Approximately 20 types of AC power plugs and sockets are in common use around the world, and many obsolete socket types are still found in older buildings.
Development
Early history
When electricity was first introduced into houses in the 1880s, it was primarily used for lighting. One common approach for other appliances (such as vacuum cleaners, electric fans, smoothing irons and curling tong heaters) was to connect to light bulb sockets using lampholder plugs. However, in Britain, there were recognisable two pin plugs and wall sockets appearing on the market as early as 1885.
British origins
As electricity became a common method of operating labour-saving appliances, a safe means of connection to the electric system other than using a light socket was needed. Thomas Tayler Smith, of London, received British patent 4162 in 1882 for an "Electric-Circuit Connection" to "enable the electric conductors conveying the current to one or more lamps, or along a flexible cord, to be rapidly and safely brought into connection with the line or main wires". Smith subsequently received for the same device in 1885. According to British Author John Mellanby there were British patents for plug and sockets granted to T.T. Smith in 1883 (No. 3883) and W. B. Sayers & G. Hookham in 1884, (No. 16655). Mellanby also writes that there were two-pin designs by 1885, one of which appears in the General Electric Company catalogue of 1889.
Gustav Binswanger, a German immigrant who founded the General Electric Company, obtained a patent (GB189516898) in 1895 for a plug and socket using a concentric (co-axial) contact system.
Hubbell's inventions
Several early American electrical plug and socket arrangements were invented by Harvey Hubbell. On 26 February 1903 he filed two patent applications featuring 2-pin plugs and adaptors for using his plugs with existing designs of lamp sockets and wall receptacles. Hubbell's first plug design had two round pins which differed from those already in use in Europe in that the tips of the pins had annular detents similar to those of present-day jack plugs to positively retain a plug in its socket. In one patent, a plug was used with a socket which screwed into a lampholder (like the early lampholder plugs). In the other patent the same type of plug was used with various three-way adaptors that could be connected to lampholders or "a receptacle of any ordinary type". Figures 2 and 4 of this patent show an adaptor plugged into what appears to be a "Chapman" receptacle.
Hubbell evidently soon found the round pin design unsatisfactory as a subsequent filed on May 27, 1904 shows lampholder adaptors similar to those of his first patent for use with plugs having coplanar (tandem) flat pins.
Hubbell's catalogue of 1906 includes various three-way adaptors similar to those shown in the US 776,326 patent, but modified for use with the coplanar flat pin plugs. The Chapman receptacle must have been in general use at the time, as it was the only type of non-lampholder receptacle for which adaptors were supplied. The 1906 catalogue says of the Chapman adaptor: "The device avoids fastening the cords together as is necessary with the ordinary Chapman plug when used for more than one purpose." This suggests that Hubbell's original invention was prompted by his observation of the problem that arose with the use of this sort of receptacle and plug. Gradually wall sockets were developed to supplement those that screwed into lampholders.
In 1912 Hubbell rotated his tandem pins by 90 degrees to arrive at the parallel flat pin configuration still widely used today (NEMA 1-15).
A feature common to all of Hubbell's patented designs is the provision of detents to retain a plug in its socket. This would have been a desirable feature in the days before wall receptacles became widespread and, for many consumers, the only source of electricity was an electric light socket.
Hubbell versus other American manufacturers
Despite Hubbell's objections, other manufacturers adopted the Hubbell pattern (omitting Hubbell's detents as these did not affect interchangeability) and by 1915 the use of Hubbell's configuration was widespread. In 1919 Hubbell unsuccessfully attempted to get an injunction to prevent other manufacturers from making receptacles and plugs to the dimensions used by Hubbell.
The report of the court proceedings[Harvey Hubbell Inc. V. General Electric Co. et.al. (262 Fed.Rep. 155) District Court of the United States, Southern District of New York, October 20, 1919. Reported in "The Trade Mark Reporter" Vol 10, 1920, Pages 164 - 175. The court observed that Hubbell's plugs were readily distinguishable from those of other makes by the provision of "nicks" in the pins of Hubbell's plug pins, designed to provide retention when mated with Hubbell's receptacles, so there was no confusion as to their origin. The report of the trial is available from Google Books here: https://books.google.co.uk/books?id=XOgsAAAAYAAJ&printsec=frontcover&dq=the+trademark+reporter+volume+10&hl=en&sa=X&ved=0ahUKEwjHgZr29YvXAhUCSRoKHWQaDOAQ6AEIJjAA#v=onepage&q&f=false includes a comprehensive review of the development of the art in the US prior to 1919, based on evidence presented to the Court. Separable plugs had been available for more than a decade prior to Hubbell's 1904 design.
