Museum of Plugs and Sockets logo, small Plug pin - Socket connection
adaptations to improve stable contact
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General info
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The crucial aspect of plug - socket connection is a reliable contact between plug pins and socket contacts. Reliable means a stable contact over a sufficiently large area to guarantee a safe and undisrupted power circuit.

Most early socket contacts were simple tubes; plugs had solid round pins, see images 1 and 2. Tube inner diameter and pin diameter has to match exactly. Accepted tolerances in diameters were in the order of magnitude of less than a millimeter.
Not really a problem if plug and socket were made by the same manufacturer, but in practice sockets are used for appliance plugs of different origin.

Quite a number of adaptations of round plug pins and socket contacts have been designed. See images 3 - 15. For straight blade and rectangular pins it is more simple to construct a reliable plug - socket connection; see images 16 - 27. Most examples show domestic plugs and sockets.


Basic round pin plugs and sockets contacts
1 2
3 4
Classic porcelain plug widh solid pins Inflexible tube socket contacts
Classic plug with split pins Socket with partially flexible tube socket contacts

1 Classic porcelain plug with two solid round pins. Pin diameter: 3.85 mm. Dating: 1900s - 1910. Manufacturer is not indicated.
2 Classic socket with porcelain base and brass tubes, with 5.57 mm inner diameter.
Manufacturer: General Accessories Co Ltd, Bristol, England. Dating: 1900s- 1910s
3 Plug with split pins that allow some adjustment of pin diameter. Split pins were introduced in the early 20st century. For half a century it was the most commonly used pin adaptation.
Enlarging split size with a screwdriver or knife resulted occasionally in breaking off a pin half. Pressing the pin halves firmly together resulted in a too small pin diameter to make stable contact.
Split pins were gradually phased out and nowadays solid pins are mandatory in all applicable standards.
Manufacturer: Felmas, Leipzig, Germany. Dating: late 1920s - early 1930s.
4 Socket split tubes have existed also. They were much less effective than split pins, because of the relative small length of the tube contact zone. Manufacturer: made in England by unknown company. Dating: 1930s (?).


Examples of improvements of plug pin and socket tube-type contact designs
5 6 7 8 9 10
Split pin with additional notch
Tube socket contact with mechanism to fix plug pins
Half round socket contact with spring
Hollow pin with internal spring
Solid pin with flexible protruding strip
Clix folded plug pin

5 Socket tube with a flexible clip, partially inside the contact zone (red arrows). Clips grab the circular indentation of the plug pin (blue arrow). A very stable plug - socket connection, but mainly used for (light) industrial appliances. Dating: 1920s.
Manufacturers: Siemens-Schuckertwerke, Berlin, Germany (plug);   Voigt-Haeffner, Frankfurt a.M., Germany (socket).
6 Socket with construction that presses a plug pin firmly toward the tube inner side (5.5 mm diameter). Allows the use of plugs with quite a range of pin diameters. Found only on sockets made by the Danish company Lauritz Knudsen. Dating: 1980s (?)
7 Socket with construction comparable to socket no. 6, but a more simple design.
Manufacturer: made in Russia by an unknown company. Dating: bought in 2014.
8 Hollow split pin with inside a firm, bended spring that adjust pin diameter when the plug is pressed in a contact tube.
Manufacturer: Voigt-Haeffner, Frankfurt a.M., Germany. Dating 1930s.
9 Pin with a raised metal strip (red arrow). The strip is flexible and can be pushed in fully. The effective pin diameter varies between 4.0 and 4.8 mm. Manufacturer: GEMA, Berlin, Germany. Dating: 1930s.
10 Plug pin having in essence the shape of a (full-) split pin or cotter pin. The ends of the two parallel tines have sufficient flexibility to make stable contact with a socket tube. The eyelet is functioning as wire connector. The plug cap (not shown) has to be closed fully to fasten wires firmly.
Manufacturer: Clix, trade name of British Mechanical Productions, London, England. Dating: 1950s - '60s.


