Telephone connectors, wiring and colours

A Master Socket is where the telephone wires come into your premises:-

LJU/1A This is often an LJU2/1A
(left: outside and inside views)
LJU/1A circuit
(right - its circuit diagram)
NB: This and the input wiring to it is BT’s property, and they get very upset if you do anything to it.
However, since deregulation, you are free to plug extension wiring and your own telephones as you wish into its output socket.

This type of socket and its associated (and now-ubiquitous) BT 431A/631A plug were introduced in the 1980s to facilitate this.  Although new installations now use type NTE5 (which was introduced to accommodate the needs of broadband), there are plenty of these LJU boxes around.  The capacitor was needed for some earlier telephones (such as the Tele 746) for the bell: it blocks the line DC, but passes the AC ringing current.  The resistor (known as the ‘out of service resistor’) simply ensures the capacitor is charged to the 50V line voltage such that if a Tele 746 or similar is plugged in, its bell doesn’t ‘ding’.  The spark gap is a protection against excessive line voltages, such as might be induced by lightning.  It is debatable whether it could provide much protection - and some think its presence upsets broadband.  However, I doubt the latter: its capacitance is immesurably low, and certainly much less than capacitance between the lines back to the exchange, so I would prefer to leave it in place.

Note the BT T logo at the bottom right of the faceplate (this might be the later ‘piper’ logo on newer sockets).  If there is no recognised BT logo on the master socket, it is ‘unapproved’, meaning if a BT technician finds it, he/she could get waspy....

Extension sockets are supplied by BT and others: these may fitted by home owners.  They may or may not have a logo.  The capacitor, resistor and spark gap are not fitted.

The LJU nomenclature is of the form LJUx/yA, where x and y are defined below:

1 55mm square surface mounted box.  All wiring is on the backplate.
2 67mm square surface mounted box.
3 85mm square box, same size as a single standard electrical box.  Can be flush-mounted.
4 Same size as LJU3, but with two sockets for separate lines.
5 Same size as LJU3, but with two sockets in parallel for one line.
6 A single socket intended to be incorporated as part of equipment.
  Only the first three will normally be encountered in a domestic situation.
Other configurations exist, up to LJU11, but are uncommon.
1 A Master Socket, including bell capacitor and resistor, with surge arrester (spark gap).
2 A Master Socket for PABXs, including bell capacitor but no resistor or surge arrester.
3 An Extension Socket, with no extra components.
4 As Master Socket type 1, with screw terminals instead of IDC forks.
5 As Master Socket type 2, with screw terminals instead of IDC forks.
6 As Extension Socket type 3, with screw terminals instead of IDC forks.
  Not all combinations of the above are available.

For example, the LJU2/6A is an extension socket with screw terminals in a 67mm square box.
The ‘A’ suffix indicates the mating plug is type 431A or 631A (qv).
An LJU2/3C ‘left-handed’ extension socket is available to accommodate plugs type 430A and 630A (qv).

Standards?  Which standard would you like, Sir???

Ever since there have been telephones there have been multiple ‘standards’ for wire colours.  Things got even more confusing when the BT 431A/631A plug and its associated LJU boxes were introduced.  Then along came the modem and third party phones, which used the American RJ11 or RJ12 connector between the various phone parts instead of having it all hardwired together.  BT and BSI even managed between them to reverse the pin numbering of the 431A/631A connector...

431a drawing rj11 drawing
BT 431A/631A plug pin numbering RJ11/12 plug pin numbering

... So throughout this article I am keeping to the way the LJU boxes are numbered, ie. pin 1 of the 431A/631A plug is nearest to the release lever.  The received wisdom for numbering the RJ11/12 is that pin 1 is on the left when viewed with the pins towards you (ie. the latch away from you) and the cable going downwards.  This is how pins are numbered in the drawings above.

If a 431A to RJ11 cable is laid flat, the pins should all be on the same side of the cable: however this does, of course, mean that a male 431A to male 431A cable where all pins are on the same side will reverse the connections.  There are too many of these around....

To add to the confusion, a variant (430A/630A) of the 431A/631A exists, which has the release lever the other side (shown dotted in the diagram above).  Pins should be numbered per the 431A, ie. pin 6 is nearest the lever side (according to this image of the mating LJ2/3C ‘left-handed’ socket).  However, reverse numbering (ie. a complete mirror image of the 431A/631A) is often found.  This plug was originally intended to connect handsets to phone bodies, but the smaller RJ11/12 types are now almost always used instead.

So what are these various connectors?

The system is designed around a flat cable with a maximum of 6 conductors laid side-by-side.  The BT 431A is simply a 631A with the outside pins missing.  Pin numbering starts at 2 to maintain some kind of consistency...  These connectors are intermateable.

Likewise, the RJ12 has all ways populated and is intermateable with the RJ11, which only uses the middle 4 (or middle 2 for modems).  The smaller RJ10, which has a maximum of 4 ways, is sometimes used.  Although strictly speaking the pin numbering of an RJ10 is from 1 thru’ 4, for the purposes of this article, pinning is the same as an RJ11 numbered 2 thru’ 5

A better convention for describing these RJ (registered jack) connectors is of the form mPnC, where m is the number of ways and n is the number of pins used - eg. 6P4C is an RJ11.

Do not get these confused with the RJ45 (or 8P8C).  This is a wider 8-pin connector used for ethernet and LAN.
Its latch is wider than the RJ10/11/12, so they are not intermateable.

