Analogue Phone Wiring

Wiring Standards
The wiring from the network into a premises for a single analogue line is a "pair" of wires, referred to as the A and B wires. The concept of a pair is important in communication systems, as it is embodied in cabling design. A pair in a multi-pair cable is two wires twisted together, usually colour coded to distinguish them from each other and from other pairs. If wires are not connected correctly according to their pairing, this can introduce cross-talk, which degrades signal quality.

The normal cable used for connecting extensions in the UK is twin pair telecommunication UTP (BT Spec. CW1308) cable. The two pairs are the blue pair and the orange pair. In each pair one wire has insulation mainly of the pair colour, interspersed with short white, the second wire's insulation is primarily white, interspersed with the pair colour. Each pair is twisted together.

The standard for wiring BT style sockets is to connect the wires as follows:

Connection (in Socket) Colours Picture
2 Blue / White Blue Wire 1
3 Orange / White Orange Wire 1
4 White / Orange Orange Wire 2
5 White / Blue Blue Wire 2

There are two methods of connecting the wires when adding extensions. The best method (electrically) is the "IDC Punchdown." This is used only with solid wires, and consists of a pair of metal jaws that the wire is pushed in to. If you are expecting to do anything more than a single extension, it may be worth purchasing a professional tool to do these. The common cheap plastic tools lose effectiveness after more than half a dozen uses. Note that no more than two wires should be connected to any single connector.

The less effective and more time consuming method uses screw terminals and requires that the insulation be stripped first. This method involves a risk of nicking the wire when stripping off the insulation, which may increase circuit resistance and degrade signal quality, it may also cause the wires to break at the nick under the stress of the screw being tightened, or flexing when the socket is placed in the mounting box. The electrical connection with this method is usually considered to be inferior to the punchdown type.

If flexible multi-strand cable is used, then screw terminations are the only option, multi-strand cable forms a very poor connection in punchdown blocks, which are designed for single core wire.