BS 1363

A typical BS 1363 plug

BS 1363 type electrical socket. Note the closed shutters (Red) blocking entry of foreign objects.

British Standard BS 1363 specifies the type of domestic AC power plugs and sockets most commonly used in the United Kingdom, Cyprus, Ireland, Malta, Malaysia, Singapore and many former British colonies. In these countries, the system is colloquially also known as the 13 amp plug/socket. Other plug types, such as IEC 309 is only seen in industrial and some outdoor applications while BS 546 is limited to old installations and specialised applications, where either the BS 1363 plug is unsuitable or where mateability with the standard variety is not desired e.g. where lamps are controlled by a switch/dimmer.

Trading standards legislation in both the UK and Republic of Ireland requires that all normal electrical goods sold in either country should be fitted with a BS 1363 / IS 401 plug.

A BS 1363 plug has two horizontal, rectangular pins for phase (commonly termed 'live' by non-specialists) 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 may substitute this contact with a plastic pin. Virtually all non-fixed domestic equipment is connected using the BS 1363 plugs, the main exceptions being equipment requiring more than 13 amperes (e.g. larger electric cookers, which are hard-wired), and remotely switched, non-fixed lighting (which use proprietary or BS 546 plugs).

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 are normally 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 switch is just for convenience, and is not a regulatory requirement. 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, for example).

Internal wiring

Safety measures

BS 1362 fuses
(for BS 1363 plugs)

Fuses

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 BS 546 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 cylindrical fuse, 25.4 mm (1") in length, with two metallic end caps of 6.3 mm (1/4") diameter and 5.5 mm long.

BS 1362 specifies breaking-time/current characteristics only for fuses with a current rating of 3 A (marked in red) or 13 A (marked in brown). Examples for the required breaking-time ranges are

• 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.

(The standard fuses react slowly at low currents in comparison to modern thermo-magnetic miniature circuit breakers. For example, the commonly used "Type B" 10 ampere ("B10") circuit breaker must break a 50 A current in less than 100 milliseconds. However, if there is a dead short from live to earth fuses can act faster than circuit breakers, and at the extremely high currents in this situation, are more likely to fail safe, as circuit breakers can be welded in-circuit by the high currents generated in these circumstances. Most domestic MCBs are not rated to break currents of greater than 10 kiloAmps, and many older MCBs are not rated to break currents above 3 kiloAmps. For this reason, it is very important to calculate the PSCC (Prospective Short Circuit Current) for the circuit the MCB is meant to protect and ensure the chosen MCB has a high enough rating to break the expected PSCC).

The 3 A fuses are intended mainly for small load (< 750 W) appliances, such as radios and desk lamps. The 13 A fuses are for larger load (<3.2 kW) appliances such as irons and electrical heaters.

BS 1362 requires that plug fuses with any other current rating are marked in black. 5 A fuses are also commonly used, for medium load (1250 W max.) appliances such as desktop computers and TV sets. Ratings of 1, 2, 7 and 10 amperes are also produced, but rarely used (1 A as found in some shaving-socket adapters is 20mm long (BS 646) but a BS 1362 version does exist.).

The fuses are mechanically compatible, but inserting a fuse with a too low current rating for the appliance will likely cause the fuse to blow prematurely, while using higher current rating than necessary will increase the time in which a fuse will blow in case of a fault, which might degrade safety.

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 or flex 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

BS 1363 type electrical plug, with a dummy (plastic) earth pin, also showing pin insulation

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.

Dimensions

The live and neutral pins have a rectangular cross section 6.35±0.13 mm wide and 3.975±0.075 mm height. They are 17.7±0.5 mm long and their centre lines are horizontally 11.115±0.065 mm from the symmetry plane of the plug. The protective-earth pin is centred on the symmetry plane, is 22.73±0.5 mm long and has a cross section 3.975±0.075 mm wide and 7.925±0.125 mm height. The centre lines of the live/neutral pins and the protective pin are vertically 22.23±0.13 mm apart.

