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Domestic sockets with an earth pin - CEE 7/5 - are the standard type in, among others, Belgium, Czech Republic, France, Poland and Slovakia (country names are listed in alphabetical order).
The origin of earth pin sockets is still a mystery, despite quite some research by Reiner Hahn and Oof Oud. Information that has been found so far is summarized below.

>>>  If you can help us to unravel the origin of earth pin sockets, please mail the museum.


A.  Introduction in Czechoslovakia and Poland,  based on paper by Josef Kunc (see source)

In the early 1930s engineers of the Czechoslovak Electrotechnical Union (ESČ) recognized the need to standardize domestic earthed plugs and sockets in Czechoslovakia.
In view of the relation between industries in Bohemia and neighboring German Sachsen, engineers proposed in 1932 to introduce the German Schutzkontakt (“Schuko”) type.

The Siemens-Schuckertwerke Schuko patent was filed in December 1929; see Origin of Schuko page. In Germany manufacturers were tied to pay royalty fees.
Czechoslovak engineers were of the opinion that no property-related provisions should apply to a generally used device. After long negotiations Siemens-Schuckertwerke renounced royalties, but only for the territory of Czechoslovakia. Export of Czechoslovak made Schuko plugs and sockets was not allowed. This was unacceptable to the Czechoslovak Electrotechnical Union.

A deadline for the introduction of the standard was already set. It had to come into force in the course of 1933. The only other system that was tested in practice was the system used in Belgium and possibly in France (see praragraph B and C). Moreover, the use of this system was not tied to any property rights. That is why it was possible to simply take over the Belgian norm then in force.

The Czechoslovak standard ČSN-ESČ 107 for earth pin sockets (the “Belgian norm”) was approved on 23 May 1933 and stipulated that from June 1, 1934 no other socket types should be used in newly build and renovated homes, offices and workplaces. Not earthed sockets were banned, but remain available only for replacement of broken items in older buildings.
Among others Kramer & Löbl (renamed Elektro Praga in 1946)  in Jablonec nad Nisou and Kontakta Gesellschaft in Liberec started production of earth pin sockets and matching plugs .
Shortly after 1934 they were introduced in Poland also.

Czechoslovak earth pin socket and plug

Earth pin socket made by
Kontakta Gesellschaft (1935).

Matching plug made by
Elektro Praga (1960).


B.  Introduction in Belgium.

The 1937 catalog of Usine Belge de Matériel Électrique Vynckier Frères & Cie shows 6A and 10A - 250V plugs and socket with earth pin (see image right). Each have a CEBEC certification mark. Earth pin sockets are absent in the Vynckier 1930 catalog. Other 1930s Vynckier catalogs have not been found. Unfortunately most of the Vynckier archives have been lost.

Niko, another reputable and still active Belgian electro-technical company started in the 1930s also production of sockets with earth pin (D. Sturtewagen, Niko; pers. comm.).

SGS CEBEC (Société Générale de Surveillance, Comité Electrotechnique Belge/Belgisch Elektrotechnisch Comité) could not provide information about the exact date of certification and/or standardization

Vynckier 1937 catalog images


C.  Introduction in France.

The ministry of Work of the French Republic enacted on August 4, 1935 the following decree: “The protection of workers using electrical equipment makes it compulsory to use outlets with an earthing device in installations where the voltage is greater than 24 Volts, for the use of electrical appliances with flexible cords, such as portable appliances, drills, tapping machines, etc.”
To conform to it, the French electro-technical company Maure started offering Schuko type plugs and sockets.
Catalogs published from 1930 to 1950 shows that Maure has never sold earth pin sockets. In 1951 Maure was acquired by Legrand.

Nowadays French earth pin socket and matching plugs are defined by Normes Françaises C 15-100. The first version of the standard dates back to 1956. The oldest NF C 15-100 examples in the museum collection have been made by L’Ébénoïd (see images). Both USE/NF centification mark and PF Bakelite quality mark have been introduced after World War II (see details).

The French company Legrand could not give information when they started production of earth pin sockets, but probably not before late 1950s. In 1949 a fire destroyed most of the porcelain production facilities. It was decided to abandon tableware production and to focus fully on electrical wiring and installation material.
At least three companies - Legrand, Electro Sécurit and L'Ébénoïd - have made 6 Amp 3-pin earthed plugs, probably from mid 1930s until late 1950s. The position of the plug earth pin was identical to the position of NF C 15-100 sockets (see images). Production may have been  discontinued, 'forced' by the 1956 French standard. A comparison with Swiss earth pin sockets and later three pin plugs is noteworthy (see D).

Maure Schuko advertisement

L'Ebenoid earth pin socket and plug

Legrand earthes socket sand 3-pin plug


D.  Earthed sockets in Switzerland.

Swiss not earthed plugs with a 19 mm pin spacing and matching sockets were classified as Type 1. Earthed, T2, was introduced in the early 1930s. T2 sockets had an earth pin. The offset of the earth pin was larger than "Belgian" (now CEE 7/5) sockets.

Earthed T2 plugs, with earth contact rather than earth pin, fit also in T1 sockets without earth connection. Through the years this was regarded as an unacceptable risk. A new earthed type (T12) was developed in 1937 and implemented in 1953. The earth pin moved from socket to plug and the offset was reduced to 5 mm. Sockets without earth contact were taken off the market.
See SN 441011 page for more information and images.

Swiss 10A sockets
Image source: Jean Steiger, Schweizer Bauzeitung (1954)

E.  Provisional conclusions.

1. It is likely that Belgium was the first country to introduce earth pin sockets according to nowadays CEE 7/5 specifications.
For that reason it seems incorrect to indicate these sockets as "French type".

2. The origin of earth pin sockets is still unknown. Possibly a Belgian company or engineer, but any proof or indication fails. There could have been a relation between Belgian and Swiss earth pin sockets; the design of one may have inspired the other (or vice versa).*

3. Another mystery is why Czechoslovakia did not had to pay royalties. Possibilities are: (a) socket patent was expired in 1933;
(b) invention has never been patented; (c) competition in a growing market for earthed domestic plugs and sockets, may have given a non-Czechoslovak organization the idea to increase market share of pin earth sockets by paying the royalties.

4. Czechoslovakia was probably the first country - worldwide - to oblige by law that only earthed sockets shoud be installed in all new homes and buildings (from mid 1934 !).

* Electrosuisse has been asked about the origin of T2, but ...?


Historie chráněných domovních zásuvek (1) by Josef Kunc, electrical engineer at ABB Electro Praga in Jablonec (Czech Republic), published in Elektrika 8.11.2002 (link).

Stand der Entwickung und Fertigung von Elektroistallatioinsmaterial in der CSSR, by Oldrich Novotny, Elektrotechnischen Prüfanstamstalt, Prag-Troja, published in
Der Elektro Praktiker 1965, issue 5, pages 165-169.
Links: p.165  p.166  p.167  p.168  p.169 

Zur Einfürung des neuen schweizerischen Steckkontaksystems, by Jean Steiger, published in Schweizer Bauzetung, Vol. 72 (1954), issue 48, pages 702-704  (link).

Hausinstallationsvoschriften (HV) des Schweizerichen Elektrotechnischen Vereins. Teil 1, published by SEV, Zürich,
1 Juni 1985.

Une prise est une prise est une pris, by Bernardo Rieder (Electrosuisse), published in, 2017 Nr. 1, pages 2-3.


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