Science Fair Project Encyclopedia
BS 1363 (British Standard) is the standard which defines what is colloquially know as the 13 Amp plug, which is the most common type of mains power plug in the British Isles. 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 it is desired to prevent mateability with the standard variety.
See also the entry in the mains power plug article on the British three-pin plug.
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 many other plugs, the earth pin is mandatory as it is needed to open the shutters. It also polarises the plug. Moulded plugs for unearthed, double-insulated appliances can substitute this contact with a plastic pin.
These plugs are required to carry a cartridge fuse, manufactured to BS 1362, which can be rated at 3, 5, 10 or 13 amperes. The maximum load that can be placed on a socket, including double and triple sockets, is 13 A. The double sockets are unfused, so it is possible to draw up to 26 A before hitting the rated current of any overcurrent protection. However when drawing up the standard it was decided to only require double sockets to be able to take 13 A total!. Most sockets can stand more than this at least in the short term but continuous running at 26 A will result in a damaged socket. Surprisingly however this has not posed a problem in practice probably because of the fact that very few domestic appliances draw the full 13 A for any significant time. 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. The UK power system is officially 230 V +10% −6%. However, in reality, voltages are generally closer to 240 volts than 230, as the old standard before European harmonisation was 240 V ±6%, and most supplies installed to the old standard meet the new standard.
This plug is often referred to as the safest in the world and to many outsiders it often seems excessively safety conscious and somewhat clunky. The high extraction force can be inconvenient, particularly to people with weak hands, such as the elderly. To counter this plugs with handles and straps to fit existing plugs and provide a handle have been produced but never really caught on. The large size can make the plugs inconvenient when there are many plugs in a small space, as on power strips.
All plugs with the exception of a few wall warts are fused. This is in addition to fuse boxes or circuit breakers in houses. This is done for two reasons: First, to better protect different types of appliance, and second, to allow high current 32-A socket circuits to be used safely. These circuits are usually but not always in the form of ring final circuits (informally known as ring mains). 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, electrical heaters and refrigerators.
- 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 not at all in the event of a fault. See fuse (electrical) for further information.
British power outlets incorporate shutters on the live 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 wall warts 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 live and neutral pins and does not depend on the earth pin. Some older sockets simply require equal pressure to be placed upon the live 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 regs permit for other non industrial socket types) means this is not recommended.
The live and neutral pins have insulated bases to prevent finger contact with pins and also to stop metal sheets (e.g. 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.
- The plug is polarised, so it should always be clear which lead or prong is live.
- 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 leads off to the side of the plug, 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 cause shorts.)
- 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 conencted before the live, and remains connected after the live is removed.
Euro converter plugs
Some manufacturers/distributers use a special type of plug for conversion of Class 2 appliances from the continent which are fitted with moulded europlugs. Unlike a travel adaptor these plugs when closed look fairly similar to normal plugs. 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.
In Britain and Ireland, conventional sockets must not be installed in bath or shower rooms. Light switches cannot be near wet zones in bathrooms, shower rooms or swimming pools and must either be a set distance away, outside the room or fitted on the ceiling and operated by an insulated cord. No such strict rules exist for kitchens or most other rooms containing water although the regulations do specify that equipment should be suitable for its environment which would preclude putting normal sockets very close to sinks as they could easily be splashed. Special "shaver sockets" compliant with BS 4573 are permitted because of their isolated design, and are now common in hotels and modern homes.
Some manufacturers produce sockets which incorporate covers and seals making them suitable for use outdoors or in other wet locations (however they are still prohibited in bathrooms). These usually incorporate or are connected through an RCD to provide additional safety. Some older designs require a specially modified plug to maintain the seal while newer designs can maintain a waterproof seal even when used with a normal plug.
There are also strict rules regarding location of sockets near swimming pools which within the pool zones specify that BS4545 (IEC 309) sockets must be used.
Countries of use
This design is in use not only in the United Kingdom, but also in Ireland, Cyprus, Botswana, Ghana, Malta, Hong Kong, Malaysia, Oman, Singapore, Kenya, Iraq, and Qatar and probably other countries where the British have had influence in the past.
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 largly similar to BS 1363. Any relevent 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.
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.
It has been stated in various comedy acts and lists of "universal truths " circulating on the internet that the most painful household incident is stepping on a 13 Amp plug whilst wearing socks. Whilst this is obviously not literally true, it is certainly the case that due to their design, with the cord exiting at the bottom rather than the back of the plug, most 13 Amp plugs will tend to lie with the pins facing upwards. Stepping on a plug in this position would undoubtedly be painful, particularly since the ends of the pins are not rounded (see picture).
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 plug to share one socket.
- Switched and unswitched fused connection units, which are the standard means of connecting permanently wired appliances to a 30 A ring final circuit. These take the same BS 1362 fuses as the plugs.
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