See also: Industrial and multiphase power plugs and sockets
AC power plugs and sockets are devices for removably connecting electrically-operated devices to the commercial power supply.
An electric plug is a male electrical connector with contact prongs to connect mechanically and electrically to slots in the matching female socket.
Wall sockets (sometimes also known as power points , power sockets , electric receptacles , or electrical outlets ) are female electrical connectors that have slots or holes which accept and deliver current to the prongs of inserted plugs. To reduce the risk of injury or death by electric shock, some plug and socket systems incorporate various safety features. Sockets are designed to accept only matching plugs and reject all others.
Terminology
There are substantial differences between American and British nomenclature related to power plugs and sockets.
In the United States, the live contact may be called live , hot or ungrounded . The neutral contact may be called cold , neutral , return , the ground ed conductor , or (in the National Electrical Code), the identified conductor . The earth contact is called ground or the ground ing conductor .
In the United Kingdom the word line is occasionally used to denote the live terminal or wire. In electrical engineering, the line voltage is that between the live conductors of the three-phase distribution system, while the phase voltage is that between live and neutral.
Live conductors are called phases when there is more than a single phase in use. Pins are also known as prongs , contacts , blades , or terminals .
In Australia, the live contact is called active .
The three contacts
Each receptacle has two or three wired contacts. The contacts may be steel or brass, and may be plated with zinc, tin, or nickel. The live contact carries current from the source to the load. The neutral returns current to the source. Many receptacles and plugs also include a third contact for a connection to earth ground, intended to protect against insulation failure of the connected device.
Polarisation
Polarised plugs and sockets are those designed to connect only in one orientation, so the live and neutral conductors of the outlet are connected (respectively) to the live and neutral poles of the appliance. Polarisation is maintained by the shape, size, or position of plug pins and socket holes to ensure that a plug fits only one way into a socket. This is so switches interrupt the live wire. If the neutral wire were interrupted instead, the device would be deactivated but its internal wiring would still be live. This is a shock hazard; if the energised parts are touched, current travels to earth through the body. Devices that especially present this hazard include toasters and other appliances with exposed heating elements, which with reversed polarity can remain live even when they are cool to the touch, and screw-in light bulbs, which with reversed polarity, exposed portions of the socket shell remain energised even though the lamp is switched off.
Interchange of the live and neutral wires in the wiring behind the walls or in the hookup of sockets defeats the safety purpose of polarised sockets and plugs; a circuit tester can be used to detect swapped wires.
Unpolarised plugs and sockets are those which can connect either way around, so live and neutral wires are connected at random. Unpolarised plug/socket systems such as the Europlug rely on device construction requirements to avoid the shock hazards created by interchange of live and neutral connections; for example, double insulation, a particular IP protection rating, or double switching.
(Further information on the configuration of specific types of polarised and unpolarised plugs is below in the section "Types in present use".)
History
Early history
When electricity was first introduced into the household, it was primarily used for lighting. At that time, many electricity companies operated a split-tariff system where the cost of electricity for lighting was lower than that for other purposes. This led to portable appliances (such as vacuum cleaners, electric fans, and hair driers) being connected to the light fitting.
However, as electricity became a common method of lighting houses and operating labour-saving appliances, a safe means of connection to the electric system other than using a light socket was needed. The original two pin electrical plug and socket were invented by Harvey Hubbell and patented in 1904. Other manufacturers adopted the Hubbell pattern and by 1915 they were widespread, although in the 1920s and even later, household and light commercial equipment was still powered through cables connected with Edison screw-base adapters to lampholders.
The three-prong plug was invented by Philip F. Labre, while he was attending the Milwaukee School of Engineering (MSOE). It is said that his landlady had a cat which would knock over her fan when it came in the window. When she plugged the fan back in, she would get an electric shock. Labre figured out that if the plug were grounded, the electricity would go to earth through the plug rather than through his landlady. He applied for and was issued a patent for grounding receptacle and plug on June 5, 1928. As the need for safer installations became apparent, earthed three-contact systems were made mandatory in most industrial countries.
Proliferation of standards
During the first fifty years of commercial use of electric power, standards developed rapidly based on growing experience. Technical, safety, and economic factors influenced the development of all wiring devices and numerous varieties were invented. Gradually the desire for trade eliminated some standards that had been used only in a few countries. Former colonies may retain the standards of the colonising country, occasionally—as with the UK and a number of its former colonies — after the colonising country has changed its standard. Sometimes offshore industrial plants or overseas military bases use the wiring practices of their controlling country instead of the surrounding region. Hotels and airports may maintain receptacles of foreign standards for the convenience of travellers. Some countries have multiple voltages, frequencies and plug designs in use, which can create inconvenience and safety hazards.
Design for safety
Design features and aspects of plugs and sockets have evolved to reduce the risk of electric shock and equipment damage. Depending on the plug/socket system, safety measures may include pin and slot configuration to permit only the correct insertion of plug into socket, earth pins longer than power pins so the device becomes earthed before power is connected, insulated pin shanks to reduce or eliminate live-contact exposure when a plug is partially inserted in a socket, socket slot shutters that open only for the correct plug, as well as inbuilt fuses and switches.
Consolidation of standards
In recent years many countries have settled on one of a few de facto standards, which became formalised as official national standards, although there remain older installations of obsolete wiring in most countries. Some buildings have wiring that has been in use for almost a century and which pre-dates all modern standards.
There has been some movement towards consolidation of standards for international interoperability. For example, the CEE 7/7 plug (see below) has been adopted in several European countries and is compatible with both Type E and Type F sockets, while the unpolarised Europlug is compatible with an even greater proportion of European and other socket types. IEC 60906-1 has been proposed as a common standard for all 230-V plugs and sockets worldwide but has only been adopted in Brazil to date.
Many manufacturers of electrical devices like personal computers have adopted the practice of putting a single world-standard IEC connector on the device, and supplying for each country a power cord equipped with a standard IEC connector on one end and a national power plug at the other. The device itself is designed to adapt to a wide range of voltage and frequency standards. This has the practical benefit of reducing the amount of testing required for approval, and reduces the number of different product variations that must be produced to serve world markets.
World maps
See also: Mains power around the worldThere are two basic standards for voltage and frequency in the world. One is the North American standard of 110-120 volts at a frequency of 60 Hz, which uses
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