AC power plugs and sockets
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 is fixed in place, often on the internal walls of buildings, and is connected to an AC electrical circuit. Inserting 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.
Overview of connections
sockets have receptacles for two power pins, one to carry the electrical current from the power source to the connected device and another one to carry it back to the power source. 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 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.History
When commercial electric power was first introduced in the 1880s, it was used primarily for lighting. Other portable appliances 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 plugs appeared. Over time other safety improvements were gradually introduced to the market. One of the first national standards for plugs and wall sockets was enacted in 1915 in the UK.Safety features
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 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. 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 covering the socket itself when not in use, or a switch to turn off the socket.Overcurrent protection
Some plugs, particularly the British Type G, have a built-in fuse which breaks the circuit if too much current flows.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.Polarization
Where a "neutral" conductor exists in supply wiring, polarization 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, polarized plugs cannot be mated with non-polarized 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 polarized 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.Extension
Various methods can be used to increase the number or reach of sockets.Extension cords
s 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
Multisocket adaptors 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.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.
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.
Cross-compatibility
Hybrid and universal sockets
"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. Safety advocates, the United States Army, and a manufacturer of sockets 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 that accepts A and C plugs and an earthed socket that accepts Type I plugs. Since the 2021 revision of the Chinese standard, the installation of such dual sockets is no longer allowed in China, but they can still be found in many existing installations.
Such Thai or Chinese dual sockets also accept plugs normally fitted to 120 V appliances. This can cause an electrical incompatibility, since both countries supply residential power at 220–230 V. These dual sockets also accept French and Schuko plugs, but unsafely: no earth contact is established and live pins may be exposed when a plug is partially inserted.
Swappable cables and plugs
Commonly, manufacturers provide an IEC 60320 inlet on an appliance, with a detachable power cord 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.Travel 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.Standard types in present use
The plugs and sockets used in a given area are regulated by local governments.The International Electrotechnical Commission maintains a guide with letter designations for generally compatible types of plugs, 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.
In Europe, CENELEC publishes a list of approved plug and socket technical standards used in the member countries.