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The flat parallel plug blades are polarized to prevent the hot and the neutral connections from being reversed. In addition, many versions have a molded obstruction bump on top of the adapter, to block the grounding prong and thus physically prevent forcible insertion of a 3-prong plug in the wrong orientation.
In older plug designs both blades were the same width, so the plug could be inserted into the receptacle either way around. Many plugs manufactured since 1948 are polarized; the neutral blade is 5 ⁄ 16 in or 7.9 mm wide, 1 ⁄ 16 in or 1.6 mm wider than the line blade, so the plug can be inserted only one way. Polarized 1-15P plugs will not ...
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. In 1885, two-pin plug designs appeared and in 1889 there were two-pin plugs and sockets in the GEC catalogue. [28] The 1893 GEC Catalogue included three sizes of what was described as Double plug ...
Harvey Hubbell had patented the parallel blade plug in 1913, and patented a polarized version in 1916. He also patented the T-slot single outlet in 1915, and a duplex T-slot outlet in 1916 both meant to take his older 1904 tandem and newer parallel plug design. (Single: U.S. patent 1,146,938; Duplex: U.S. patent 1,210,176). Prior to the 1930s ...
IEC 60309 (formerly IEC 309 and CEE 17, also published by CENELEC as EN 60309) is a series of international standards from the International Electrotechnical Commission (IEC) for "plugs, socket-outlets and couplers for industrial purposes".
Ratings of NEMA 5-15 (type B) plug marked on the label (left: 7 A 125 V), engagement face, and IEC 60320 C13 connector at the appliance end of the cord (centre, right: both 10 A 125 V) 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 ...