|Around 1979 Post Office Telecommunications introduced the Plug and Socket (PST) method of
connection. This document explains how the concept works.
700 type telephones always had their bells or tone ringers connected in series (max of four) and this system needed four or five wires to each phone as the bell circuit, itself, used two wires. Each bell ringer was 1000 ohms.
PST changed this by making telephones work in a three wire system that allowed for the
disconnection or reconnection of phones without causing interruption of the ringing
circuit. The Plan 4 (old style plug and socket - Plug No. 420
) arrangement involved the use of special
sockets and specially wired telephones when more than one telephone was used.
The system starts with a master socket and this contains a capacitor (dc blocking for the bell circuit), a resistor (allows the network provider to test the line and ascertain whether the line is alright even though every telephone may be unplugged) and a gas discharge tube (transient voltage surge protector). The resistance of each telephone bell is now 4000 ohms.
Every additional socket is an secondary/extension socket and contains no electrical components whatever (there can be as many extension as one wants, as long as they do not leave the building). The extension sockets are wired to the master socket in parallel.
The sockets are wired in series with each other, effectively joining terminals 2 to 2, 3 to 3, 4 to 4 and 5 to 5 (using 6 or 4 wire cable).
The original PST sockets were grey oblong sockets with screw terminal connections and called Jack No. 620. The white sockets appeared when the system was nationally introduced to households. Click here for more information
Tip: Cut back the cable sheath about one inch and locate a nylon draw string. Pull this draw string and split the cable sheath for about five inches and then remove the sheath exposing the wires. Fix the cable to the socket using the cable strap supplied, then wrap the wires a couple of times around the socket housing in the middle of the socket, then fan out the wires and finally terminate the wires. This leaves some slack in the wiring in case of maintenance.
Only use white sheathed telephone cable for internal use - black sheathed should be used externally.
The wiring should be connected to socket terminals using the following colours:-
If six wire cable is used the Green wires can be connected as follows (these are not used but should be terminated):
If an extension bell is fitted then this must be a 4000 ohm magneto type and should be wired to 3 and 5 on any socket.
Each telephone cable is terminated with a Plug No. 431A which has a latch on the right. Some system phones have a latch on the left to stop people inadvertently plugging them into non-system extension sockets. The telephone lead is flat so as to fit into the plug. A special tool is required to connect the plug and cable together. Plug No. 431A is used with 4 wire cordage, whilst Plug No. 631A is used with 6 wire cordage.
Do not attempt to connect the old style round telephone lead to a Plug No. 431A or 631A.
This is the normal colour code on BT supplied cords (Cord 4/502 shown).
TELEPHONE INSTRUMENT WIRING
All telephones must now have 4000 ohm ringers - a mix of ringers can really cause problems.
for instructions on how to convert BPO telephones.
Last revised: April 29, 2020