page_title: Industrial and multiphase power plugs and sockets
section_title: Twist-locking connectors
hierarchical_section_title: Industrial and multiphase power plugs and sockets / North America / NEMA connectors / Twist-locking connectors
context_page_description: Industrial and multiphase plugs and sockets provide a connection to the electrical mains rated at higher voltages and currents than household plugs and sockets. They are generally used in polyphase systems, with high currents, or when protection from environmental hazards is required. Industrial outlets may have weatherproof covers, waterproofing sleeves, or may be interlocked with a switch to prevent accidental disconnection of an energized plug. Some types of connectors are approved for hazardous areas such as coal mines or petrochemical plants, where flammable gas may be present.
  Almost all three-phase power plugs have an earth connection, but may not have a neutral because three-phase loads such as motors do not need the neutral. Such plugs have only four prongs. An example of a socket with neutral is the L21-30 and the L21-20 both of which have five pins.
  While some forms of power plugs and sockets are set by international standards, countries may have their own different standards and regulations. For example, the colour-coding of wires may not be the same as for small mains plugs.
context_section_description: Twist-locking connectors were first invented by Harvey Hubbell III in 1938 and Twist-Lock remains a registered trademark of Hubbell Incorporated to this day, although the term tends to be used generically to refer to NEMA twist-locking connectors manufactured by any company. Twist-locking connectors all use curved blades that have shapes that conform to portions of the circumference of a circle. Once pushed into the receptacle, the plug is twisted and its now-rotated prongs latch into the receptacle. To unlatch the plug, the rotation is reversed. The locking coupling makes for a very reliable connection in commercial and industrial settings.
  Like non-locking connectors, these come in a variety of standardized configurations and follow the same general naming scheme except that they all begin with an L for locking. The connector families are designed so that 120 V connectors, 208/240 V connectors, and various other, higher-voltage connectors can not be accidentally intermated.
caption: L21-30 plug
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