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AC power plugs and sockets connect electric equipment to the alternating current (AC) mains electricity power supply in buildings and at other sites. Electrical plugs and sockets differ from one another in voltage and current rating, shape, size, and connector type. Different standard systems of plugs and sockets are used around the world.

Plugs and sockets for portable appliances became available in the 1880s, to replace connections to light sockets with wall-mounted outlets. A proliferation of types developed for both convenience and protection from electrical injury. Today there are about 20 types in common use around the world, and many obsolete socket types are found in older buildings. Coordination of technical standards has allowed some types of plug to be used across large regions to facilitate trade in electrical appliances, and for the convenience of travellers and consumers of imported electrical goods.

Some multi-standard sockets allow use of several types of plug; improvised or unapproved adaptors between incompatible sockets and plugs may not provide the full safety and performance of an approved socket-plug combination.

Concepts and terminology

A plug is the movable connector attached to an electrically operated device, and the socket is fixed on equipment or a building structure and connected to an energised electrical circuit. The plug is a ''male'' connector, often with protruding pins that match the openings and ''female'' contacts in a socket. Some plugs have female contacts that are used only for an earth ground connection. Some plugs have built-in fuses for safety.

To reduce the risk of electric shock, plug and socket systems have safety features in addition to the recessed contacts of the energised socket.

A socket may be surrounded by a decorative or protective cover which may be integral with the socket.

Single-phase sockets have two current-carrying connections to the power supply circuit, and may also have a third pin for a safety connection to earth ground. Depending on the supply system, one or both current-carrying connections may have significant voltage to earth ground.

History

When commercial electric power was first introduced in the 1880s, it was used primarily for lighting. Other portable appliances (such as vacuum cleaners, electric fans, smoothing irons, and curling-tong heaters) were connected to light-bulb sockets.

As early as 1885 a two-pin plug and wall socket format was available on the British market. By about 1910 the first three-pin earthed (grounded) plugs appeared. Over time other safety improvements were gradually introduced to the market. The earliest national standard for plug and wall socket forms was set in 1915.

Application issues

Protection from accidental contact

Designs of plugs and sockets have gradually developed to reduce the risk of electric shock and fire. Plugs are shaped to prevent finger contact with live parts. Sockets may be recessed and plugs designed to fit closely within the recess to reduce risk of a user's fingers contacting the live pins. Contact pins may be sheathed with insulation over part of their length, so as to reduce exposure of energized metal during insertion or removal of the socket. Sockets may have automatic shutters to stop foreign objects from being inserted into energized contacts. Some types can also include fuses and switches.

Earthing (grounding)

A third contact for a connection to earth is intended to protect against insulation failure of the connected device. Some early unearthed plug and socket types were revised to include an earthing pin or phased out in favour of earthed types. The plug is often designed so that the earth ground contact connects before the energized circuit contacts.

The assigned IEC appliance class is governed by the requirement for earthing or equivalent protection. Class I equipment requires an earth contact in the plug and socket, while Class II equipment is unearthed and protects the user with double insulation.

Polarisation

Where a "neutral" conductor exists in supply wiring, polarisation of the plug can improve safety by preserving the distinction in the equipment. For example, appliances may ensure that switches interrupt the line side of the circuit, or can connect the shell of a screw-base lampholder to neutral to reduce electric shock hazard. In some designs, polarised plugs cannot be mated with non-polarised sockets. Wiring systems where both circuit conductors have a significant potential with respect to earth do not benefit from polarised plugs.

Universal sockets

"Universal" or "multi-standard" sockets are intended to accommodate plugs of various types. In some jurisdictions, they violate safety standards for sockets.

Safety advocates, the United States Army, and a manufacturer of sockets point out a number of safety issues with universal socket and adapters, including voltage mismatch, exposure of live pins, lack of proper earth ground connection, or lack of protection from overload or short circuit. Universal sockets may not meet technical standards for durability, plug retention force, temperature rise of components, or other performance requirements, as they are outside the scope of national and international technical standards.

A technical standard may include compatibility of a socket with more than one form of plug. The Thai dual socket is specified in figure 4 of TIS 166-2549 and is designed to accept Thai plugs, and also Type A, B, and C plugs. Chinese dual sockets have both an unearthed socket complying with figure 5 of GB 1002-2008 (both flat pin and 4.8 mm round pin), and an earthed socket complying with figure 4 of GB 1002-2008. Both Thai and Chinese dual sockets also physically accept plugs normally fitted to 120 V appliances (e.g. 120 V rated NEMA 1-15 ungrounded plugs). This can cause an electrical incompatibility, since both states normally supply residential power only at 220 V.