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.
• 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.

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 accommodate both types, though the terminal screws inside are nearly always flat-headed. Most modern products come fitted with moulded plugs with removable 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, Scotland's Standard Grade, Ireland's Junior Certificate and Leaving Certificate (along with the concept of a ring circuit and a radial circuit), and Hong Kong's Certificate of Education Examination include the procedure for wiring BS 1363 plugs.

Euro converter plugs

euro converter plug, empty, europlug inserted and closed.

Some manufacturers/distributors use a special type of plug for conversion of Class 2 appliances from mainland Europe which are fitted with moulded europlugs. Unlike a travel adaptor these plugs when closed look fairly similar to normal plugs, although larger and squarer. Inside are two metal clips into which the metal ends of the europlugs pins are clipped. The body of the converter plug is shaped to grip the europlug. The hinged lid is then shut to cover the connections to the europlug and is screwed shut. These plugs have a plastic earth pin and a fuse accessible from the outside and in some cases are fitted with screws that are made to be difficult to unscrew. Similar converters are available for a variety of other plug types but do not seem to be seen much in practice.

These adaptors are usually supplied fitted on all recent Class 2 Sony electrical products with fixed wiring (and not IEC C7 cables) sold in the British Isles instead of typical moulded plugs, probably as a measure to reduce costs .

Countries of use

This design is not only in use in the United Kingdom and Ireland, but also in Sri Lanka, Bahrain, UAE, Qatar, Yemen, Oman, Cyprus, Malta, Gibraltar, Botswana, Ghana, Hong Kong, Macau, Brunei, Malaysia, Singapore, Kenya, Uganda, Nigeria, Iraq, and Zimbabwe. BS 1363 is also standard in several of the former British Caribbean colonies such as Belize, Dominica, St. Lucia, St. Vincent and Grenada. It is also used in Saudi Arabia in 230v installations although 110V installations using the NEMA connector are more common.

In the Republic of Ireland the National Standards Authority of Ireland (NSAI) issued the Irish Standard I.S. 401 Safety Requirements for Rewireable 13A Fused Plugs for Normal and Rough Use Having Insulating Sleeves on Line and Neutral Pins which is largely similar to BS 1363. Any relevant plugs originating or sold, in the Republic, must pass the Irish Standard or an equivalent standard of a member of European Union, which includes BS 1363.

The UK, and some of these other countries, also use the older BS 546 round-pin socket standard.

In some continental European countries (e.g., Germany), the BS 1363 system is today often used for low-voltage (12 V or 24 V) DC connections in photovoltaic systems, because it provides easily available high-current connectors with fixed polarity that are incompatible with the schuko system and europlugs.

Variants

Several manufacturers have made deliberately incompatible variants for use where intermatability with standard plugs and sockets is not desired. Examples include filtered supplies for computer equipment and cleaners' supplies in public buildings/areas (to prevent visitors plugging things in). The most commonly seen variant is one made by MK which has a T-shaped earth pin. The Walsall Gauge 13 A plug was used by the BBC for technical supplies.

• 
  Walsall Gauge 13 A plug (bottom) compared to regular BS 1363 plug
• 
  Walsall Gauge 13 A plug

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).

See also

• Electrical wiring (UK)
• Common electrical adaptors in Hong Kong and the United Kingdom
• Common electrical plug and socket problems in Hong Kong
• Domestic AC power plugs and sockets

References

• British Standard BS 1363: 13 A plugs, socket-outlets, adaptors and connection units.
  • Part 1: Specification for rewirable and non-rewirable 13 A fused plugs. BS 1363-1:1995.
  • Part 2: Specification for 13 A switched and unswitched socket-outlets. BS 1363-2:1995.
  • Part 3: Specification for adaptors. BS 1363-3:1989.
  • Part 4: Specification for 13 A fused connection units switched and unswitched. BS 1363-4:1995.
• British Standard BS 1362: General purpose fuse links for domestic and similar purposes (primarily for use in plugs). 1973.