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Lampholder plug from 1893 GEC Ltd (London) catalogue. 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. [1]
The general term is socket, but there are numerous common alternatives, including power point, [7] plug socket, [8] wall socket, [9] and wall plug. [10] Modern British sockets for domestic use are normally manufactured as single or double units with an integral face plate and are designed to fit standard mounting boxes.
The plug and socket are rated 16 A 250 V AC and are intended for use only on systems having nominal voltages between 200 V and 250 V AC The plug pins are 4.5 mm in diameter, line and neutral are on centres 19 mm apart. The earth pin is offset by 3.0 mm. The line pin is on the right when looking at a socket with the earth pin offset up.
A three or four-pole isolator or circuit breaker is used for three-phase loads, for devices with both permanent and switched supplies (such as bathroom extractor fans) and also at the distribution board to isolate all the phases. A three-phase installation with a TT earthing system is an example where four-pole devices would be required.
The TT-30R receptacle is commonly available in nearly all RV parks in the United States and Canada, and all but the largest RVs manufactured since the 1970s use this plug to connect to power feeds. [citation needed] The appearance of this plug is sometimes confused with a NEMA 10 connector, rated for 240 V, but the NEMA TT-30 is a 120 V device.
UFAD systems use an underfloor supply plenum located between the structural concrete slab and a raised floor system to supply conditioned air to supply outlets (usually floor diffusers), located at or near floor level within the occupied space. Air returns from the room at ceiling level or the maximum allowable height above the occupied zone.
The neutral is connected to the center tap of the power company transformer of a split-phase system, or the center of the wye connection of a polyphase power system. United States electrical codes require that the neutral be connected to earth at the "service panel" only and at no other point within the building wiring system.
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. [citation needed]