A photograph of a WT-4 plug with four metal prongs and a plastic pin protruding from the top, and an RJ-11 socket on the side. The metal prongs are labelled with the letter A, while the RJ-11 socket is labelled with the letter B.
A WT-4 plug (A) with an RJ-11 adapter (B)
A photograph of a plastic GTN-6 socket (with seven holes) and GTN-4 socket (with five holes) side-by-side.
GTN-6 (left) and GTN-4 (right) sockets produced by Telos-Kraków.

WT-4 (adopted under the name РТШ-IV 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 It has since been replaced by the RJ-11 standard.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.

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
A GTN-6 plug disassembled into two parts, the plastic cover on the left and the internal electronics on the right. Above the terminal block is visible a blue capacitor.
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

References

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