AC power plugs and sockets connect devices to mains electricity to supply them with electrical power. A plug is the connector attached to an electrically operated device, often via a cable. A socket (also known as a receptacle or outlet) is fixed in place, often on the internal walls of buildings, and is connected to an AC electrical circuit. Inserting ("plugging in") the plug into the socket allows the device to draw power from this circuit.

Plugs and wall-mounted sockets for portable appliances became available in the 1880s, to replace connections to light sockets. A proliferation of types were subsequently developed for both convenience and protection from electrical injury. 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, and many obsolete socket types are still found in older buildings.

Coordination of technical standards has allowed some types of plug to be used across large regions to facilitate the production and import of electrical appliances and for the convenience of travellers. Some multi-standard sockets allow use of several types of plug. Incompatible sockets and plugs may be used with the help of adaptors, though these may not always provide full safety and performance.

Single-phase sockets have receptacles for two power pins, one to carry the electrical current from the power source to the connected device (called live, line, or phase pin) and another one to carry it back to the power source (called neutral pin). Many sockets have a third receptacle or pin for a safety connection to earth ground. The plug is a male connector, usually with protruding pins that match the receptacles (female contacts) in a socket. Some plugs also have a female contact, used only for the earth ground connection. Typically no energy is supplied to any exposed pins or terminals on the socket, hence the power pins are always part of the plug rather than the socket. In addition to the recessed contacts of the energised socket, plug and socket systems often have other safety features to reduce the risk of electric shock or damage to appliances.

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 plugs and wall sockets was enacted in 1915 in the UK.^{[citation needed]}

Designs of plugs and sockets have gradually developed to reduce the risk of electric shock and fire. Plugs are shaped to prevent bodily contact with live parts. Sockets may be recessed and plugs designed to fit closely within the recess to reduce risk of a user 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 plug. Sockets may have automatic shutters to stop foreign objects from being inserted into energized contacts.^[1] Sockets are often set into a surround which prevents accidental contact with the live wires in the wall behind it. Some also have an integrated cover (e.g. a hinged flap) covering the socket itself when not in use, or a switch to turn off the socket.

Some plugs, particularly the British ones, have a built-in fuse which breaks the circuit if too much current is passed.

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.

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 live 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. In most NEMA 1 plugs, for example, the neutral blade is slightly wider than the hot blade, so it can only be inserted one way. Wiring systems where both circuit conductors have a significant potential with respect to earth do not benefit from polarised plugs.

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.

Various methods can be used to increase the number or reach of sockets.

Extension cords (extension leads) are used for temporary connections when a socket is not within convenient reach of an appliance's power lead. This may be in the form of a single socket on a flexible cable or a power strip with multiple sockets. A power strip may also have switches, surge voltage protection, or overcurrent protection.

Multisocket adaptors (or "splitters") allow the connection of two or more plugs to a single socket. They are manufactured in various configurations, depending on the country and the region in which they are used, with various ratings. This allows connecting more than one electrical consumer item to one single socket and is mainly used for low power devices (TV sets, table lamps, computers, etc.).

They are usually rated for 6 A, 10 A, or 16 A at 250 V or for 15 A or less at 125 V. This is the general rating of the adaptor, and indicates the maximum total load in amperes, regardless of the number of sockets used (for example, if a 16 A 250 V adaptor has four sockets, it would be fine to plug four different devices into it that each consume 2 A as this represents a total load of only 8 A, whereas if only two devices were plugged into it that each consumed 10 A, the combined 20 A load would overload the circuit).

In some countries such adaptors are banned and are not available in shops, as they may lead to fires due to overloading them or can cause excessive mechanical stress to wall-mounted sockets. Adaptors can be made with ceramic, Bakelite, or other plastic bodies.

"Hybrid", "multi-standard", or "universal" sockets are intended to accommodate plugs of two or more types. In some jurisdictions, they violate safety standards for sockets.^[3][4] Safety advocates, the United States Army,^[5] and a manufacturer of sockets^[6] point out a number of safety issues with universal socket and adaptors, 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. The latter was removed from the 2021 revision of the Chinese standard,^[7] but such dual sockets can still be found in many existing installations.

