Twist-locking Connectors
Twist-locking connectors were first invented by Harvey Hubbell III in 1938 and "Twist-Lock" remains a registered trademark of Hubbell Incorporated, although the term is used generically to refer to NEMA locking connectors manufactured by any company. Locking connectors use curved blades. Once pushed into the receptacle, the plug is twisted and its now-rotated blades latch into the receptacle. To unlatch the plug, the rotation is reversed. The locking coupling makes for a more reliable connection in commercial and industrial settings, where vibration or incidental impact could disconnect a non-locking connector.
Locking connectors come in a variety of standardized configurations that follow the same general naming scheme except that the designations all begin with an "L" for "locking". Locking connectors are designed so the different voltages and current ratings can not be accidentally intermated. Many specific types exist; only a few are listed below. Other types include miniature locking connectors, and special purpose connectors for boats, 400 Hz circuits such as used for aircraft, and direct-current use.
One disadvantage of twist-lock connectors is that in the event that the cable is accidentally pulled too hard, rather than the plug falling out of the receptacle, the exposed conductors may come out of the plug, causing dangerous shorts or shock hazards if the circuit is live.
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