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Early GEC 2-pin plug and socket as depicted in the 1893 GEC catalogue. The earliest domestic plug and socket is believed to be that patented by T. T. Smith in 1883. [27] This was shortly followed by patents from W. B. Sayers and G. Hookham; these early designs had rectangular plugs with contact plates on either side.
U.S. patent 774,250.The first US power plug and socket dated 1904. 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.
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 15 A 7-15 plug has the crowsfoot current carrying pins of the Type I plug, but with a U-shaped earth pin. The 7-20 version has an enlarged line/hot pin. 7-30 is a larger diameter connector, with an L-shaped neutral, while the 7-50 has an enlarged neutral pin, compared with the hot.
On the male plug, three 6 mm pins each 20.5 mm long are equally spaced around a 15.4 mm diameter circle, starting at the 12-o'clock position. They are surrounded by a 23 mm long shroud with an inner diameter of 36 mm and an outer diameter of 42 mm. The pin opposite the major key is optional and may be omitted to make a 2-pin variant.
BS 73:1915 is the original standard for 5 amp 2 pin plugs and sockets, superseded by BS 372. The correct place for old British 2 pin plug information is probably AC power plugs and sockets in a new sub-section under "Obsolete Types", and I propose to start that. Based on the poor quality and inappropriateness of the current paragraph in BS 546 ...
Schuko sockets can accept two-pin unearthed Europlug (CEE 7/16) and CEE 7/17. Less safely, Schuko plugs can be inserted into many two-pin unearthed CEE 7/1 sockets and into some sockets with a different form of earth connection that will not mate with the earth contacts on the Schuko plug (e.g., some variants of the Danish socket).
IEC 60906-1-plug. IEC 60906-1 (IEC designation "Type N") [1] is an international standard designed "to provide a standard for a safe, compact and practical 16 A 250 V AC system of plugs and socket-outlets that could be accepted by many countries as their national standard, even if not in the near future."