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Internal Wiring

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Revision as of 04:16, 18 April 2012 by Burakkucat (talk | contribs) (Draft. Work in progress.)
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>>> DRAFT -- This page is currently under construction. <<<


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. One twisted pair of two PVC insulated wires. One wire's insulation coloured black and the other one coloured blue.

The 1960s

An outer sheath of either woven cotton or PVC. Four PVC insulated wires. Either twisted as a quad or as two pairs. (Failure of the author's memory.)

The individual wires were coloured blue, brown, green and orange and were connected as follows:


Colour Usage Note
Blue B-wire
Brown Bell Wire When extra bells or telephones were connected
Green Earth Wire For shared-service lines
Orange A-wire


The 1980s

An outer sheath of PVC. Six PVC insulated wires, twisted as three 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 and the introduction of DIY wiring, all three pairs were connected as follows:


Wire Colour IDC Connector Usage
Green/White Stripe 1 Signalling
Blue/White Stripe 2 B-wire
Orange/White Stripe 3 Bell Wire
White/Orange Stripe 4 Earth Wire
White/Blue Stripe 5 A-wire
White/Green Stripe 6 Signalling


The Present Day

Blah, blah, etc.


--Burakkucat 04:16, 18 April 2012 (BST)