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Three-prong plugs do not fit into the older, two-prong receptacles. [7] When used as intended, the ground pin of the 3-wire receptacle is to be connected to a grounded cover screw, or to an external ground. In 1969, Underwriters Laboratories mandated three-prong plugs on major appliances for safety. [7]
The Electrical Safety Foundation International has stated: "Never remove the ground pin (the third prong) to make a three-prong plug fit a two-prong outlet". [18] In addition to the dangers of breaking a ground connection, removing the ground pin to make it fit a 1-15R receptacle or extension cord , may result in the live–neutral polarity ...
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.
BS 546, Two-pole and earthing-pin plugs, socket-outlets and socket-outlet adaptors for AC (50–60 Hz) circuits up to 250 V is an older British Standard for three-pin AC power plugs and sockets: four sizes with current capacities from 2 A to 30 A. Originally published in April 1934, it was updated by a 1950 edition which is still current, [1 ...
The pilot pin is located in the centre of main contact circle on 4- and 5-pin connectors. On 3-pin (2P+E) connectors, it is located on the contact circle opposite the ground pin. The other connectors are located 105° on either side of the earth pin, rather than 120° as in the smaller variants, to make room for the pilot pin.
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.
Semko 17 were 3/4-pin 3-phase connectors, with (4-pin) or without (3-pin) a neutral connector. Earth were provided via the shield. The connectors were available in different sizes, 16 A with rounded corners; 25 A and 63 A were rectangular. These connectors were used mainly in industrial and agricultural installations.
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).