Museum of Plugs and Sockets logo, small Socket Safety Shutters
examples of three mechanisms
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Safety shutters have been designed to avoid that children and thoughtless adults put other objects than plugs in sockets. Three types of shutter mechanisms block holes that give access to socket contacts.

(1) The most frequently used type of shutters are opened by pressing down simultaneously both line and neutral pins of a plug. The method can be applied to all types of sockets.
In 1928 the British company MK Electric has developed an alternative procedure; the earth pin shifts the shutters away that covers the line and neutral contacts (see image no. 14).

(2) The locking mechanism consists of a round plate that has to rotated by plug pins. See examples shown in images 1-3 and 4, below.

(3) An uncommon method that has been found on socket no. 5. Shutters are shifted away by a coordinated movement of L and N pins in a direction parallel to the socket top plane, rather than pressing down plug pins. The system works nicely, but is less intuitive than method 1. Moreover, the method is not suitable for (deep) recessed sockets and less practical for 3-pin plugs.


NIKO socket with shutters NIKO socket with shutters, detail
NIKO socket with shutters, detail
Child-safe 2-pin adapter

1 - 3
Bakelite 10A-250V socket with child-safe locking mechanism. The lock that has to be rotated to insert a plug (images 2 an 3), is an integral part of the socket construction, rather than add-on types of shutters. After releasing the plug, a spring ensures that the plate automatically rotates back to its safe position.
This 1950s socket was a significant safety improvement. Manufacturer: NIKO, a still existing company, established in 1918 in Sint-Nikolaas, Belgium.
4 Bakelite safety adapter with a child-safe locking mechanism comparable to NIKO socket no. 1. Adapter pins have a length of 19 mm, identical to common plug pin length*. The adapter can be used on most two-pole socket, but is incompatible with Schuko sockets. Name of manufacturer is not indicated.

* this is an important feature. The 19 mm pins make sufficient contact between adapter pins and socket contacts. Simple add-on shutters, without pins, exist also. They can be fastened on top of a socket with double sided adhesive tape. These shutters are at least 3 mm thick, which means that a plug can be inserted 3 mm less deep in a socket and the contact zone of plug pins is 3 mm shorter. That may result in overheating, depending on the current that needed for the attached appliance.


EL-JO duplex socket with shutters


Duplex socket with shutters. Rating 10A - 250V. Cast and shutter are made of a thermosetting plastic, socket body of steatite.
The socket has three marks: (1) EL- -JO and, (2) DB refer to the name of the manufacturer*, and (3) Swedish Semkocertification mark. Dating: 1950s.   {WN}

Shutters have to be shifted aside by a sliding movement of both plug pins. Images 6-13 below show that access to a contact hole it is not possible with a single pin. Images of artificially coloured shutters (in Photoshop) have been added to explain the action of pins and shutters.

EL-JO socket with shutters, detail 1

No. 6: each outlet has two shutters. The green shutter is operated by the right plug pin; the blue shutter by the left pin.

No.7: when moving the right (fake) pin upwards, the green shutter - in both the right and left slot - is sliding to the right.

EL-JO socket with shutters, detail 2

EL-JO socket with shutters, detail 3

No. 7: right pin is moved fully upwards. Green shutter does not block the slots anymore, but pin can not be inserted because of the blue shutter, positioned under the green shutter (see insert).

No 8: The left (fake) pin has been added.

EL-JO socket with shutters, detail 4

EL-JO socket with shutters, detail 5

No. 10: when moving the left pin upwards, the blue shutter moves to the left in both the left and right slot.

No. 11: when to left pin in in top position there is no obstruction anymore to insert both pins.

EL-JO socket with shutters, detail 6

EL-JO socket with shutters, detail 7

No. 12: when only the left pin is used the pin can move upwards to a position halfway only. Green shutter blocks further movement.

No 13: only a simultaneous upward movement of both pins will result in sliding away both shutters.

EL-JO socket with shutters, detail 8

* DB and EL-JO are abbreviations related to the name of the manufacturer. Joel Olsson (Båstad, Sweden) bought Dala Bakelit in 1945 and renamed the company in 1953 to Eljo Plastindustri AB, and in 1971 to Eljo AB. The company has been taken over by Schneider Electric in 2003.


BS 1363 socket safety shutters explained

14 Shutter system designed by MK Electric in 1928 and still in use for BS1363 and BS 546 sockets.
Shutter (grey) covers line (L) and neutral (N) contacts (a, b). When inserting a plug, the long earth pin pushes the shutters aside (c). A spring (not visible) pushes the shutter back into safe position (b) when the plug is released.
From 1994 BS 1363 plugs must have partially insulated line and neutral pins. Earth pins may not be insulated.


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