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Domestic AC power plugs and sockets allow a connection between the mains (domestic, usually single-phase, AC electrical power) and the appliances commonly used in homes.
A power plug (mains plug) is a mechanical connector that fits into a power point or electrical socket. It has male features, usually brass and often tin or nickel plated, that interface mechanically and electrically to the mains. Such plugs have a live contact, a neutral contact, and an optional earth contact.
A power socket (electrical socket, power point, mains socket, plug-in, outlet, receptacle, or female power connector) is a connection point that delivers mains electricity when a plug is inserted into it. It is the opposite of a plug, and usually has only female features.
Large appliances with higher power requirements may use three-phase current and have phase 1, phase 2, and phase 3 contacts, an optional neutral and an earth contact. However, this article concentrates on single-phase plugs and sockets for common domestic use where three-phase power is not usually available — for these, see Industrial & multiphase power plugs & sockets.
| Internal wiring | |||
|---|---|---|---|
| Wiring diagram for British BS 1363 plug | |||
| Standard wire colours for plugs | |||
| live | neutral | earth | |
| EU & Australia | brown | blue | yellow & green |
| UK & Australia before 1969 | red | black | green |
| United States and Canada (screw colour) | black (gold) | white (silver) | green (green) |
| Standard wire colours for wall sockets | |||
| live | neutral | earth | |
| EU & Australia | black | blue | yellow & green |
The live contact has an AC (generally sinewave) voltage relative to the neutral, the exact voltage varying by country. In a few cases (older installations in Scandanavian countries, the outputs of British site isolation transformers and probably a few other cases) both main conductors may be live, either being two phases from a three-phase system or being from a single-phase transformer with centre-tapped output. Some socket designs do not provide for polarisation and some that do are commonly wired either way round.
The neutral contact is in most (but not all) cases referenced to the earth and except under fault conditions generally does not pose a danger but is nevertheless treated as live in most installation practices.
The earth contact is only intended to carry current when connected to a faulty instrument (except for EMI/RFI filters which do cause a small current down the earth): if a bare live wire in a device gets loose and touches the metal casing of the appliance, somebody touching this part may receive an electric shock. Hence, according to the law in many countries, devices with metal outer casing must use a three-contact plug, and the metal casing must be connected to the earth contact. So, in the event of this kind of fault, the ground will carry off the current and drag the case to earth potential. Also, as this is a short circuit, the circuit breaker will open, or the fuse will blow. On first sight, it might seem that one can get the same protection by connecting the casing to the neutral instead of the earth wire and this was tried in the US at one point. However one then has the additional danger of a live case if there is a fault in the wiring of the neutral wire of the fixed wiring or in extreme cases even the electricity suppliers wiring. Furthermore when grounding to the neutral was introduced in the US people commonly replaced the new polarised plugs with unpolarised ones to fit their existing wiring with the result that in 50% of cases the casing ended up bonded to live. With a seperate earth if there is no provision for earthing it is much more likely to get left unconnected (which is undesirable but not an imminent danger) than to get connected to a live accidently. In situations where earth loop impedence is high because there is no direct connection to the supply transformer a residual-current circuit breaker must be used to protect against ground faults. Such breakers are also used for supplementary protection in other situations.
In the United States, "mains" power is referred to as AC (for alternating current). The live contact may be called line, hot, or line-hot. The neutral contact may be called return, cold, or line-cold. The earth contact is called ground. In Australia, the live contact is called active. Live contacts are usually called phases when there is more than one of them (in three-phase systems). Pins are also known as prongs, contacts or terminals.
When electricity was first introduced into the domestic environment, it was primarily for lighting. However, as it became a viable alternative to other means of heating and also the development of labour-saving appliances, a means of connection to the supply other than via a light socket was required. In the 1920s, the two-pin plug made its appearance.
At that time, some electricity companies operated a split-tariff system where the cost of electricity for lighting was lower than that for other purposes, which led to low-power appliances (e.g. vacuum cleaners, hair driers, etc.) being connected to the light fitting. The picture to the right shows a 1909 electric toaster with a lightbulb socket plug. As the need for safer installations grew, earthed three-contact systems were developed.
The reason we now have over a dozen different styles of plugs and wall outlets is that many countries preferred to develop plug designs of their own, instead of adopting a common standard. In many countries, there is no single standard, with multiple plug designs in use, creating extra complexity and potential safety problems for users.
The outline maps below show the different plug types, voltages and frequencies used around the world, colour-coded for easy reference.
See also List of countries with mains power plugs, voltages & frequencies for specific places.
Electrical plugs and their sockets differ by country in shape and size. We designate each type by a letter, following US government practice, plus a short comment in parentheses giving its country of origin and number of contacts. Subsections then detail the subtypes used in various parts of the world.
Note that Class 1 refers to earthed equipment, usually with higher current. Class II refers to unearthed equipment. See Appliance classes.
