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Locking plugs and sockets
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Locking coupling provides for protection against inadvertent disconnection by accidentally pulling the cable, which might have frustrating (or worse) consequences when cables are hidden in places out of sight or reach. A large range of locking plugs and sockets - from 15A-125V up to 30A-600V - has been designed for the North American market. Images 1-5 show examples.

More locking devices can be found in the heavy duty / three-phase section.

Japanese locking devices are different from U.S. types, see page on plugs and sockets used in Japan.



NEMA ML-2P plug NEMA ML-3R wiring

NEMA ML-2R wiring

1, 2 Non-grounding Midget flanged locking socket (no.1: NEMA ML-1R) and plug (no. 2: NEMA ML-1P), rated at 15A, 125V. The devices have been made industrial grade rugged nylon. The Midget series is considerably smaller than the standard locking devices; compare nos. 2 and 4. After insertion the plug has to be rotated 30 degrees clockwise. Manufacturer: Leviton
3, 4

Hubbell Twist-Lock 15A, 125V plugs (Insulgrip quality), according to NEMA L5-15R (no. 3) and NEMA L5-15P (no. 4) specifications. Plugs have to be rotated 30 degrees to lock, or unlock them.
Also available, but not in the museum collection: NEMA L5-15R socket.

5 15A Midget type 3-pole locking plug (NEMA ML-3P), rated at 125V (X+W or Y+W) 15A, or 250V (X+Y) 10A. ML-3 Receptacle wiring is shown right. Manufacturer: Hubbell.   {RW}

NEMA ML-2 is a very similar 2-pole, grounding model, rated at 125V only. The wiring scheme is shown far right. The round center pin provides for equipment grounding, a connection that is not available on ML-3.
Internal inspection of plug No. 5 shows that the top and bottom, bended pins are indicated as X and Y, which means that it must be an ML-3 plug.


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