Author Topic: UK telephone instractions  (Read 4954 times)


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UK telephone instractions
« on: October 08, 2011, 07:13:13 AM »
This article explains how to wire a UK telephone extension. You are allowed to install extension wiring so long as the connection to the master socket is via a plug, or in the case of the master socket with a detachable lower half, via the connectors on that section. If you decide to connect directly to the master socket or install/replace a master socket you should be aware that you are in breach of your licence to use the phone service and your service provider may charge if they have to come to rectify a fault caused by your efforts (it is rare for them so to do unless you have been really ham fisted).

NEVER connect un-approved devices which take power from the mains to the telephone system. Even if you survive the mains on the cabling the poor wireman sitting in a muddy hole some way away trying to find a fault may not.

Master Socket

This is what a typical master socket looks like. You must have one of these at the point at which BT wiring ends and yours begins.

Master sockets should not normally be used for extensions.

Extension (Slave) Socket.

All extension sockets are like this. Note absence of ringing capacitor and surge arrrester.

The connectors shown are Insulation displacement types (IDC), Extension sockets (especially if bought in DIY sheds) often have screw terminals.

Fixed Upper section
This is the BT supplied NTE5 master socket found in many installations, this particular one is made by Austin Taylor ( Internally it is functionally similar to the master socket above but the components are protected by a plastic cover.

User removable lower section

When the front screws are removed the complete lower portion of the socket including the IDC connectors for user wiring can be pulled out. They connect to the back plate by means of a fixed plug. The fixed plug is a standard BT plug and socket and allows all the internal wiring to be easily isolated for fault finding. A normal phone can be plugged directly into the socket remaining in the wall to test if the line is OK.

The Wiring

Wiring inside a master socket

Although pin 4 has no function in a domestic installation it is usually connected for the sake of neatness. The quality of wiring carried out by many older BT wiremen is often approaching art in its perfection.

The BT Drop Cable (The cable coming from the outside world)

This often has Orange, White, Green and Black wires. Usually (but not always) Orange and White are the active pair and go to connections 2 and 5. In some master boxes (such as the type with a removable front section) they go to two connectors marked A and B.

Which way around they are connected usually doesn't matter but as Rick Hughes kindly pointed out some modems (especially older USA sourced ones) and some answering machines are fussy about polarity, so it's wise if possible to check the voltage on the line and connect -48V to the B leg (J2) and 0V to the A leg (J5).

If you have underground wiring with a small grey connection box by the door the internal cabling will usually be the same type and colour as the extension cabling.

Internal Cabling

Conn No.
Usual Cable Colour

No connection

Blue with White Bands
Speech and Ringing

Orange with White Bands

White with Orange Bands
Not used but usually connected for neatness

White with Blue Bands
Speech and Ringing

No connection

An Important Note on Colour Codes

The colour code shown above is the one which would normally be used by BT. HOWEVER it isn't always adhered to, especially if internal wiring in a new house has been installed by an electrician or it's been done or modified by previous occupants.

You should never rely solely upon the colour code - always check both ends of the cable.

The Nice Thing About Standards Is That There Are So Many To Choose From

Just when you thought you understood it all, a brief word on plugs. The standard plugs encountered are the small RJ11 found on line cords at the telephone or modem end and the BT431A plug at the socket end. On RJ11 it is usual for the centre two pins to carry the signal, on BT431A it is the outer two.

By a masterpiece of technical superiority between them the BSI and BT managed to number the 431A plug in the British Standard as a mirror image of the socket, so when inserted pin 1 on the plug goes to pin 6 on the socket, pin 2 to pin 5 and so on.

As the pin numbering isn't shown on the plug you might ask why I'm confusing the issue. Simply, many packets of plugs in the DIY sheds come with a little diagram on the back which often follows the BS numbering. If you do the logical thing and put a plug on assuming the numbers are as on the socket the phone may