This class II unearthed plug with two flat parallel pins is fairly standard in most of North and Central America. Plugs may or may not be polarised and distinguish between the live wire and the return wire. The neutral pin is slightly wider at the tip in a polarised installation.
These class II unearthed plugs with two flat parallel pins are 250 V variants of the 1-15. The 2-15 has both pins rotated 90 degrees; the 2-20 has one pin rotated 90 degrees, and the 2-30 is rotated. These are all fairly rare types.
At first glance, the Japanese plug and socket seem to be identical to NEMA 1-15. However, the Japanese system incorporates tighter dimensional requirements, different marking requirements, and mandatory testing and approval by MITI or JIS. Furthermore, standard wire sizes and the resulting current ratings are different than those used elsewhere in the world.
This is a class I plug with two flat parallel pins and an earthing pin (American standard NEMA 5-15/Canadian standard CS22.2, Nº42). It is rated at 15 amps. This is the plug in the illustration. The earthing pin is longer than the two parallel pins so that the device is earthed before the supply is connected.
Note that the US and Canada are unusual in having three-phase power commonly available in homes. This article deals only with simple domestic plugs and sockets. The article Industrial & multiphase power plugs & sockets has more information about connectors used in North America. There is also valuable information at National Electrical Code (U.S.)
Japan uses a type B that differs from its American counterpart in the same way that the type A one does. It is, however, much less common.
An unearthed version of the North American NEMA 5-15 plug is commonly used in Central America and parts of South America. It is therefore common for users who have bought North American appliances to simply cut off the earthing pin so that the plug can be mated with a two-pole unearthed socket.
This two-wire plug is unearthed and has two round, 19 mm pins, which usually converge slightly. It is popularly known as the Europlug which is described in CEE 7/16. This is probably the single most widely used international plug. It will mate with any socket that accepts 4.0 mm round contacts spaced 19 mm apart. It is commonly used in all countries of Europe except the UK, Ireland, and (former) UK dependencies such as Malta. It is also used in various parts of the developing world. This plug is generally limited for use in class II applications that require 2.5 amps or less. It is, of course, unpolarised. This plug is also defined in Italian standard CEI 23-5.
This peculiar unpolarised plug might easily be categorised under E or F. It has two pins like 7/16 does, but they are 4.8 mm in diameter like types E and F, and also a round, plastic or rubber base that stops the plug being inserted into small sockets that 7/16 can fit into. Instead, only large round sockets such as those intended for types E and F can take it. The base has holes in it to accommodate both side contacts and socket earth pins. Class II applications. Also defined in CEI 23-5.
In the United Kingdom and Ireland, there is a special version of the type C plug for use with shavers (electric razors) in bathrooms (where normal sockets cannot be used close to wet zones). It has 5 mm diameter pins on 16.6 mm pitch, and can often take CEE 7/16, US and/or Australian plugs. They are also often capable of supplying either 230 V or 115 V. In wet zones, they must contain an isolating transformer compliant with BS 3535.
India has standardised on a plug which was originally defined in British Standard 546 (one of the standards in Britain). It has three large round pins in a triangular pattern, and is now almost exclusively used in India, Sri Lanka, Nepal, Namibia and Hong Kong. However, this 5 amp plug, along with its 2 amp cousin, is sometimes used in the UK for centrally-switched domestic lighting circuits, in order to distinguish them from normal power circuits.
There is a 15 A version of this plug, known as type M, which we have classified under Industrial & multiphase power plugs & sockets.
France, Belgium and some other countries have standardised on a socket which is not compatible with the CEE 7/4 socket (type F) that is standard in Germany and other continental European countries. The reason for incompatibility is that earthing in the E socket is accomplished with a round male pin permanently mounted in the socket. The plug itself is similar to C except that it is round and has the addition of a female contact to accept the earthing pin in the socket. It has two round pins measuring 4.8 × 19 mm, spaced 19 mm apart.
The pins are slightly thicker than those of type C; as a consequence, they do not fit into type L sockets, but may be generally forced into them (care must be taken when pulling out the plug not to pull the socket as well).
Plug F, known as CEE 7/4 and commonly called a "Schuko plug", is like E except that it has two earthing clips on the sides of the plug instead of a female earth contact. Because the CEE 7/4 plug can be inserted in either direction into the receptacle, the Schuko connection system is unpolarised (i.e. live and neutral are connected at random). It is used in applications up to 16 amps. Above that, equipment must either be wired permanently to the mains or connected via another higher power connector such as the IEC 309 system.
"Schuko" is an abbreviation for the German word Schutzkontakt, which means "Protective (i.e. earth) contact".