File:Multi plug.jpg, A so-called "universal socket", which meets no official standard but is intended to accept a number of different plug types.
File:Thai socket.jpg, An earthed Thai socket that appears to comply with figure 4 of TIS 166-2549. Although it may accept NEMA plugs, the Thai voltage is 220 V and thus is electrically incompatible with devices designed only for 110 V.
File:Chinasocket.jpg, Chinese dual socket accepting both unearthed 2-pin (upper) and earthed 3-pin (lower) plugs.

Voltage rating of plugs and power cords

Plugs and power cords have a rated voltage and current assigned to them by the manufacturer. Using a plug or power cord that is inappropriate for the load may be a safety hazard. For example, high-current equipment can cause a fire when plugged into an extension cord with a current rating lower than necessary. Sometimes the cords used to plug in dual voltage 120 V / 240 V equipment are rated only for 125 V, so care must be taken by travellers to use only cords with an appropriate voltage rating.

Appliance connections and extensions

So that manufacturers need not build many similar appliances differing only in the type of plug fitted, a common strategy is to provide an IEC 60320 inlet on the appliance and a detachable power cord (mains flex lead) and appropriate plug. The appliance need only to be tested to the power inlet. Some appliances have a switch for selection of voltage.

Extension cords (extension leads) are used for temporary connections when a socket is not within convenient reach of an appliance's power lead. A power strip with multiple sockets may also have a switch, surge voltage protection, or over-current protection.

Special purpose plugs and sockets

Special purpose sockets may be found in residential, industrial, commercial or institutional buildings. Examples of systems using special purpose sockets include:
* "Clean" (low electrical noise) earth for use with computer systems,
* Emergency power supply,
* Uninterruptible power supply for critical or life-support equipment,
* Isolated power for medical instruments, tools used in wet conditions, or electric razors,
* "Balanced" or "technical" power used in audio and video production studios,
* Theatrical lighting,
* IEC 60309 (CEE 17) "plugs, socket-outlets and couplers for industrial purposes", used as a weather-resistant connector for Caravans, Motorhomes, camper vans and tents for mains hook-up at camp-sites.
* Sockets for electric clothes dryers, electric ovens, and air conditioners with higher current rating.

Special-purpose sockets may be labelled or coloured to identify a reserved use of a system, or may have keys or specially shaped pins to prevent use of unintended equipment.

Shaver supply units

National wiring regulations sometimes prohibit the use of sockets adjacent to water taps, etc. A special socket, with an isolation transformer, may allow electric razors to be used near a sink. Because the isolation transformer is of low rating, such outlets are not suitable to operate higher-powered appliances such as hair dryers.

An IEC standard 61558-2-5, adopted by CENELEC and as a national standard in some countries, describes one type of shaver supply unit. Shaver sockets may accept multiple two-pin plug types including Europlug (Type C), Australian (Type I) and BS 4573. The isolation transformer often includes a 115 V output accepting two-pin US plugs (Type A). Shaver supply units must also be current limited, IEC 61558-2-5 specifies a minimum rating of 20  VA and maximum of 50 VA. Sockets are marked with a shaver symbol, and may also say "shavers only."

Isolation transformers and dedicated NEMA 1-15 shaver receptacles were once standard installation practice in North America, but now a GFCI receptacle is used instead. This provides the full capacity of a standard receptacle but protects the user of a razor or other appliance from leakage current.

Types in present use

The plugs and sockets used in a given area are regulated by local governments.

The International Electrotechnical Commission (IEC) maintains a guide with letter designations for generally compatible types of plugs,World Plugs
Retrieved on 27 March 2015. which expands on earlier guides published by the United States Department of Commerce. This is a ''de facto'' naming standard and guide to travellers. Some letter types correspond to several current ratings or different technical standards, so the letter does not uniquely identify a plug and socket within the type family, nor guarantee compatibility. Physical compatibility of the plug and socket does not ensure correct voltage, frequency, or current capacity. Not all plug and socket families have letters in the IEC guide, but those that have are noted in this article, as are some additional letters commonly used by retail vendors.The letter system first appeared in such guides in 1967
''Electric Current Abroad''.
United States Department of Commerce, 1967. Retrieved on 14 December 2013.) which designated types A 2-pin US, B (BS 546), and C (BS 1363). The 1984 edition
''Electric Current Abroad''.
United States Department of Commerce, 1984. Retrieved on 14 December 2013.) uses the letters A to G in the same manner as they are used today, and the last revision of the print series was in 1998 (). The International Trade Administration of the US Department of Commerce now publishes a web version
Electric Current Worldwide
which differs slightly from the IEC list and from Wikipedia's list at Mains electricity by country.

