BS 1363

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A typical BS 1363 plug

British Standard BS 1363 specifies the most common type of domestic AC power plugs and sockets that are used in the United Kingdom, Cyprus, Ireland, Malta, Malaysia, Singapore, and many former British colonies. Of these, Ireland, Malaysia and Singapore have equivalent standards—IS 401 & 411, MS 589, and SS 145, respectively. The standard was introduced in 1946, shortly after the Second World War, as part of a new standard for electrical wiring in the United Kingdom.

Trading standards legislation in both the UK^[1] and Ireland requires that all domestic electrical goods sold in either country should be fitted with a BS 1363 or IS 401 (as applicable) plug.

Other plug types used in the same area include IEC 309, only used in industrial and some outdoor applications, and BS 546, 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 (for example, where lamps are controlled by a switch or dimmer).

[edit] Origins

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

This style of plug appeared in 1946, shortly after the Second World War. Britain had previously used a combination of 5 amp and 15 amp round pin sockets, where each had to be wired to the distribution board and each provided with its own fuse. As a result of high wartime demands, Britain suffered from a chronic shortage of copper. Further, as a result of enemy action a considerable amount of housing stock had been destroyed. Britain was in the position of having to rapidly rebuild housing for its population and electrically wire it using as little copper as possible.

The solution was the ring circuit system (often informally called "ring main");, where, instead of each socket being individually wired, a cable was brought from the fuse or circuit breaker in the distribution board and wired in sequence to a number of sockets before being taken back to the distribution board and wired to the same fuse or circuit breaker. This provided considerable savings in the copper required to wire the circuit. Since the fuse or circuit breaker had to be rated for the maximum current the ring could carry (30 A or 32 A for breaker), it was required that the plugs used to connect to the ring each contained their own fuse rated for the appliance that it connected. At first there were several competing designs of plug, but the BS 1363 design displaced all of the others.

Although the need to economise on the use of copper has long passed, the ring main system has proven so successful that Britain retains the system to this day, even in new build houses.

Fuses were originally available in 1, 2, 3, 5, 7, 10, and 13 amps but by the 1960s, this range had reduced to 2 (blue), 5 (grey), 10 (yellow), and 13 (brown) amps. Plugs are normally supplied fitted with a 13 A fuse, and householders tended not to replace them with a more appropriate size. Eventually^[when?] the range was officially reduced to just 3 (red) and 13 (brown) amps. 5 amp fuses have remained freely available but are coloured black (which does not denote a 5 amp fuse but an unofficial size) and may be used for appliances that draw a large current at switch-on but a lower steady load (for example older CRT television sets). Other fuse sizes remain available albeit hard to find.^{[citation needed]}

[edit] Design

Internal wiring. There is more slack in the neutral wire than the live, ensuring that if the lead is forcibly removed, the live wire will disconnect first.
1 - Cable grip; 2 - Neutral terminal; 3 - Earth terminal; 4 - Live terminal; 5 - Fuse

A BS 1363 plug has two horizontal, rectangular pins for phase (commonly termed "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 necessary for use of the BS 1363 plug, as it is needed to push open a shutter in the socket to allow the live and neutral pins to be inserted. It also polarises the plug, ensuring that the live pin is connected to the correct terminal in the socket. Moulded plugs for unearthed, double-insulated appliances may substitute this contact with a non-conductive plastic pin to open the shutter. Most non-fixed domestic equipment is connected using the BS 1363 plugs, the main exceptions being equipment requiring more than 13 amps (e.g. larger electric cookers, which are hard-wired); remotely switched, non-fixed lighting (which use proprietary or BS 546 plugs); and low-power portable equipment, such as shavers, which may be used in several countries. Many bathrooms, particularly in hotels, have 2-pin standard "shaver sockets", which usually accommodate both European and US 2-pin plugs.

Because typical British circuits (especially ring circuits) can deliver more current than many appliance power cords can safely handle, BS 1363 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.

[edit] Shuttering

British power outlets incorporate shutters on the phase and neutral contacts to prevent someone from pushing a foreign object into the socket. Sockets from most manufacturers have shutters which are opened by the earth pin alone. This is longer than the others and must always be present (though on double insulated equipment it may be plastic). Sockets manufactured by MK use a proprietary system which requires that both live and neutral pins of correct dimensions must be inserted simultaneously. On older versions of MK sockets the latter condition was sufficient, but current versions also require that the earth pin is inserted.

The requirement for sockets to be child safe and include a shutter mechanism dates back to a wartime committee which resulted in the publication of "Post War Building Study No. 11 – Electrical Installations" in January 1944. British Standard 1363 followed in 1947 as a result of this initiative.^[2]

Shows how an inverted socket cover opens shutters to expose live parts.

