WT-4

A WT-4 plug (A) with an RJ-11 adapter (B)

GTN-6 (left) and GTN-4 (right) sockets produced by Telos-Kraków.

WT-4 (adopted as ВТ-4 in the USSR) is a Polish telephone plug used to connect telephone sets to the network. Introduced in the second half of the 20th century and adopted in several Eastern Bloc countries as a standard. It has since been replaced by the RJ-11 standard. Sockets are labelled with the text GTN-4 (РТШ-4 in the USSR).

WT-4 plugs consist of 4 metal pins with an additional plastic pin at the bottom to prevent inserting the plug the wrong way round. When the plug is inserted into a socket, the plastic pin also disconnects a 1μF capacitor built into the socket. When connected, the capacitor simulates a telephone set with the handset hung up. This allows for the testing of the line even when the subscriber doesn't have a telephone connected to the network. In the mid-1990s, installations of WT-4 plugs began to be phased out, and the standardised shape of the socket was used to install RJ-11 connectors instead.

Pin number	Function	Notes
3	Telephone line A
4	Bridged with pin 5 in the socket	Used to connect an additional bell after removing a jumper wire in the socket
5	Telephone line B
6	Ground	Used in sets with grounding buttons when working with internal telephone exchanges

The inside of a GTN-6 plug. Visible above the contacts is the 1μF capacitor.

A rare 6-pin version also existed, called WT-6. Extra pins (numbered with the missing 1 and 2) were located between the existing pins 3 and 5, and 4 and 6, respectively. These extra pins were used for powering telephone sets with illuminated rotary dials, as well as some more advanced telephone installations.

See also

• Telephone jack and plug

References

• Biuletyn Informacyjny Teleelektroniki, nr 2 / 1974
• Schemat telefonu Aster (prod. Radomska Wytwórnia Telefoniczna)
• Schemat telefonu CB-662 z podświetlaną tarczą