In Europe, CENELEC publishes a list of approved plug and socket technical standards used in the member countries.

Argentina IRAM 2073 and 2071 (Type I)

The plug and socket system used in Class 1 applications in Argentina is defined by IRAM standards. These two standards are; IRAM 2073 "Two pole plugs with earthing contact for domestic and similar purposes, rated 10 A and 20 A, 250 V AC" and IRAM 2071 "Two pole socket – outlets with earthing contact for 10 A and 20 A, 250 V AC., for fixed installations." The plug and socket system is similar in appearance to the Australian and Chinese plugs. It has an earthing pin and two flat current-carrying pins forming an inverted V-shape (120°). The flat pins for the 10 A version measure and for the 20 A version, and are set at 30° to the vertical at a nominal pitch of . The pin length is the same as in the Chinese version. The earthing pin length is for the 10 A version and for the 20 A version. On the plugs, the pole length is for the 10 A version and for the 20 A version. The most important difference from the Australian plug is that the Argentinian plug is wired with the live and neutral contacts reversed.

In Brazil, similar plugs and sockets are still commonly used in old installations for high-power appliances like air conditioners, dishwashers, and household ovens. Although being often called "Argentinian plug," it is actually based on the American NEMA 10-20 standard, and is incompatible with Argentinian IRAM plugs. Since Brazil adopted the NBR 14136 standard which includes a 20 A version, the original motivation to use the NEMA 10-20 plug has ceased to exist.

Australian/New Zealand standard AS/NZS 3112 (Type I), used in Australasia

This Australian/New Zealand standard is used in Australia, New Zealand, Fiji, Tonga, Solomon Islands, and Papua New Guinea. It defines a plug with an earthing pin, and two flat current-carrying pins which form an inverted V-shape. The flat pins measure and are set at 30° to the vertical at a nominal pitch of . Australian and New Zealand wall sockets (locally often referred to as ''power points'') almost always have switches on them for extra safety, as in the UK. An unearthed version of this plug with two angled power pins but no earthing pin is used with double-insulated appliances, but the sockets always include an earth contact.

There are several AS/NZS 3112 plug variants, including ones with larger or differently shaped pins used for devices drawing 15, 20, 25 and 32 A. These sockets accept plugs of equal or lower current rating, but not higher. For example, a 10 A plug will fit all sockets but a 20 A plug will fit only 20, 25 and 32 A sockets. In New Zealand, PDL 940 "tap-on" or "piggy-back" plugs are available which allow a second 10 A plug to be fitted to the rear of the plug. In Australia these piggy-back plugs are now available only on pre-made extension leads.

Australia's standard plug/socket system was originally codified as standard C112 (floated provisionally in 1937, and adopted as a formal standard in 1938), which was based on a design patented by Harvey Hubbell and was superseded by AS 3112 in 1990. The requirement for insulated pins was introduced in the 2004 revision. The current version is ''AS/NZS 3112:2011, Approval and test specification – Plugs and socket-outlets''.

Brazilian standard NBR 14136 (Type N)

Brazil, which had been using mostly Europlugs, and NEMA 1-15 and NEMA 5-15 standards, adopted a (non-compliant) variant of IEC 60906-1 as the national standard in 1998 under specification NBR 14136 (revised in 2002). These are used for both 220-volt and 127-volt regions of the country, despite the IEC 60906-2 recommendation that NEMA 5-15 be used for 120 V connections. There are two types of sockets and plugs in NBR 14136: one for 10 A, with a 4.0 mm pin diameter, and another for 20 A, with a 4.8 mm pin diameter. This differs from IEC 60906-1 which specifies a pin diameter of 4.5 mm and a rating of 16 A. NBR 14136 does not require shutters on the apertures, a further aspect of non-compliance with IEC 60906-1. NBR 14136 was not enforced in that country until 2007, when its adoption was made optional for manufacturers. It became compulsory on 1 January 2010.

Few private houses in Brazil have an earthed supply, so even if a three-pin socket is present it is not safe to assume that all three terminals are actually connected. Most large domestic appliances were sold with the option to fit a flying earth tail to be locally earthed, but many consumers were unsure how to use this and so didn't connect it. The new standard has an earth pin, which in theory eliminates the need for the flying earth tail.