Such Thai or Chinese dual sockets also 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 countries supply residential power at 220 V. These dual sockets also accept French and Schuko plugs (Types E and F), but unsafely: no earth contact is established and live pins may be exposed when a plug is partially inserted.^[8]

Commonly, manufacturers provide an IEC 60320 inlet on an appliance, with a detachable power cord (mains flex lead) and appropriate plug in order to avoid manufacturing whole appliances, with the only difference being the type of plug. Alternatively, the plug itself can often be swappable using standard or proprietary connectors.

Adaptors between standards are not included in most standards, and as a result they have no formal quality criteria defined. Physical compatibility does not ensure that the appliance and socket match in frequency or voltage. Adaptors allow travellers to connect devices to foreign sockets, but do not change voltage or frequency. A voltage converter is required for electrical compatibility in places with a different voltage than the device is designed for. Mismatch in frequency between supply and appliances may still cause problems even at the correct voltage. Some appliances have a switch for the selection of voltage. Many modern devices use AC adapters that accept 100–240 V voltage at 50 or 60 Hz, meaning they can be used anywhere in the world as long as a suitable plug adapter is used.

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,^[9] 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.^[10]

In Europe, CENELEC publishes a list of approved plug and socket technical standards used in the member countries.^[11]

A number of countries in Oceania as well as China and Argentina use a plug that has two flat pins at an angle, forming a V-shape, as well as a flat vertical grounding pin. While the exact pin dimensions and the polarity vary between different countries, these plugs and sockets are all compatible to each other and are known as Type I.

The plug and socket system used in Argentina is defined by several standards of the Argentine Normalization and Certification Institute (IRAM). IRAM 2071 defines two sockets that have the same form factor, but differ in size and rating.^[12] The smaller, more common socket is rated for 10 A. The larger variant, rated for 20 A, is mostly used for higher-power appliances such as air conditioners. IRAM 2073 defines the corresponding plugs used for Class I appliances (that require earthing).^[13] The plug and socket system has an earthing pin and two flat current-carrying pins forming an inverted V-shape (120°).

The pins for the 10 A version measure 6.25 by 1.55 mm (0.246 by 0.061 in). The power pins are set at 30° to the vertical with a distance of 7.92 mm (0.312 in) from their centres to the centre of the plug; they are 18.2 mm (0.717 in) long. The earthing pin is 21.4 mm (0.843 in) long and placed below the plug centre at a distance of 10.31 mm (0.406 in). The pins of the 20 A version are larger – 8.0 by 1.9 mm (0.315 by 0.075 in) – and placed further away from the centre of the plug – 9.53 mm (0.375 in) for the power pins, 11.1 mm (0.437 in) for the earthing pin. At 21.8 mm (0.858 in), the earthing pin is a bit longer, while the power pins are a bit shorter at 17.8 mm (0.701 in).

There is also an unearthed version of the 10 A plug, defined by IRAM 2063, that lacks an earthing pin and can be used with Class II appliances.^[14] 20 A plugs, on the other hand, must always provide earthing.

Insulating sleeves around the upper parts of the power pins are allowed, but not required. When used, they significantly reduce the risk of electric shock from accidentally touching the pins of a partially inserted plug. Because the pins of the 20 A plug are placed farther apart than those of the 10 A plug, the two Argentine plug types are mutually incompatible. It is not possible to insert a 20 A plug into a 10 A socket or vice versa.^{[citation needed]}

The most important difference from the Australasian and Chinese plugs is that the Argentine plug is wired with the live and neutral contacts reversed.^[15] In Argentina, when the earth contact is positioned downward, the live (line) contact is on the right side of the socket,^[16] while in Australasia and China it is on the left (assuming the same orientation).