In order to bridge the differences between sockets E and F, the CEE 7/7 plug was developed: it has earthing clips on both sides to mate with the CEE 7/4 socket and a female contact to accept the earthing pin of the type E socket. Nowadays, when appliances are sold with type E/F plugs attached, the plug is CEE 7/7 and non-rewirable (see second photo). This means that the plugs are identical in countries like France and Germany: only the sockets are now different. One is only likely to come across type E/F plugs that are not compatible with the other type if for some reason a cheap replacement plug has been attached to a cord that originally had another plug.
Note that the CEE 7/7 plug is polarised when used with a type E outlet. The plug is rated at 16 amps. Above that, equipment must either be wired permanently to the mains or connected via another higher power connector such as the IEC 309 system.
The countries of the CIS use a standard plug and socket defined in Russian Standard Gost 7396 which is similar to the Schuko standard. Contacts are also 19 mm apart, but the diameter of these pins is 4.0 mm (like C) instead of 4.8 mm (E and F). It is possible to mate Russian plugs with Schuko outlets, but Russian sockets will not allow to connect type E and F plugs as the outlets have smaller hole diameters than the pins of those two plugs mentioned. Many official standards in Eastern Europe are virtually identical to the Schuko standard. Furthermore, one of the protocols governing the reunification of Germany provided that the DIN and VDE standards would prevail without exception. The former East Germany was required to conform to the Schuko standard. It appears that most if not all of the Eastern European countries generally use the Schuko standard internally but, until recently, they exported appliances to the Soviet Union with the Soviet standard plug installed. Because the volumes of appliance exports to the Soviet Union were large, the Soviet plug has found its way into use in Eastern Europe as well.
This plug has three rectangular prongs that form a triangle. Live and neutral are 4 × 6 × 18 mm with 9 mm of insulation (the insulation ensures that even small fingers cannot touch any live part of the prongs while unplugging an appliance), and spaced 22 mm apart. Earth is 4 × 8 × 23 mm. British Standard BS 1363 requires use of a three-wire earthed and fused plug for all connections to the power mains (including class II, two-wire appliances).
The fuse in the plug is chosen to match the current taken by the appliance, from 3–13 amps. BS 1363 was published in 1962 and since that time it has gradually replaced the earlier standard plugs and sockets (type D) (BS 546).
This very safe system is used in the UK and many of its former colonies. See BS 1363 for further information on safety features, where it is used, and more.
This plug, defined in SI 32, is unique to Israel and is incompatible with all other plugs. It has two flat pins like the type B plug, but they form a V-shape rather than being parallel like B plugs. Rated at 16 amps, it also has an earthing pin.
This plug, used in Australia and New Zealand, has also a earthing pin and two flat pins forming a V-shape. There is an unearthed version of this plug as well, with only two flat V-aligned pins. These flat blades measure 6.5 by 1.6 mm and are set 30° to the vertical on a nominal pitch of 13.7 mm. It is easy to bend them straight with pliers to force them into American sockets.
Although the above plug looks very similar to the one used in Israel (type H), both plugs are not compatible. Australia's standard plug/socket system is described in SAA document AS 3112 and is used in applications up to 10 A. As of 2003, the latest major update is AS/NZS 3112:2000, which mandates insulated pins by 2005.
A variant plug with a slightly longer, wider and thicker earth pin is used for devices drawing up to 15 ampères; sockets supporting this pin will also accept 10 A plugs.
Wall sockets almost always have switches on them for extra safety, as in the UK.
New Zealand and Papua New Guinea have the same system as Australia.
Although there are slight differences (the pins are 1 mm longer) the Australian plug mates with the socket used in the People's Republic of China (mainland China). The standard for Chinese plugs and sockets was set out in GB 2099.1–1996 and GB 1002–1996. As part of China's commitment for entry into the WTO, the new CPCS (Compulsory Product Certification System) has been introduced, and compliant Chinese plugs have been awarded the CCC (China Compulsory Certification) Mark by this system. The plug is three wire, grounded, rated at 10 A, 250 V and used for Class 1 applications.
The Argentine plug is a three-wire, earthed plug rated at 10 A, 250 V used in Class 1 applications in Argentina and Uruguay.
This plug is similar in appearance to the Australian and Chinese plugs. The pins are 1 mm longer than those of the Australian version and there are slight differences in the specified body dimensions.
The most important difference lies in how the Argentinian plug is wired: the positions of the live and neutral contact pins are reversed from those of the Australian plug. With devices conforming to current standards this should not matter too much in practice as neutral is generally treated with the same care as live in appliance design. However with older or non-complying equipment, using for example single pole switches to break only the live conductor rather than both live and neutral, this difference can be dangerous.
Switzerland has its own standard which is described in SEV 1011. This plug is similar to C, except that it has the addition of an earth pin off to one side. Swiss sockets can take europlugs (CEE 7/16). This connector system is rated for use in applications up to 10 amps. Above 10 amps, equipment must be either wired permanently to the electrical supply system with appropriate branch circuit protection or connected to the mains with an appropriate high power industrial connector.
