Museum of Plugs and Sockets: locking plugs

Locking Plugs
overview of types in museum

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Locking plugs prevent accidental disconnection that may have unwanted and possibly dangerous consequences because lighting fails or equipment comes to an untimely or unsafe stop.
Various mechanisms have been designed to secure special plugs in combination with adapted sockets. An overview of classic and modern locking plugs in the museum collection is given below.

→ Click on an image for additional information and matching socket.

1	The US National Electrical Manufacturers Association (NEMA) has defined a large number of amperage, voltage and grounding specific locking plug configurations. Pins of twist lock plugs are rounded and have a flange to lock the plug (see green arrow).
2	Danish twist lock plug for ceiling sockets, designed to connect lamps. Locking mechanism is comparable to US plugs (see no. 1).
3	Japanese locking sockets use the holes in flat blades to fix a plug after rotation. See also nos 17 and 18.
4	British EPOS (Electric Point of Sale) plug. After insertion in a matching socket the plug is locked by rotating 90°.

5	Neutrik PowerCON plug. After insertion in a PowerCON socket and 30° rotation, the yellow clip locks the plug.
6	British bayonet cap plug. Bayonet plugs were initially meant for lighting only, but adapters for BS 372 2-pin plugs existed also.
7	Three-phase plug with bayonet-type locking mechanism, made by the German company Kontakt AG in the 1930s.

8	Plug with a screwed, restraining ring to secure the plug after inserting in a socket with matching spiral thread.
9	Concentric plug with locking mechanism similar to no. 8. The German Maehler & Kaege plug dates back to the 1950s.
10	A cutout of the earth pin (green arrow) enables the matching socket to secure the plug with a bar. Locking mechanism has been designed also by Maehler & Kaege. Dating: early 1930s.
11	British plug with slotted earth pin, made in the 1950s by General Electric Company. Locking mechanism is comparable to no. 10.

12	IEC 60329-C13 connector plug. When pushing the connector into an appliance inlet, the metal plate (green arrow) glides easily around the inlet earth pin but blocks attempts to retract the connector.
13	French DCL plug (Dispositif de Connection Luminaire). A clip (green arrow) locks the plug in a DCL lamp socket.
14	Wylex clock connector. A long screw keeps socket and plug together. Dating: 1950s.
15	Plug with hollow pins. The cavity is slightly conical. Pin tips can be pushed away from each other by a (black) plastic bar; triangles (green arrowheads) fix pins in the socket contact.

16	Plug is a standard Schuko model inserted into a socket with rotary switch. In switched on position, the knob of the switch locks the plug. It is essential that the plug has a collar (green arrowhead). Nearly all classic Schuko plugs had a collar.
17	Connector with mechanism to lock CEE 7/4, -7/6, -7/7 and -7/17 plugs. Rotate connector collar to lock / unlock plug.
18	Adapter plug with locking mechanism connected to tube around earth pin. Press red button to unlock plug. Click image for details.

In the true sense of the word, plugs 19 - 22 are not locking plugs. Straight blades of plugs 19 and 20 have holes. Matching sockets have small balls that fit partially in the holes. Balls help to keep plugs in position.
Pins of plugs 21 and 22 show other types of plug fixation; see captions.

19	US 15A-125V non grounding plug. Although not compulsory, many US straight blades have holes that help to fix plugs. See nos. 1 (US) and 3 (Japan) for genuine locking plugs.
20	Classic Danish plug designed for lamp sockets. Image No. 2 shows a modern, locking type Danish lamp plug.
21	Classic Australian 10A plugs often had recesses at the top of each of the blades (green arrows). Their function was comparable to holes in straight blades (see nos 17 and 18). Already in 1904 Harvey Hubbell patented plug blades with recesses and matching outlet contacts with indentations adapted to engage blade recesses (see images A, C and D on Origin of US plugs and sockets page). Ultimately the Hubbell method of fixing plugs has been outstripped by the "hole and ball" method shown in image no. 17. The origin of Australian plugs is a 1915 design by George Knapp, working at Hubbell Inc. (see image E). Whether Australian sockets initially had contacts with indentations is not clear. None of the classic outlets in the museum collection have the provision. Modern Australian plugs have flat blades without recesses or holes.
22	Porcelain round pin plug made in Germany in the 1910s. They often had round pin tips, which helps to insert plugs into socket contacts. The additional circular recess is either a decorative feature, or - occasionaly - a provision to fix plugs. An example of the latter function is shown on Classic Uncommon Plugs page (see Voigt & Haeffner socket no. 28).

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