Museum of Plugs and Sockets logo, small Round pin earthed sockets
configuration of contacts
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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).

Quite a number of domestic sockets have a spacing of 19 mm between N and L. The earth contact offset varies varies; 10 mm is a not uncommon value, see table below.

It is a tempting speculation that French, Greek and Israeli earth pin offset have a common "ancestor", see row of round sockets below.
A hypothetical "socket tree" shows possible relations.

Earth pin offset of Thai TIS 166-2549 plugs has a very different origin, visualized at the bottom of page.
configuration earthed socket

Earthed domestic plugs and sockets with 19 mm N - L spacing
Standard Amp Spacing
Offset Details
Italy CEI 23-50
19 mm
0 mm
IEC 60906-1 10A 19 mm
3 mm  intentionally smaller than Swiss*
Switzerland SN 441011
10A 19 mm
5 mm
Israel SI 32
16A 19 mm
9.8 mm
CEE 7/5
19 mm
19 mm
10.0 mm
10.0 mm
 From 1930s until late 1950s
 Mid 1950s earth pin moved to socket
Greece "tripoliki" 10A 19 mm
10.2 mm
 Until 1989 used in parts of Greece
Denmark DS107-2-D1 13A 19 mm
15 mm
Thailand TIS 166-2549
16A 19 mm
11.9 mm

 * IEC 60906-1 should have been the universal domestic socket. It is based on Swiss 10A. To avoid foavour one country (Swizerland) abovw all other, a 3 mm rather than 5 mm offset was chosen.


Seven earthed sockets have a similar configuration of contacts: 19 mm N - L spacing and earth contact offset of 10 mm +/- 0.2 mm. Table below image gives relevant details. Click >info< for additional information.

Similar earthed cont. European sockets

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 (see note below) 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.
note about KPI
Vorschriften, Regeln und Normen für die Konstruktion und Prüfung von Installationsmaterial
bis 750 V Nennspannung (more info at origin of Schuko page).

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

Origin of TIS 166-2549 earthed plugs

Origin of Thai earthed plugs

Prior to a nation wide electricity standard, foreign appliance manufacturers, from Japan, United States and Europe used, to save costs, their own “home” plug types.
This explaines why Thai sockets accept straight blade NEMA 1-15 and earthed NEMA 5-15 plugs rated at 15A - 125V.
To connect not earthed round pin plugs sockets have also contacts that accept plugs with 19 mm pin spacing and 4.8 mm pin diameter.
Due to an increasing demand for electric power upgrading mains electricity was raised to 220 Volt in 1960. The ihigher voltage applied to US-type plugs also.

Appliances imported from Europe often have a Schuko (CEE 7/4) plug that fits in Thai socket round pin slots, but Schuko earth clips are not connected to earth.
An adapter was needed to make contact betweem earth clips and the earth slot of earthed straight blade plugs (see images 6-9 on  Thai_3polePlugs.html page).

In 2008 appliance cords were introduced with a fixed TIS 166-2549 standard earthed plug (see above image). Plugs have an “European” L - N pin spacing (19 mm) and NEMA earth pin offset (11.9 mm). Rewirable plugs became available in 2022
The museum is grateful to Maethasit Hongmanee for providing information about TIS 166-2549


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