AC power plugs and sockets connect electric equipment to the alternating current (AC) 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.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 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.
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.