Science Fair Project Encyclopedia
Domestic AC power plugs & sockets
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. In many types of plugs there is no distinction between live and neutral and in a few cases both main pins may be live (see section on live under "the three contacts" for more details on this).
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.
Most common household power is "single phase". In some countries two (which may be split phase two phase or two phases from three phase) or even three phases are wired into a home. However in most places only one phase conductor along with the neutral is connected to each normal socket.
Large appliances with higher power requirements may use three-phase current and have phase 1, phase 2, and phase 3 contacts, an earth contact and in some cases a neutral contact. Sockets with two live connections and a neutral are also common in parts of North America but not elsewhere.
This article concentrates on single-phase plugs and sockets for common domestic use. for other types of mains plugs and sockets see Industrial & multiphase power plugs & sockets and unusual and obsolete plugs and sockets.
Those planning to travel with electrical equipment should also review the electrical travellers' guide
|Wiring diagram for British BS 1363 plug|
|Standard wire colours for plugs|
|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|
|EU & Australia||brown or black||blue||yellow & green|
|UK||brown or red||blue or black||green/yellow (core is usually bare and should be sleeved at terminations)|
|United States and Canada||black (or red)||white||green or bare|
The three contacts
The live contact carries an alternating current (generally in the form of a sine wave as voltage is measured relative to the neutral). The exact voltage varies by country, as set by national regulations and industry standards. In a few cases (older installations in Scandinavian 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 main danger that can be posed by the neutral is it becoming live after a broken neutral in the wiring. Neutral and earth are strongly related and more info on this can be found in Ground (power)
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 (called a "short"), 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 but this has safety problems of its own more info on this can be found in Ground (power)
- Alternative terminology
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.
History of plugs & sockets
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.
However, as shown below, most countries have settled on one of a few common de facto standards; though there are legacy installations of obsolete wiring conventions in most regions of the world. Some buildings have wiring that has been in use for almost a century and which pre-date all modern standards.
World maps by plug/socket & voltage/frequency
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.
Types of plug & sockets
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 I refers to earthed equipment, usually with higher current. Class II refers to unearthed equipment. See Appliance classes.
Type A (American 2-pin)
- NEMA 1-15
This class II unearthed plug with two flat parallel pins is standard in most of North America (including Central America and the Caribbean) on devices not requiring an ground connection, such as lamps and "double-insulated" small appliances. The corresponding sockets are now restricted in Canada and much of the U.S. in favour of the earthed class-I sockets, below, but remain in place in many older homes. Early examples were not polarized, but later ones distinguish the neutral (return) conductor by making it slighly broader than the hot one. It should be borne in mind that America went through a period of grounding appliance casings to the neutral and therefore great care should be taken about polarity when wiring such plugs and sockets and American two-pin plugs should not be changed for any unpolarised type without checking the appliance is not grounded to the neutral. Attention should also be paid to this when running American appliances off a centre-tapped supply such as British site isolation transformers.
- NEMA 2-15, and 2-20
These class II unearthed plugs with two flat parallel pins are 240 V variants of the 1-15. The 2-15 has both pins rotated 90 degrees and the 2-20 has one pin rotated 90 degrees. These are both fairly rare types.
- JIS 8303, Class II
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 from those used elsewhere in the world. These outlets are also unpolarized - the holes in the sockets are the same size as the smaller hole on newer North American type A (and also B) sockets, and the blades on the plugs match this. Japanese devices should be able to fit into a North American outlet without trouble, but most devices from North America would require an adapter to be able to plug into a Japanese outlet (especially if the device in question has a grounding pin).
Type B (American 3-pin)
- NEMA 5-15 / CS22.2, Nº42
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 amperes. 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.
Homes in the U.S. and Canada are normally supplied with both legs and the neutral of a centre-tapped 240 V transformer, making both (nominal) 120 V and 240 V readily available. Lighting circuits and general-purpose outlets, including those illustrated here, are connected to either, but not both, legs (which in common usage are referred to as "phases") and thus supply 120V. Some of the specialized outlets used for 240 V appliances are illustrated at Industrial & multiphase power plugs & sockets, and there is also valuable information at National Electrical Code (US)
- JIS 8303, Class I
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.
- Latin America
In Latin America it is common for people to have North American class 1 appliances with type B plugs and only have type A sockets in their property. A common response to this is simply to cut the earth pin off.
Type C (European 2-pin)
- CEE 7/16 (Europlug)
This two-wire plug is unearthed and has two round, 4 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 A or less. Because it can be inserted in either direction into the socket, it is unpolarised (i.e. live and neutral are connected at random). This plug is also defined in Italian standard CEI 23-5.
- CEE 7/17
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.
- BS 4573
In the United Kingdom and Ireland, there is a special version of the type C plug for use with shavers (electric razors) in bath or shower rooms. It has 5 mm diameter pins on 16.6 mm pitch, and the sockets for this plug 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.