Common socket cover type allows paper clips etc. to be inserted into live socket.

Extraction hole in domed socket cover allows access to live parts (cutaway cover).

Despite the built-in shuttering, socket covers are sold and marketed as preventing children from inserting objects into sockets. Unfortunately the majority of these plug-in devices can be inserted upside down, which serves to disable the internal shutters and result in decreasing safety, not improving it. It has been demonstrated that some of the most popular types on the market actually allow objects such as paper clips to be inserted alongside the line pin of the socket cover. The FatallyFlawed.org.uk campaign was founded in August 2008 to raise awareness of this issue and publishes photographic and video evidence of the dangers.^{[3][4] [5]}

[edit] Pin insulation

BS 1363 type electrical plug, with a dummy (plastic) earth pin, also showing 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 window blind slats) from becoming live if lodged between the wall and a partly pulled out plug.

[edit] 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 on either side of 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.

(These dimensions may be more easily remembered in the original imperial units, which are not mentioned in the current standard: 1/4±0.005 inch wide by 5/32±0.0025 inch high, and 0.695±0.02 inch long. They are 7/8±0.005 inch horizontally from each other, and the same distance vertically from the earth pin, which is 0.895±0.02 inch long. The pin lengths were presumably originally 0.7 and 0.9 inches, and assigned an asymmetric tolerance of +0.005/−0.015.)

[edit] Other safety 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 that it cannot be inserted with the phase and neutral pins reversed. This feature is important to make sure the fuse is in the phase side of the supply.
• 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 design of the earth pin being of a larger dimension prevents it from being inserted into the phase or neutral sockets should this be attempted in error
• The plug is firmly fitting and therefore difficult to dislodge by accidental knocks or strains on the cord.
• The cable always enters the plug from the bottom, thus making it difficult for people to unplug the plug by tugging on the cable. This practice, possible with many other plug designs but difficult with BS 1363, 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.
• If the cable is forcibly pulled from the plug, the phase wire disconnects first, the neutral second, and the earth wire last. This is ensured by forcing the different wires to have varying amounts of slack in them by design of the internal channel of the plug.

[edit] Fuses

BS 1362 fuses (for BS 1363 plugs)

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, while 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 to use 5 A plugs in 15 A sockets and to use 2 A plugs in 5 A sockets, so 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 while 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, and the plug/socket combination had to provide the fuse. It was decided to place the fuse in the plug and to use a new plug configuration so that older, unfused plugs could not be used.

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 13 A. 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.

3 A fuses are intended mainly for small load (< 750 W) appliances, such as radios and lights. 13 A fuses are for larger load (<3.2 kW) appliances, such as heating and heavy-duty electric motors.

BS 1362 requires that plug fuses with any other current rating be 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 20 mm long (BS 646), but a BS 1362 version does exist).

The fuses are mechanically compatible; it is up to the user or appliance manufacturer to use the most appropriate rating.

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.

[edit] 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 cross-head screwdriver—modern plugs often have external screws which accommodate both types, but the terminal screws inside are more often flat-headed; the cross-head and combination screw-heads tend to be made to accommodate Pozidriv rather than Phillips screwdrivers, but this varies between manufacturers. 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 secondary school physics syllabuses in the British Isles and Hong Kong include the procedure for wiring BS 1363 plugs, as well as the concept of a ring circuit and a radial circuit (except in Hong Kong).

[edit] Compatibility

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 by fitting the same plug for all European markets^{[citation needed]}.

[edit] Adoption

The BS 1363 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, Jordan, Macau, Brunei, Malaysia, Singapore, Kenya, Uganda, Nigeria, Iraq, Tanzania 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 230 V installations, although 110 V installations using the NEMA connector are more common.

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 country must pass the Irish Standard or an equivalent standard of a member of the 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 cannot accidentally be connected into higher-voltage Schuko system and Europlug sockets.

[edit] Variants

Several manufacturers have made deliberately incompatible variants for use where intermateability with standard plugs and sockets is not acceptable. Examples include filtered supplies for computer equipment and cleaners' supplies in public buildings and areas (to prevent visitors plugging things in). The most commonly seen variant is one made by MK that has a T-shaped earth pin. Sockets made by MK use a proprietary method of opening shutters that seems to depend on the shape of the phase and neutral pins and does not depend on the earth pin.

The Walsall Gauge 13 A plug was used by the BBC for technical supplies, and sockets for these plugs can be seen in some London Underground stations.

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

Walsall Gauge 13 A plug

MK 13 A Plug with a T-shaped earth pin

[edit] Other devices in the standard

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 or switch (or both) 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 otherwise concealed 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).

[edit] See also

• Common electrical adaptors in Hong Kong and the United Kingdom

[edit] Footnotes

[edit] References

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