The following table is an attempt to sort out this mess:-

Function Rsvd Line B Bell Earth Line A Rsvd
BT431A/631A pin number(1)2345(6)
RJ11/12 pin number (phone)(1)2345(6)
(Pins not used in 431A and RJ11 connectors are shown in brackets.)
RJ11/12 pin number (modem)nc3(2)(5)4nc
Perversely, some phones use modem connections (extra pins are shown bracketted).
Clearly a phone designed for one system will not work with a cable designed for t’other,
so it’s always safer to use the cable supplied with the phone!
BT flat flexible cables ORN RED BLU GRN WHI BLK
US flat flexible cables BLU YEL GRN RED BLK WHI
This coding is often used in imported phones.
3-wire flat flexible cables nc WHI RED nc BLU nc
BT Incoming cable - ORN - - WHI -
(Auxiliary colour pairs that might be present are:  Green & Black, Blue & Brown and Red & Grey.
These are active when two or more lines come into a building.)
Additionally, there are 3 steel wires (usually coloured Yellow) that are only used for cable support: they do not carry signals.
Perversely, the Orange wire is Line A here, but is Line B in the Openreach ‘standard’ for wiring within premises...
Older systems used a heavy-duty figure-of-8 twin with no way of telling which wire was which without using a volt meter.
BT fixed wiring within buildings (new(er)) GRN/WHI BLU/WHI ORN/WHI WHI/ORN WHI/BLU WHI/GRN
(The secondary colour is usually rings, or could be a tracer, or could be absent altogether on wires whose base colour is not White.)
Openreach fixed wiring (and GPO pre-1980) - BLU GRN BRN ORN -
(The brown and green wires could be swapped.)  Whether coincidence or not, Openreach reverted to the old GPO colour scheme...
An alternative sometimes encountered... - ORN BLU GRN BRN -
I’ve come across this several times.  It is not recognised AFAIA, but is much more logical since the Orange wire is Line B - the same as in incoming cables, and the Blue and Green wires are the same colours as their equivalents in flexible cables.  Oh well...

Wire Functions:

So the bottom line is: in a modern system (ie. just two wires - Line A and Line B - and no bell wire), it doesn’t really matter is these wires get swapped.  But in any system incorporating a rotary dial phone with a bell, it is important to get it right.  So do so anyhow to avoid any hassle in the future.


With the advent of the ‘LJU’ system, any number LJU boxes can be daisy-chained in parallel around a house.  There is, however, a limit on the number of phones that may be connected at any one time.  This is imposed by the exchange on ringer impedance: if there are too many ringers (or bells) in parallel, only some (or none) will work.  Most phone systems allow up to 4, as defined by the Ringer Equivalence Number (REN).  A bell impedance of 4kΩ (such as the bell type 59D) represents an REN of 1.  The older 1kΩ bells originally fitted to Tele 746s and 706s therefore had an REN of 4, so no further phones could be added.

Cordless phones usually have an REN of 1 (though it is usually less than this in practice): this is a function of the base unit only, so you can have as many handsets as the base unit will permit and still maintain an REN of 1.

In days of of the Tele 746 and earlier, fitting extensions wasn’t simple, and required an expensive visit by a GPO person to intall them: the default 746 configuration was one hard-wired phone with its 1kΩ bell.  To ensure all phones would ring when more were added, a complicated ‘series’ system (known as Plan 4) was devised:

746 extensions

The circuit above is my interpretation of fig. 2 of the dreadfully presented and confusing GPO circuit number N4506.  If a phone is plugged into one of the sockets, its jack opens the shorting switch within the socket, which breaks the connection between pins 4 and 4A, and inserts the phone’s bell in series with other bells.  If no phone is connected, the bell in the fixed bell set (no.26, which is wall-mounted near the incoming cable), will still ring.  Thus additional bells increase ringer impedance, and so do not exceed the permitted maximum REN of 4.  The maximum number of phones that may be thus connected is 4.

To add further to the confusing plethora of cable ‘standards’, wire colours in wiring and extension cables differed from each other and from those listed above.  Moreover, unlike the convention for most jack plugs, the tip is the highest number.

A 5-pin version is used on shared (‘Party’) lines.  In this case, pin 4 is the ‘Party’ pin and the bell link is via pins 5 and 5A.

4-pin plug (420) and socket (95A) pin number: 1 2 3 4 - 4A
Function: Bell rtn. Line A Line B Bell -  
Fixed wiring: GRN * ORN BLU GRN -
 Terminal 1 is wired internally to terminal 4A
5-pin plug (505) and socket (96A) pin number: 1 2 3 4 5 5A
Function: Bell rtn. Line A Line B Party Bell  
Fixed wiring: GRN * ORN BLU BRN GRN
 Terminal 1 is wired internally to terminal 5A
* The Green wires terminate at pin 1 and a different green wire carries on to pin 4 (or 5) of the next socket in the chain.
Other wires carry on through.
The Orange wire connects to pins 1, 2 and 4A (or 5A in Party systems) in the last socket of the chain.

These old connectors and junction boxes were beautifully made:-

  Differences between
Old hardwired junction box 5-pin extension jack and socket 4- and 5-pin jack sockets

BT8  BT4    The incoming line is connected to the outside pins of the hard-wired box (above left).
BT7  BT3    The terminals are not normally numbered, but on the GPO circuit diagram N846
BT6  BT2    for the Tele 746, they correlate with the junction box photo
BT5  BT1  ← as shown here.

Note the colour coded dots on the 5-pin plug (above centre, inset: 4-pin plug is similar) to facilitate wiring.
You can see from the ‘difference’ photo (above right) that the plastic moulding is the same for both versions of the socket.  The switch contact terminals are also the same; they’re merely placed differently according to type.
Production engineering at its best!

Steve Glennie-Smith   25th August 2020