same type. Industrial devices may also be constructed to be dust or water-tight. NEMA wiring devices are made in current ratings from 15–60 A, and voltage ratings from 125–600 V. There are two basic configurations of NEMA plug and socket: straight-blade and locking. Numbers prefixed by L are twist-lock, others are straight blade. Locking type connectors are found mostly in industrial applications and are not common in residential and light commercial use. 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 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. Stage pin connectors A stage pin connector (SPC), or grounded stage pin (GSP), is a connector used primarily in the theatre industry for stage lighting applications in the United States. Stage pin connectors are generally used for conducting dimmed power from a dimmer to stage lighting instruments, although occasionally they can power other equipment. The primary advantage of the stage pin connector over the NEMA 5-15 connector (commonly known as an Edison connector in the theatre industry) is its increased durability and resistance to damage due to its more robust construction and the ability to compensate for wear with a pin splitter. Having a distinct connector designated for dimmable power also helps prevent confusion of dimmed and non-dimmed circuits which could lead to equipment damage. Even the smallest stage pin connectors are rated for 20 A, which translates to 2.4 kW at 120 V, compared to the 15 A and 1.8 kW of the NEMA 5-15. In applications where cables are on the floor, the low profile of the connector allows for connections that are only