In Brazil, Type I–style plugs and sockets are still commonly used in old installations for high-power appliances like air conditioners, dishwashers, and household ovens. These are often called "Argentine plugs" and are indeed physically identical to the IRAM plugs rated for 20 A, though they might have been developed independently on the basis of the American NEMA 10-20 standard (which uses the same form factor). While these plugs are sometimes used for normal single-phase power and wired in the same way as in Argentina, some are wired for split-phase power with two "hot" (or phase) wires and a neutral wire, but no earth connection. Care must therefore be taken that the same wiring scheme is used in the socket and in the plug, since otherwise the equipment would surely be damaged. In newer installations, the 20 A version of the Type N plug, adopted by Brazil as national standard NBR 14136, is generally preferred.^[17]

This Australian/New Zealand standard is used in Australia, New Zealand, Fiji, Tonga, Solomon Islands, and Papua New Guinea.^[18] The plug shape and basic dimensions are identical to the 10 A plug used in Argentina. The Australasian standard defines a series of plugs and sockets for devices drawing up to 10, 15, 20, 25 and 32 A that are one-way compatible. All sockets accept plugs of equal or lower current ratings, 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 all plugs, the power pins are set at 30° to the vertical with a distance of 7.92 mm (0.312 in) from their centres to the centre of the plug, while the earthing pin is placed below the plug centre at a distance of 10.31 mm (0.406 in) – exactly the same dimensions as used by the Argentine 10 A plug. The three flat pins all measure 6.35 by 1.63 mm (0.250 by 0.064 in) – very slightly larger than in Argentina. The length of the power pins is 17.06 mm (0.672 in), while the earthing pin has a length of 19.94 mm (0.785 in) – somewhat shorter than in Argentina.

In the 15 A plug, the height of the earth pin is increased from 6.35 to 9.08 mm (0.357 in), while the dimensions of the power pins remain the same. In the 20 A plug, however, their height is increased in the same way, so that the surface dimensions of the three pins are again identical. In the 25 A version, the earthing pin takes the shape of an inverted L (an extra bar is added at the top end), and in the 32 A version it takes the shape of a sideways U (extra bars are added at both ends).

In general, only 10 A and 15 A socket outlets are likely to be encountered in domestic or commercial installations. 20 A sockets are rare and higher rated ones are very rare. An unearthed version of the 10 A plug with two angled power pins but no earthing pin is used with double-insulated appliances, but sockets and higher-rated plugs always include an earth contact. 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.

A novel feature of modern Australian socket outlets is a snap-fitting surround for the socket, covering the screws that affix the socket to the wall, and extending all the way around the socket periphery. Australia is unique in having electrical rules that stipulate that only licenced electricians may remove the socket from the wall, so the easily removed outer cover facilitates repainting the wall without exposing power and incurring the expense of engaging an electrician to remove and replace the sockets.

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.^[19] The current version is AS/NZS 3112:2017.^[20]

The standard for Mainland Chinese plugs and sockets (excluding Hong Kong and Macau) is set out in GB 2099.1 and GB 1002. 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 Mark by this system. The plugs are three-wire, earthed, rated for 10 or 16 A, 250 V, and are chiefly used for Class I applications. The pins of the 10 A version are 1.5 mm (0.059 in) thick by 6.4 mm (0.252 in) wide; the centres of the power pins are placed 7.9 mm (0.311 in) from the plug centre, while the centre of the earth pin is 10.3 mm (0.406 in) away. In both plug types, the live and neutral pins are 18 mm (0.709 in) long, while the earth pin is 21 mm (0.827 in) long. This makes the 10 A version compatible with Argentine and Australasian 10 A sockets, as their dimensions are nearly identical.^[21]

Except for the pin length, all dimensions of the 16 A version are slightly larger. Its pins are 1.8 by 8.1 mm (0.071 by 0.319 in); the centres of the power pins are placed 9.55 mm (0.376 in) from the plug centre, while the centre of the earth pin is 11.1 mm (0.437 in) away.^[21] This makes this plug compatible with the 20 A sockets used in Argentina. It is, however, incompatible with Chinese 10 A sockets as well as all Australasian sockets because of the different pin spacing.