Examples of socket contact clip designs
11 12      13 14 15
MK patented socket connector
Swiss socket contact
two part crossed contact clips
Contact clip with spring
Socket with flexible folded clips

11 Socket contact designed by MK Electric in 1933 can be regarded as a first step from tube to clip contact. Top of contact has splayed out jaws, facilitating plug pin entry. The red annulus limits the widening of socket entry, The spring ring below the annulus presses plug pin and contact halves together.
Manufacturer: MK Electric, Edmonton, Middlesex, England.
12 Compared to no 11 a next step towards clip contact. The main features are similar to the MK socket contact.
Manufacturer: Feller AG, Horgen (ZH), Switzerland. Dating: 1960s.
13 Simple but effective type of clip consisting of two crossed parts kept together by the wire connector. Dating: 1960s(?).
Manufacturer: Presto-Vedder, Altena, Westfalen, Germany. Crossed clips have been used by several more manufacturers.
14 Socket contact comparable to no. 13 but clips are a single, not crossed unit. Pin contact zone is kept together by a strong spring.
Manufacturer: NIKO, Sint-Niklaas, Belgium; also used by other manufacturers. Dating: 1970s - '80s.
15 Nowadays most common type of socket contact. Clips are boxed up which limits the degree of widening out.
Various manufacturers have designed similar variants. Example shown has been made by
Merten GmbH, Gummersbach, Germany. Dating: from 2000s.


Straight blade plugs and matching sockets contacts (1)
16 17 18
Straight blade 3-pin plug
US straight blade socket Classic Japanese straight blade socket
Sperryn straight blade socket

16 - 18
Standard US earthed NEMA 5-15 plug (no. 16) and two examples of sockets (17, 18). Japan uses similar plugs and sockets.
Manufacturers: Leviton Manufacturing Company, Manhattan, USA (16, 17) and National (Panasonic), Osaka, Japan (18).
19 Uncommon type of standard not-earthed NEMA 1-15 plug. Straight blades have clips to enhance blade - socket contact.
Image is based on a picture taken by Scot Kienzlen. The plug is attached to the appliance cord of a Lionel Electric trains transformer (type 1035; late 1940s). It is questionable whether the plug is a Lionel factory original. No other Lionel transformers in Scot Kienzlen's collection have this type of plug. It could be a replacement plug. Unfortunately it doesn't have any clues about the manufacturer. If somebody recognize the plug, please inform the museum (see link on start page).


Straight blade plugs and matching sockets contacts (2)
20 21 22 23
Australian classic straight blade plug Straight blade plug made by Elto Straight blade socket made by PDL
Straight blade contact clips made by HPM

20, 21
Classic (no. 20) and modern (21) Australasian straight blade plugs. From 2015 pin sleeves are compulsory.
Manufacturer: Elmaco Industries, Melbourne, Australia (20) and Elto Electrical Accessories, Auckland, New Zealand.
22 Most common Australasian straight blade socket. Either both clip contacts are flexible, or only the "outer" clip (red arrow).
Manufacturer: PDL Industries, Christchurch, New Zealand.
23 A very different type of contact clips (red arrows) have been used on some sockets made by HPM Industries, Sydney, Australia. The socket has such clips for each of the three straight blades, but for clarity only the earth pin connection is shown.


Plugs with rectangular pins and matching socket contacts
24 25
26 27
BS1363 plug
BS1363 socket contacrt, detail

Swiss T23 plug pins
Swiss T23 connector contact, detal

24, 25
BS 1363 plugs and sockets were introduced in Great Britain in 1947. Safety shutters have always been obligatory; insulating sleeves from 1984. Socket contacts has a shape comparable to image no. 22, but are necessarily wider. Occasionally a rectangular brace was used to control the distance between clip halves.
Manufacturers:  Volex (brand name used by Ward and Goldstone), Basingstoke, England (plug) and MK Electric Edmonton, Middlesex, England (socket).
26, 27
Swiss T23 16A - 250V plug and socket - introduced 1998 - have square pins, to prevent the use of a 10A round pin socket for a 16A appliance. Sockets and connector plugs (shown) have slightly conical contacts (see T23 clip).
Manufacturers: Steffen AG, Würenlos (AG), Switzerland (plug) and Tschudin & Heid AG, Waldenburg (BL) Switzerland  (connector).


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