Museum of Plugs and Sockets logo, small Special plugs and sockets
alternatives to CEE 7/4 (Schuko)
navigation icon navigation icon
navigation icon


Material shown below is used in special, local networks that have to clearly separated from a standard network with CEE 7/1 (Schuko) sockets. For example powering electronic data processing systems via a separate circuit that is protected against voltage spikes, or situations in which interchanging live and neutral poles has to be avoided. Special plugs can also be used for local networks with a different voltage or frequency.


Terko socket Terko socket for outdoor use Terko plug Terko plug and connector

1 - 4 Terko type socket, rated at 16A-250V. Terko is a trademark of Busch-Jaeger Electro GmbH (Lüdenscheid, Germany). There are two variants, sockets with a notch near the eath contact (no. 2) and without notch (nos. 1 and 4). Each of the models in the colection that have a notch have a certain degree of protection against dust and water ingression and are more suitable for use in rough and outdoor conditions. However, the 'notch issue' is not documented in Busch-Jaeger catalogs. Terko system probably dates back to 1950s. It has been told (in 2016) that Busch-Jaeger will stop production of Terko material within a couple of years.
{ES} - nos 1 and 4;  {WN} - nos. 2 and 3


The HNA system is an older German type of special plugs and sockets.
HNA stands for Handelsschiff-Normen-Ausschuss (cargo vessel standard committee). This German shipbuilding standard dates back to the 1920s. The committee later became a part of the Deutsches Institut für Normung (DIN).

A domestic version of HNA plugs and sockets have been used from the 1930s until early 2010s. See nos. 5-7 below.
A 1930s HNA plug, made by Dr. Deisting (Kierspe, Germany) is shown on a page about classic uncommon plugs (see no. 15).

HNA plugs and connectors are still used in marine industries. See nos. 8-12 below.

Merten HNA socket Merten HNA plug Merten HNA plug with cable side entry

5 - 7
HNA type socket and plugs, rated at 10A-250V, made by Merten (Gummersbach, Germany). Plug no. 6 has the older Gebrüder Merten (GMG) logo, while no. 7 shows the full name of the company. Merten 2015 catalog doesn't mention HNA material anymore.
The domestic HNA variant has flat pins (2.5 x 4.8 mm). Marine version (no. 10) has round pins. which probably correspond to the original HNA standard. A likely reason for not having round pins is a KPI 1928 regulation stating that "Earthed sockets are not allowed to accept unearthed plugs"
. A socket with relative narrow, flat slots cannot be used with unearthed plugs with round 4 mm pins. Moreover, angled flat pins is a feature that distinguishes HNA from plugs made by other companies.
Marine and domestic variants have the same pin configuration.

Note that HNA type devices are similar to old SI 32 plugs with angled flat pins. Find more information on the SI 32 page.

WISKA HNA plug and connector

Pin confuguration of HNA plugs

WISKA HNA connector
WISKA HNA plug and connector details

8 - 12
Marine HNA plug and connector, with round pins. Diameter is 6.0 mm for power pins and 7.5 mm for the earth pin. Plug and socket are protected against dust and water ingression according to IP56 level. Image no. 9 shows that despite a difference in pin size domestic HNA plugs (GMG, no. 6) fit in marine outlets (WISKA, no. 11).

There are two marine versions, [v.1]: 10A (DC) /16A (AC) - 150V and [v.2]: 10A (DC) / 16A (AC) - 250V. Pins have a groove and contact tubes have a pin. Position of groove and pin differ for 150 and 250 Volt. The shown plug and connector are 250V models; groove and pin are indicated with green arrows in image no. 12. The lower voltage version has groove and pin at the opposite side.
HNA standard is laid down in DIN 89267 version 1 (150 Volt) and version 2 (250 Volt).

Plug and connector have been donated to the museum by WISKA Hoppmann & Mulsov GmbH in Kaltenkirchen (north of Hamburg, Germany).

Plugs comparable to HNA In France, earthed, 6A plugs with three round pins have been used prior to the introduction of CEE 7/6 (sockets with earth pin); see page on classic French material for details. The configuration of the three pins is similar to HNA. These French plugs fit  in modern SI 32 sockets (with round slots), vice versa. Another plug that is closely comparable to the French plugs is the old Greek Tripoliki plug.

Conclusion: five types of plugs have a similar 3-pin configuration:
HNA and old type SI 32 (angled, flat pins), and old 6A French, Tripoliki and modern SI 32 (round pins).
HNA could be the original type, given the date of introduction (1920s).


Connectors for special use

Connectors, older type Connectors, new type Connectors, old and new type

13, 14 Male and female 16A-250V connectors, specially designed for use in cable ducts, or installations under raised floors or dropped ceilings. No. 13 is an older type, which is now replaced by the safer model no. 14. The green model has been replaced because the contacts of the outlet can be touched relatively easy with fingers or tools. The black model is better protected against direct access. Both models are polarized; L, N and earth poles are differently shaped, making exchange of live and neutral impossible.
15 Top, left: older type connector outlet and plug facing each other; top,right: connectors fully slided into one another.
Bottom, left: newer type connector partially connected; bottom, right: plug with cap removed.
Wieland Electric logo
Manufacturer of nos. 13-15: Wieland GmbH in Bamberg, Germany. A family-owned company, founded in 1910 by Fritz Wieland. Today it is a worldwide leading manufacturer of pluggable electrical installation technology.


Navigation iconCEE7/4
D i g i t a l   M u s e u m   o f
navigation icon
P l u g s   a n d   S o c k e t s
Navigation iconspecials