British and compatible standards

BS 546 and related types (Type D and M)

BS 546, "Two-pole and earthing-pin plugs, socket-outlets and socket-outlet adaptors for AC (50-60 Hz) circuits up to 250V" describes four sizes of plug rated at 2 A, 5 A (Type D), 15 A (Type M) and 30 A. The plugs have three round pins arranged in a triangle, with the larger top pin being the earthing pin. The plugs are polarised and unfused. Plugs are non-interchangeable between current ratings. Introduced in 1934, the BS 546 type has mostly been displaced in the UK by the BS 1363 standard. According to the IEC, some 40 countries use Type D and 15 countries use Type M. Some, such as India and South Africa, use standards based on BS 546.

BS 1363 (Type G)

BS 1363 "13 A plugs, socket-outlets, adaptors and connection units" is the main plug and socket type used in the United Kingdom. According to the IEC it is also used in over 50 countries worldwide. Some of these countries have national standards based on BS 1363, including: Ireland, Malaysia, Malta, Singapore, Bahrain, Sri Lanka and Saudi Arabia.

This plug has three rectangular pins forming an isosceles triangle. The BS 1363 plug has a fuse rated to protect its flexible cord from overload and consequent fire risk. Modern appliances may only be sold with a fuse of the appropriate size pre-installed.

BS 4573 (UK shaver)

The United Kingdom, Ireland, and Malta, use the BS 4573 two-pin plug and socket for electric shavers and toothbrushes. The plug has insulated sleeves on the pins. Although similar to the Europlug) was a standards body which published ''Specification for plugs and socket-outlets for domestic and similar purposes'' as CEE Publication 7 in 1951. It was last updated by Modification 4 in March 1983. CEE 7 consists of general specifications and standard sheets for specific connectors.

Standard plugs and sockets based on two round pins with centres spaced at 19 mm are in use in Europe, most of which are listed in IEC/TR 60083 "Plugs and socket-outlets for domestic and similar general use standardized in member countries of IEC." EU countries each have their own regulations and national standards; for example, some require child-resistant shutters, while others do not. CE marking is neither applicable nor permitted on plugs and sockets.

CEE 7/1 unearthed socket and CEE 7/2 unearthed plug

CEE 7/1 unearthed sockets accept CEE 7/2 round plugs with pins. Because they have no earth connections they have been or are being phased out in most countries. Some countries still permit their use in dry areas. Older sockets are so shallow that it is possible to accidentally touch the live pins of a plug. CEE 7/1 sockets also accept CEE 7/4, CEE 7/6 and CEE 7/7 plugs without providing an earth connection. The earthed CEE 7/3 and CEE 7/5 sockets do not allow insertion of CEE 7/2 unearthed round plugs.

CEE 7/3 socket and CEE 7/4 plug (German "Schuko"; Type F)

The CEE 7/3 socket and CEE 7/4 plug are commonly called ''Schuko'', an abbreviation for ''Schutzkontakt'', ''Protective contact'' to earth ("Schuko" itself is a registered trademark of a German association established to own the term). The socket has a circular recess with two round holes and two earthing clips that engage before live pin contact is made. The pins are . The Schuko system is unpolarised, allowing live and neutral to be reversed. The socket accepts Europlugs and CEE 7/17 plugs. It is rated at 16 A. The current German standards are DIN 49441 and DIN 49440. The standard is used in Germany and several other European countries and on other continents. Some countries require child-proof socket shutters; the DIN 49440 standard does not have this requirement.

The plug is used in most countries of Europe, Asia, and Africa, as well as in the countries of Peru, Chile and Uruguay. The few European countries not using it (or not using it predominantly) are Denmark, Switzerland, UK, Italy and Ireland.

CEE 7/5 socket and CEE 7/6 plug (French; Type E)

French standard NF C 61-314 defines the CEE 7/5 socket and CEE 7/6 plug, (and also includes CEE 7/7, 7/16 and 7/17 plugs). The socket has a circular recess with two round holes. The round earth pin projecting from the socket connects before the energized contacts touch. The earth pin is centred between the apertures, offset by . The plug has two round pins measuring , spaced apart and with an aperture for the socket's projecting earth pin. This standard is also used in Belgium, Poland, the Czech Republic, Slovakia and some other countries.

Although the plug is polarised, CEE 7 does not define the placement of the live and neutral and there is no universally observed standard.

CEE 7/2 and 7/4 plugs are not compatible with the CEE 7/5 socket because of the round earthing pin permanently mounted in the socket.

CEE 7/7 plug (compatible with E and F)

The CEE 7/7 plug fits in either French or Schuko sockets.
The CEE 7/7 plug has earthing contacts to connect to either the CEE 7/3 socket or the CEE 7/5 socket. It is polarised when used with a French CEE 7/5 socket, but can be inserted two ways into a CEE 7/3 socket. The plug is rated at 16 A. Appliances are sold with non-rewireable CEE 7/7 plugs attached. This plug can be inserted into a Danish Type K socket, but the earth contact will not connect.

