Wiring for UK Telephone Sockets
B - W = Blue with thin white bands
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 the master socket. In some master boxes (such as the type with a removable front section) they go to two connectors marked A and B.
Which way round they are connected usually doesn't matter but 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 (5) and 0V to the A leg (2) in the master socket.
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.
Usual Cable Colour
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
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 or modified by previous occupants.
You should never rely solely upon the colour code - always check both ends of the cable.
No ringing -
Terminal 3 disconnected
Phone ringing continuously.-
Terminals 2 and 5 swapped (2 at one socket connected to 5 on another and vice versa)
Very poor speech quality, possibly poor bell. -
Terminal 3 and 2 or 3 and 5 transposed
Ringing but no speech (or very poor speech) and can't dial out. -
Wire between terminals 2 or 5 broken.
Testing the cabling
You have an installation where the main socket works and the remote in the garage roof doesn't. You need to test the continuity of the circuit. So you can use a very long lead with and a test meter - or cheat.
- Firstly, disconnect the BT line completely.
- In the remote end bridge any two terminals (make a note of which two).
- Measure continuity between these two wires at the master socket end - should be no more than a few ohms.
- Repeat for the second pair of wires.
- If either show a fault swap the combinations - so if you tried 2 and 4 and that was OK, and then 3 and 5 and that failed, you know 2 and 4 are both good so trying 2 and 5 and 2 (or 4) and 3 will show you the faulty wire.
OK - so you have tested the cables and there is 150 yds of cable buried in the newly decorated wall and only 2 wires have continuity - what do I do??
This is about the only occasion where you cheat and use a second master socket.
The second master gives you back your ring signal so connect the two working wires to terminals 2 and 5 on the new master and hope for the best!
Ringer Equivalence Number (REN)
REN measures the load a device places on the line when ringing. A normal BT line will support a REN of at least 4, in other words at least a total of 4 phones/fax/modems should work on any line so long as their REN figures added together don't exceed 4.
The REN is normally found on a label at the base of the machine
(near the green approval symbol).
In practice you can quite often exceed this number because devices with a REN of 1 may actually have a real REN of only a fraction of 1.This is an anomaly of the test procedure used. Moreover many lines can drive a REN of more than 4.
Note that some elderly fax and answering machines can have very high REN's (and they really are high!). If some or all of your phones fail to ring or some ring very anaemically then its possible you have exceeded the REN. Try unplugging devices until they work.
You can get REN Boosters which will increase the ringing capacity of a line if desired, although if you get to this stage you should probably be thinking of installing a small PABX.