This page documents the internal wiring used by The GPO, Post Office Telecommunications, British Telecommunications and Openreach at domestic customers' premises.
Some fundamental background information --
A telephone "line" consists of a pair of wires and is, unsurprisingly, referred to as the "pair". Each of the two wires making up the pair have a name: the A-wire and the B-wire.
Relative to each other the A-wire is more positive than the B-wire. At the telephone exchange, the A-wire is connected to earth. The voltage present between the two wires is nominally 50 V DC.
Given two wires making up a pair without any markings, how are the A- and B- wires distinguished from each other? With a modern digital volt meter (DVM) set to an appropriate range, one could just apply the probes to the wires. The display will then either read ~ +50 V DC or ~ -50 V DC, thus allowing the wires of the pair to be correctly identified. Fifty years ago, when the average voltmeter was (typically) a moving coil instrument, it was bad practice to connect one the wrong way round -- simply because driving the movement against its end-stop could damage the device. The technique used was to earth the positive probe and test each of the wires, in turn, with the negative probe. That wire which showed ~ 50 V DC was therefore the B-wire and, logically, the other was the A-wire. Obviously, the "earthing of the positive probe technique" is equally valid with a modern-day DVM.
The Late 1940s - Early 1950s
An outer sheath of either woven cotton or PVC. Two PVC insulated wires, twisted as a pair.
One wire's insulation was coloured black and the other wire's insulation was coloured blue.
An outer sheath of either woven cotton or PVC. Four PVC insulated wires, twisted as a quad.
The individual wires were coloured blue, brown, green & orange and were connected as follows:
|Brown||Bell Wire||When extra bells or telephones were connected|
|Green||Earth Wire||For shared-service lines|
An outer sheath of PVC. Six PVC insulated wires, twisted as three separate pairs. Each pair is referred by its major colour -- hence the green, blue and orange pairs.
The green pair consists of a wire with green insulation & a white stripe and a wire with white insulation & a green stripe.
The blue pair consists of a wire with blue insulation & a white stripe and a wire with white insulation & a blue stripe.
The orange pair consists of a wire with orange insulation & a white stripe and a wire with white insulation & a orange stripe.
When first introduced, co-incident with the current BT 431A plug & socket system, insulation displacing connectors (IDCs) and DIY wiring, all three pairs were connected as follows:
|Orange/White Stripe||3||Bell Wire|
|White/Orange Stripe||4||Earth Wire|
The Present Day
An outer sheath of PVC. Four PVC insulated wires, twisted as two separate pairs.
The individual wires are coloured blue, brown, green & orange and are connected as follows:
The blue & orange wires constitute one twisted pair and the brown & green wires constitute the other.
-- Burakkucat 21:43, 19 April 2012 (BST)