Some Type C sockets can only take 4 mm pins or have plastic barriers in place to prevent Schuko or French plugs entering however many can take 4.8 mm pins and have plenty of room for a 4.8 mm pin round Schuko or French plug to be inserted
Type D (Old British 3-pin)
- BS 546, 5 A
India has standardised on a plug which was originally defined in British Standard 546. 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 ampere plug, along with its 2 A cousin, is sometimes used in the UK for centrally-switched domestic lighting circuits, in order to distinguish them from normal power circuits.
- BS 546, 15 A
This plug is sometimes referred to as type M, but it is in fact merely the 15 A version of the plug above, which it resembles, though its pins are much larger: 7.05 mm × 21.1 mm. Live and neutral are spaced 25.4 mm apart, and earth is 28.6 mm away from each of them. Although the preceding type is standard in India, Sri Lanka, Nepal and Namibia, the 15 A version is also used for larger appliances. Some sockets over there can take both types of plugs. Some countries like South Africa use it as the main domestic plug and socket type, where sockets almost always have an on-off switch built into them. The Type M is almost universally used in the UK for dimmable theatre and architectural lighting installations. It is also often used for non-dimmed but centrally controlled sockets within such installations. The main reason for doing this is that fused plugs, while convenient for domestic wiring (as they allow 32 A socket circuits to be used safely), are not convenient if you have extension cords powering equipment in difficult-to-access locations
Type E (French 2-pin, female earth)
- French type E
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).
Type F (German 2-pin, side clip earth)
- CEE 7/4
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. The Schuko connection system is, of course, unpolarised. 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".
- Gost 7396
The countries of the CIS use a standard plug and socket similar to the Schuko standard. The CIS stanadard is in Russian Standard Gost 7396. 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 mate with type E and F plugs as the outlets have smaller hole diameters than the pins of type E and F.
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.
Type E & F hybrid
- CEE 7/7
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. 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 A. Above that, equipment must either be wired permanently to the mains or connected via another higher power connector such as the IEC 309 system.
Type G (British 3-pin)
- BS 1363
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 pin fused plug for all connections to the power mains. On plugs for class II, two-wire appliances the earth pin is often plastic.
The fuse in the plug is chosen to match the current taken by the appliance, from 3 A to 13 A.
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.
Type H (Israeli 3-pin)
- SI 32
This plug, defined in SI 32, is unique to Israel and is incompatible with all other sockets. 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 A, it also has an earthing pin. Visitors to Israel will find that in practice, sockets are manufactured with widenings in the middle of the V-shape-oriented slots for the energised prongs. This allows the type H socket to accommodate type C plugs.
Type I (Australian 2/3-pin)
- AS 3112
This plug, used in Australia, New Zealand and Papua New Guinea, has an 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 this is not advised due to the differences in both voltage and frequency. Wall sockets almost always have switches on them for extra safety, as in the UK.
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.
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 mandated insulated pins by 2005.
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.
- IRAM 2073
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 Argentine 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.
Type J (Swiss 3-pin)
- SEV 1011
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 amperes. Above 10 A, 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 two-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 A.
Type K (Danish 3-pin)
- Afsnit 107-2-D1
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.
Type L (Italian 3-pin)
The Italian earthed plug/socket standard, CEI 23-16/VII, includes two styles rated at 10 A and 16 A 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. An adaptor is sometimes supplied to fit the large-diameter pin CEI 23-16/VII sockets, in the case of washing machines for example.
- CEI 23-16/VII, 10 A style
The 10 ampere 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 A type L plugs can also be inserted. These plugs are otherwise incompatible with any other socket. This is the plug in the illustrations.
- CEI 23-16/VII, 16 A style
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 A 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") although this is not a correct definition. Sockets are also sold that accept all three types of plug: CEE 7/7 ('German'), CEI 23-16/VII 16 A (large-diameter pins), and 23-16/VII 10 A (small-diameter pins).
Type M (see D)
Type M is used to describe the 15 A version of D.
- Safety note for mainland European plug types C, E, F, J, K, & L
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, especially in old buildings where earthed sockets have been installed on older electrical installations that did not have earth connections.
- Safety note for developing countries
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.
- DC plugs
- Double insulated
- Electrical power
- Extension cable
- Telephone plugs
- Power connector
- Power cord
- Protective multiple earthing
- The original content for this article came from http://users.pandora.be/worldstandards/electricity.htm.
- IEC/TR 60083, a 359-page technical report that summarizes the national standards for domestic AC connectors up to 440 V used in IEC member countries.
- CEE Publication 7, 1963.
- IEC Zone: Plugs and sockets
- AC power cords
- Glossary of standards organisations
- Steve Kropla's World Electric Guide
- Change to UK electrical wire colours 2004
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