaying recommended wire lengths. They can be assembled or opened with a flat-head and/or Phillips screwdriver - modern plugs have external screws which acoomodate both types, though the terminal screws inside are always flat-headed. Most modern products come fitted with moulded plugs with removeable fuse sections (these plugs can only be easily removed by cutting off), with some exceptions such as most lamps which usually have normal hand-wired plugs along with a label with wiring instructions wrapped around the flex.

The physics syllabi of the UK's GCSE, Ireland's Junior Certificate and Leaving Certificate, and Hong Kong's Certificate of Education Examination include the procedure for wiring BS 1363 plugs.

Others

• 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 phase and which is neutral. This feature is also important to make sure the fuse is in the phase side of the supply.
• Wall sockets usually incorporate switches to turn off the power. Some interpret the regulations 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 pins, and remains connected after the live pins are removed.
• Should the cable be tugged from the plug, the designed lengths of the internal wires should mean that the phase wire is disconnected first, followed by the neutral and finally the earth. This ensures that an abused plug will fail safely.

Criticism of Design