Many three-pin sockets in China include a physical lockout preventing access to the active and neutral terminals unless an earth pin (which is slightly longer than the other two pins) is entered first. Hence such sockets will not accept the 10 A-rated unearthed Class II plugs sold in Argentina and Australasia. The Chinese 16 A plug exists, however, also in an unearthed Class II variant that includes an unwired dummy earth pin used only to open shutter. No such variant exists of the 10 A plug.^[21]

China also uses American/Japanese NEMA 1-15 sockets and plugs for Class II appliances (however, polarized plugs with one prong wider than the other are not accepted). Older versions of GB 1002 also defined a socket type that accepts Europlugs (type C), but this hybrid socket was removed from the 2021 revision of the standard.^[7] The voltage at a Chinese socket of any type is 220 V.

In China, sockets typically have the earth contact above the power pins, while in Argentina and Australasia it is usually positioned below them. While Australasian plugs have insulating sleeves around the upper part of their power pins, that is not the case for most existing Chinese plugs, hence their pins may become live while there is still a large enough gap between the faces of the plug and socket to allow a finger to touch the pin. Since 2022, insulating sleeves are also allowed on plugs sold in China;^[22] they will be mandatory from 2027.^[23]

The polarity of all grounded British sockets is standardized: earth is at the top and live is at the right of the socket. This convention is also followed in other countries using these plugs and sockets.^[24]

BS 546, "Two-pole and earthing-pin plugs, socket-outlets and socket-outlet adaptors for AC (50-60 Hz) circuits up to 250 V" 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,^[25] 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 "13 A plugs, socket-outlets, adaptors and connection units"^[26] is the main plug and socket type used in the United Kingdom. According to the IEC^[9] it is also used in over 50 countries worldwide. Some of these countries have national standards based on BS 1363, including: Bahrain, Hong Kong,^[27] Ireland, Cyprus, Malaysia, Malta, Saudi Arabia, Singapore, Sri Lanka, and UAE.

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.

The United Kingdom, Ireland, and Malta use the BS 4573 two-pin plug and socket for electric shavers and toothbrushes.^[28] The plug has insulating sleeves on the pins. Although similar to the Europlug Type C, the diameter and spacing of the pins are slightly different and hence it will not fit into a Schuko socket. There are, however, two-pin sockets and adaptors which will accept both BS 4573 and Europlugs.

The International Commission on the Rules for the Approval of Electrical Equipment (IECEE) 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.^[29] 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."^[30] 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 sockets accept CEE 7/2 round plugs with 4.8 by 19 mm (0.189 by 0.748 in) 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, while others allow their sale for replacements only. 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.

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 4.8 by 19 mm (0.189 by 0.748 in). The Schuko system is unpolarised, allowing live and neutral to be reversed. The socket accepts Europlugs and CEE 7/17 plugs and also includes CEE 7/7. 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 or many countries of Europe, Asia, and Africa, as well as in the countries of South Korea, Peru, Chile and Uruguay.^{[citation needed]} The few European countries not using it at all are Belgium, Czech Republic, Cyprus, Ireland, Liechtenstein, Switzerland, and the UK, or not using it predominantly are Denmark, Faroe Island, France, Italy, Monaco, San Marino, Slovakia.

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 10 mm (0.394 in). The plug has two round pins measuring 4.8 by 19 mm (0.189 by 0.748 in), spaced 19 mm (0.748 in) 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 different countries have conflicting standards for that. For example, the French standard NF C 15-100 requires live to be on the right side, while Czech standard ČSN 33 2180 requires it to be on the left side of a socket. Thus, a French plug when plugged into a Czech socket (or a Czech plug when plugged into a French socket) will always have its polarity reversed, with no way for the user to remedy this situation apart from rewiring the plug. One approach for resolving this situation is taken in Poland, where CEE 7/5 sockets are typically installed in pairs, the upper (upside-down) one having the "French" polarity and the lower one having the "Czech" polarity, so that the user can choose what to plug where.

CEE 7/4 (Schuko) plugs are not compatible with the CEE 7/5 socket because of the round earthing pin permanently mounted in the socket; CEE 7/6 plugs are not compatible with Schuko sockets due to the presence of indentations on the side of the recess, as well as the earth clips. CEE 7/7 plugs have been designed to solve this incompatibility by being able to fit in either type of socket.^[31]

Sales and installations of 7/5 sockets are legally permitted in Denmark since 2008, but the sockets are hard to find in physical stores,^[32] and installation is exceedingly rarely performed.