Museum of Plugs and Sockets logo, small Earthed sockets with
similar configuration of contacts
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The three contacts (Neutral, Line, Earth) of single phase earthed sockets are often positioned in an isosceles triangle. The L and N contacts form the triangle basis (red line). Distance from N to E is identical to L-E and depends on the earth contact offset (blue line).

Continental European domestic sockets have a spacing of 19 mm between N and L. The earth contact offset varies*. For the sockets, shown below, it is 10 mm +/- 0.2 mm.

Whether or not these sockets have a common "ancestor" and are in one or another way related to each other is an intriguing question. Pure coincidence seems less likely. A hypothetical "socket tree" shows possible relations.
configuration earthed socket


Similar earthed cont. European sockets

Seven earthed sockets have a similar configuration of contacts: 19 mm N - L spacing (center to center) and an earth contact offset of 10 mm +/- 0.2 mm. Indicated position of N and L does not necessarily apply to each of the sockets.
For each socket an off-white duroplast example is shown; older sockets were initially made of Bakelite or other material. Table below gives relevant details. Click >info< for additional images and details.

HNA maritime Handelsschiff-Normen-Ausschuss (cargo vessel standard committee). In 1917 ship builders in Hamburg, Germany decided to promote standardization of various components of ships, among others illumination and electrical accessories. HNA maritime sockets are still in production. Configuration of contacts has never been changed.
6 mm plug pins fit in N and L contacts and 7.5 mm pins in E contact. Earth contact offset is 10 mm.  >info<
HNA domestic Domestic variant of HNA maritime socket. Contact configuration is identical to maritime socket, but contacts accept 2.5 x 4.8 mm flat pins only. KPI 1928** regulation states that earthed socket are not allowed to accept unearthed plugs. Domestic HNA socket design dates back to end of 1920s.  >info<
French 6A Socket for 6A plugs with 4.0 mm pins. Earth contact offset = 10 mm. Design dates probably back to 1930s and have been produced by several French manufacturers, notably Legrand. Production in France ended late 1950s, except for export to Greece.  >info<
Greek 10A Socket, formerly used in Greece, for 10A plugs with 4.0 mm pins. Earth contact offset is 10.2 mm. In 1989 Greece switched to CEE 7/3-7/4 (Schuko) standard for earthed plugs and sockets.  >info<
Israeli 10A Until the early 1980s Israeli sockets accepted only 10A plugs with flat, 2.5 x 4.9 mm, pins. Offset of earth contact is 9.8 mm. In 1989 sockets were introduced that could be used for flat and round pins plugs (see below). The socket shown is such transition model.  >info<
Israeli 16A Israel upgraded 10A flat pin plugs to 16A without any change of design. Because of the risk of overheating 4.1 mm round pin plugs were introduced in 1989. From 2000 sockets have round, 5.2 mm outlets.  >info<
earth pin socket CEE 7/5 sockets have an earth pin rather than internal contact. Earth pin offset is 10 mm.
Origin of earth pin sockets is unclear. First known mention dates back to early 1930s (>info<). CEE 7/5  became the standard domestic earth socket in Belgium, Czechoslovakia, Poland and from 1956 in France.  L and N contacts and earth pin have a diameter of 5 mm. Socket rating was initially 10A; in the 1950s upgraded to 16A.

HNA domestic plug and old French earthed plug
Plug compatibility

HNA domestic plugs.
Plugs fit in both HNA maritime and domestic sockets, and are also compatible with Israeli 10A and 16A sockets. The 0.2 mm difference in offset between HNA earth pin and Israeli socket contact remains within socket contact tolerance.
Round pin 16A outlets have a sufficient large diameter to accept 4.8 mm wide HNA flat pins. Israeli 10A plugs fit in HNA domestic sockets, as to be expected.

Old French 6A plugs.
Beside French 6A sockets, plugs with three 4.0 mm round pins are accepted also by Greek 10A, Israeli 10A and 16A sockets. Plugs fit in HNA maritime sockets, but pins do not touch the much wider HNA contacts sufficiently.

HNA maritime plugs.
Because of the large diameter of L, N and E pins, plugs fit in HNA maritime sockets only.

CEE 7/6 plugs, designed for CEE 7/5 sockets.
Plugs have 4.8 mm L and N pins. Therefore they fit only in CEE 7/5, Israeli 16A and (loosely) in HNA maritime sockets. However, earth connection is restricted to 7/5 earth pin sockets.


The image below shows a hypothetical relation between the sockets mentioned on this page. The HNA maritime socket has been taken as common ancestor. Possibly an even older "19 mm L-N spacing  / 10 mm earth offset" socket have existed, but the HNA socket is the oldest in the museum collection.

Similar cont,.European sockets, relations


*    Earth contacts of Swiss T12 and IEC 60906-1 sockets have a ≤ 5 mm offset. Danish,
     British BS 546 and 6A sockets made by former German company Felmas have
     ≥ 12 mm offset. Matching plugs are incompatible with the seven sockets shown above.
**  KPI: Vorschriften, Regeln und Normen für die Konstruktion und Prüfung von
     Installationsmaterial bis 750 V Nennspannung (more info at origin of Schuko page).


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