The earliest presented to the court was the "Weston" ( issued August 16, 1892, for which Hubbell had taken out a licence), another (unidentified) type following in 1897. Plugs per se with parallel flat pins, such as the "Fort Wayne" design were in common use by 1886, and flush receptacles, such as the "Bryant Electrical Company" design, by 1902. Hubbell had introduced its own parallel flat pin configuration in 1912. In 1915 there were ...from 15 to 20 different types of blades and from 15 to 30 different types of receptacles. The line of each was not interchangeable with competing lines. .... The existence of both "tandem" and parallel pin configurations had led to the introduction in 1914 of some receptacles having both configurations of slots and others having "T" slots.
By 1915, Hubbell had sold about 13 million receptacles and plug bases/caps with tandem slots/pins, and about 1.25 million with parallel slots/pins, most of which were then still in active use, meaning that Hubbell's configurations were by far the most widely used.
Following the lead set by the lamp manufacturers in standardising lamp bases, a conference of the plug and receptacle manufacturers, including Hubbell, was arranged with a view to agreeing a standard configuration. It was a time of great expansion in the use of electrical appliances such as fans, heaters, and cookers, as well as portable devices such as hair curlers and irons, and the public wanted interchangeability. Hubbell's parallel pin configuration was preferred, but Hubbell rebelled against standardization, and refused to agree, asserting that it had common law rights in the dimensions of its line. The other conference members pressed on regardless, and agreed to standardize on Hubbell's parallel flat pin configuration and dimensions. Hubbell's court action only served to confirm the legality of their activities, to the great benefit of the U.S. public.
Earthed (grounded) plugs
The earthed (grounded) consumer plug has several claimants to its invention.
In Britain, a 1911 book"SMALL SWITCHES, etc., and their CIRCUITS" (1911), MAYCOCK, W. Perren, London: S. Rentall & Co. dealing with the electrical products of A. P. Lundberg & Sons of London describes the "Tripin" earthed plug available in 2.5 amp and 5 amp models. The pin configuration of the "Tripin" appears virtually identical to modern BS 546 plugs. In her 1914 book Electric cooking, heating, cleaning, etc. Maud Lucas Lancaster mentions an earthed iron-clad plug and socket by the English firm of Reyrolle and Co.
The earliest American patent application for an earthed plug appears to be 11 January 1915 by George P. Knapp, on behalf of the Harvey Hubbell company. covers the use of an earthing pin which extends further than the other two contacts to ensure that it is engaged first. Knapp's design was obsoleted in the U.S. before the modern NEMA designations, but is still used in some other countries including China, Argentina and Australia. The configuration of the socket was not operable with existing two-contact unearthed plugs. Other earthed sockets that are widely used in the U.S. today are operable with unearthed plugs. It is sometimes claimed (e.g., in Illumin) that the modern American version of the earthed plug, was invented by Philip F. Labre who was issued a for an earthed socket and plug in 1928. However, Labre's design is no more similar to the modern version than Knapp's earlier design.
The German Schuko-system plug is believed to date from 1925 and is attributed to Albert Büttner. As the need for safer installations became apparent, earthed three-contact systems were made mandatory in most industrial countries.
Proliferation During the first 50 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. After the two-prong electric plug was introduced in the 1920s, the three-pin outlet was developed. This format was introduced in order to mitigate the effect of a short circuit event, as the supply would be neutralised with earth. Gradually the desire for trade eliminated some standards that had been used in only a few countries. Former colonies may retain the standards of the colonising country. Sometimes offshore industrial plants or overseas military bases use the wiring practices of their controlling country instead of the surrounding region. Some countries have multiple voltages, frequencies and plug designs in use, which can create inconvenience and safety hazards. Hotels and airports may maintain sockets of foreign standards for the convenience of travellers. By 2018, there were 15 plug and socket types around the world.
Consolidation De facto standards became formalised as official national and international standards. The earliest is believed to be British Standard 73 Wall plugs and sockets (five ampere two-pin without earthing connection) which was first published in 1915. The International Electrotechnical Commission in 1934 established technical committee TC 23 for electrical fittings. Only two meetings were held before the outbreak of the Second World War. In mainland Europe, since 1951 the International Commission on the Rules for the Approval of Electrical Equipment (CEE) has published a standard (CEE 7 Specification for Plugs and Socket-Outlets for Domestic and Similar Purposes) describing the plugs and sockets used. In 1953 the CEE published Technical Report 83 (later 60083), which was a listing of plugs and sockets then in use. In North America the National Electrical Manufacturers Association (NEMA) publishes standards for plugs and sockets.