This type of socket can also be sporadically encountered in buildings in Spain, where they may be erroneously referred to as enchufes americanos — American sockets.
Switzerland also has a 2 pin plug, with the same pin shape, size and spacing as the SEV 1011's live and neutral pins, but the hexagonal form factor is more flattened. It fits into both Swiss sockets (round and hexagonal) and CEE 7/16 sockets and is rated for up to 10 amps.
The Danish standard is described in Afsnit 107-2-D1. The plug is similar to F except that it has a earthing pin instead of earthing clips. The Danish socket will also accept the CEE 7/4, CEE 7/7, CEE 7/16 or CEE 7/17 plugs; however, there is no earthing connection with these plugs because a male earth pin is required on the plug. The correct plug must be used in Denmark for safety reasons. A variation of this plug intended for use only on surge protected computer circuits has been introduced. The current rating on both plugs is 10 A.
The Italian earthed plug/socket standard, CEI 23-16/VII, includes two styles rated at 10 and 16 amps and differ in terms of contact diameter and spacing. Because they can be inserted in either direction at random, both are unpolarised. CEE 7/16 (type C) plugs are also in common use. Appliances with CEE 7/7 plugs are often sold in Italy, but not every socket can take them.
The 10 amp style is like C except that it is earthed by means of a central earthing pin. Italian sockets designed to accept the Schuko plugs often have an extra hole in the centre so that 10-amp type L plugs can also be inserted. These plugs are otherwise incompatible with any other socket. This is the plug in the illustrations.
This plug is relatively standardised in Libya, Ethiopia, and Chile and is found randomly throughout North Africa, and occasionally in old buildings in Spain.
The 16-amp style is even more idiosyncratic. The pins are a couple of mm further apart, and all three are slightly thicker. The sockets for this often have special holes that can take the 10-amp plugs and CEE 7/16 as well. The packaging on these plugs in Italy may claim they are a "North European" type. They were also referred to as industriale ("industrial") altough this is not a correct definition.
Type M is used to describe the 15 A version of D.
It is classed under Industrial & multiphase power plugs & sockets.
They do not convert electricity. They simply allow an appliance (dual-voltage if necessary), a transformer or a convertor from one country to be plugged into the wall outlet of another country. Adaptors sometimes do not make an earth connection, and so may be unsafe.
These can be made up from appropriate plugs out of extension sockets (or power strips if you want more than one socket) and cable. This method has the advantage of having the exact right parts as opposed to adaptors which tend to try and generalise a bit. Another possibility is to make an adaptor lead with an IEC male plug (with pins like on the cables used to connect power from older PC PSUs to monitors) and an appropriate power strip. Then you just have to source an IEC power cord locally, which should be trivial, and if you used a rewirable IEC (rather than one molded to a cord) you can always unscrew it and replace temporarily with a locally sourced plug if needed.
Transformers are used with "electronic" products. Electronic products have a chip or circuit. Examples are radios, CD or cassette players, shavers, camcorder battery rechargers, computers, computer printers, fax machines, televisions and answering machines. Transformers can also be used with electric appliances and may be operated continuously for many days.
Transformers are available for appliances of different power. Most products have the power printed somewhere on them. Some products list only amps. A rough conversion for the USA and other 110 V countries is 1 amp equals 100 watts and for European and other 240 V countries 1 amp equals 200 watts. The power of your appliance must fall within the range of the convertor or transformer being used (allow a margin of at least 10 watts).
Computers are electronic devices and therefore they must be used with a real transformer. It appears that there may be other conversion methods around which may damage electronic equipment. Most PC power supplies have a voltage switch, on the back of the case, for 110 V or 220 V — this should never be set incorrectly, and should always be checked if a PC has come from an unknown source or may have been tampered with. Laptop battery chargers and AC adaptors generally have a universal input which can take a wide range of voltages without manual switching, so they can be used with only a plug adaptor for the country you will be visiting. In general if an appliance has an input that can accept the local voltage it is generally better to connect it directly than to mess around with transformers or other forms of voltage conversion.
Many European countries use the same basic two-pin plug designs but extended them to be earthed in different ways. Thus it is fairly common to find plug and socket combinations where the live and neutral pins will mate, but the earth pin will not. This also applies if a European plug is forced into a UK socket. Earth connections on European sockets are also sometimes absent or unreliable.
No precise information can be given about electrical systems in developing countries. Even information in this article is to be taken with a pinch of salt due to the fact that formal standards, where they even exist, tend to be ignored. Voltages and frequencies may vary greatly, with different contractors installing different systems at different times. Villages may have no power supply; in towns and cities, different voltages and frequencies may even be supplied to the same building. Earth contacts, where they exist, may not really be connected to the ground, and should not be trusted. Caution is advised.
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