See also - How to work with telephone wiring

Plug and Socket Telephone system (PST) or New Plan was introduced into the UK in the early eighties.  This was to allow the easy connection (no engineer needed) to the telephone line of portable customer apparatus.

PST afforded a new way of wiring sockets as well, if you could call it new!  The wiring was similar to the way that 200 and 300 type telephones were connected together and effectively used three wires in parallel.

The original PST sockets were:-
Jacks No. 620A - master, surface, with captive lid
Jacks No. 621A - master, surface, with non-captive lid
Jacks No. 622A - secondary, surface, with captive lid
Jacks No. 623A - secondary, surface, with non-captive lid

Circuit diagram of Jacks (Line Jack Units (LJU's) are wired the same except secondary has no components)
Master type is shown to the left and Secondary type to the right  


These jacks (shown above) were coloured light grey and had screw terminations.  The socket outlet was on the side and not the top.  The captive lid variant stopped removal of the telephone plug, which could only be released by removing the lid with the aid of a screwdriver.  See picture below.

These above jacks were quickly superseded by Line Jack Unit (LJU) No's 1, 2, 3, 4, 5, 6, 7, 8, 10.
Each version comprised of master and secondary sockets.  Some LJU's also had  a PBX master variant (cheapened version of master - with the lightning arrestor removed).

Note that the outlet jack is numbered in reverse compared to the strip connection on the back of the jack.  i.e. outlet 6 is connected to strip connection 1 (the picture below shows jack/strip numbering).  This information is included for design purposes only and in all normal cases the strip connection numbers should be used throughout this document.

Some LJU's have screw terminations whilst most have Insulation Displacement Connections (IDC). IDC allows for quick termination as one does not have to solder or use screw connections.  The wire is forced, by the use of a special tool (Wire Inserter), into the slot of a strip of metal. The wire must be terminated with the insulation intact and two wires can be terminated in one IDC connector.

Many manufacturers make IDC connectors but BT decided to use those made by Krone.  This effectively standardised this make throughout the Telecommunications industry in the UK.

The sockets are wired together using three pair cable of twisted pair design and the old quad cable was dispensed with.  Twisted pair was standardised because these meant that the same cable could be used for telephone system featurephones as well as standard analogue phones.  System featurephones in general use a pair in the cable to transmit data to and from the telephone system.  If the old quad cable was used on a modern telephone system, then there is a good chance that the data signal will be induced on the other wires in the cable causing overhearing and data noise (a ticking sound).  The twisted pairs neutralise this problem and allow for faster transmission of data, with minimal crosstalk.  Twisted pair cable normally has a bright wire sheath, whilst quad cable came with a grey or ivory sheath.

Every PST system must have one master socket, but can have any number of slave/secondary sockets (subject to cable lengths and whether the wiring goes out of the premises).  The Master LJU contains a capacitor, a resistor and a surge protector. PBX master sockets used internally have the surge protector and resistor missing. The capacitor is part of the ringing circuit, the resistor is for line testing purposes and the surge protector is for arresting high voltage discharges (not lightning!).  Secondary sockets on the other hand have no electrical components.

These master sockets stopped being used in customer premises in the late 1980's and were replaced by the NTE 5 socket (see below) which legally allowed the customer to connect wiring to the carriers wiring.

Sockets are connected together by means of 6 wire cable.  All the secondary sockets are `daisy chained' off the master.

Only connect two wires maximum to any IDC connector.

A Master socket can be made into a secondary socket by cutting off all the components!

Extension bells are wired off connections 3 and 5.



	 	              1  ----- GREEN/WHITE ----------  1  -------

	  A	------------  2  ------ BLUE/WHITE ----------  2  -------

			      3  ----- ORANGE/WHITE ---------  3  -------

	PBX EARTH    -------  4  ----- WHITE/ORANGE ---------  4  -------

	  B     ------------  5  ------ WHITE/BLUE ----------  5  -------

	                      6  ----- WHITE/GREEN ----------  6  -------

The incoming BT line wires can be any colour and polarity is not a problem.

Socket terminal 4 is used generally as an earth connection for telephones connected to old fashioned PBX's.  Terminals 1 and 6 were for future use and never really used.

In the late 1980's the Network Termination Equipment No. 5 (NTE 5) was introduced.  This allowed customers to legally connect their own internal wiring to the Master Socket.   The NTE No. 5 was a master socket with a removable plate on the front.  The customer wiring connected to this plate and would only be in circuit once the plate was fitted back in place.  Effectively, this was a demarcation point.

Be warned........
In the UK you can only wire to the Network provider if you have a Network Termination Socket (NTE No. 5 shown to the right).  This is the type where you can release the front plate, to find connectors on the rear.  The network provider owns the NTE and the incoming wiring to the socket.

The customer can remove the front plate and connect their internal wiring to the plate.  The plate is then refitted.  The plate locates in an internal socket which is also used as a test point.

If your phone line goes wrong - you remove the front plate and plug a known working telephone instrument in the socket that is now exposed.  If the line works, then it is your wiring at fault.  You are responsible for this wiring - do not call the Network provider!

If your house is fitted with any other type of line jack, or you have to hard wire directly onto the incoming line - then this is illegal as only the carrier can connect any wiring into it!  Your carrier will fit an NTE 5 if requested to do so.

For more information please contact OFCOM.


Go to the lightning page to find out how!


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Last revised: November 17, 2021