CEE 7/16 plugs

The CEE 7/16 unearthed plug is used for unearthed appliances. It has two round 4 by 19 mm (0.157 by 0.748 in) pins, rated at 2.5 A. There are two variants.

=CEE 7/16 Alternative I

=
Alternative I is a round plug with cutouts to make it compatible with CEE 7/3 and CEE 7/5 sockets. (The similar-appearing CEE 7/17 has larger pins and a higher current rating.) This alternative is seldom used.

=CEE 7/16 Alternative II "Europlug" (Type C)

=

Alternative II, popularly known as the Europlug, and is still in use on parts of the London Underground for 110VAC voltage supply. In the 1960s they were used for 240VDC in the Power laboratory of the Electrical Engineering department of what was then University College, Cardiff. Power was supplied by the public 240VDC mains which remained available in addition to the 240VAC mains until circa 1969, and thereafter from in-house rectifiers. They were also used in the Ministry of Defence Main Building on circuits powered from the standby generators to stop staff from plugging in unauthorised devices. They were also known to be used in some British Rail offices for the same reason.

Italian BTicino brand Magic security connector

The Italian manufacturer BTicino in the 1960s introduced an alternative to the Europlug or CEI 23-16 connectors then in use. The socket is rectangular, with lateral key pins and indentations to maintain polarisation, and to prevent insertion of a plug with different current ratings. Three single-phase general purpose connectors were rated 10 A, 16 A and 20 A; and a three-phase industrial connector rated 10 A; all of them have different key-pin positioning so plugs and sockets cannot be mismatched. The socket is closed by a safety lid (bearing the word ‘'Magic'’ on it) which can be opened only with an even pressure on its surface, thus preventing the insertion of objects (except the plug itself) inside the socket. The contacts are positioned on both sides of the plug; the plug is energised only when it is inserted fully into the socket.

The system is not compatible with Italian CEI plugs, nor with Europlugs. Appliances were never sold fitted with these security plugs, and the use of adaptors would defeat the safety features, so the supplied plugs had to be cut off and replaced with the security connector. Even so, the ''Magic'' security system had some success at first because its enhanced safety features appealed to customers; standard connectors of the day were considered not safe enough. The decline of the system occurred when safety lids similar to the Magic type were developed for standard sockets.

In Italy, the system was never definitively abandoned. Though very rarely seen today, it is still marked as available in BTicino’s catalogue, (except for the three-phase version, which stopped being produced in July 2011).

In Chile, 10 A Magic connectors are commonly used for computer/laboratory power networks, as well as for communications or data equipment. This allows delicate electronics equipment to be connected to an independent circuit breaker, usually including a surge protector or an uninterruptible power supply backup. The different style of plug makes it more difficult for office workers to connect computer equipment to a standard unprotected power line, or to overload the UPS by connecting other office appliances.

In Iceland, Magic plugs were widely used in homes and businesses alongside Europlug and Schuko installations. Their installation in new homes was still quite common even in the late 1980s.

Single phase electric stove plugs and sockets

The plugs and sockets used to power electric stoves from a single-phase line have to be rated for greater current values than those used with three-phase supply because all the power has to be transferred through two contacts, not three. If not hardwired to the supply, electric stoves may be connected to the mains with an appropriate high power connector. Some countries do not have wiring regulations for single-phase electric stoves. In Russia, an electric stove can often be seen connected with a 25 or 32 A connector.

In Norway and parts of Sweden a 25 A grounded connector, rectangular shaped with rounded corners, is used for single phase stoves. The connector has three rectangular pins in a row, with the grounding pin longer than other two. The corresponding socket is recessed to prevent shocks. The Norwegian standard is NEK 502:2005 – standard sheet X (socket) and sheet XI (plug). They are also known as the two pole and earth variants of CEE 7/10 (socket) and CEE 7/11 (plug).

See also

* DC connector
* History of AC power plugs and sockets
* IEC 60309 high-power industrial and polyphase connectors
* IEC 60320 appliance connectors
* Industrial and multiphase power plugs and sockets
* Mains electricity
* Mains electricity by country lists voltage, frequency, and connector types for over 200 countries
* Perilex
* Plug load
* Polyphase system
* Smart plug
* Stage pin connector
* Anderson Powerpole

References

External links

Digital Museum of Plugs and Sockets
(comprehensive collection of plugs and sockets)

Glossary of standards terms

{{Authority control

Electrical standards
Electrical wiring
Mains power connectors