Wire your product Correctly


So often people are unsure about the way in which the telephone wall socket is wired to the BT network.

This page will show you how the wall socket is configured and how a typical piece of equipment is attached to this installation.

The diagram below depicts the typical wiring arrangement of a wall socket and typical apparatus.


The socket and apparatus wiring is coded as follows:
BK - Black
W - White
G - Green
B - Blue
R - Red
WG - White with Green banding
WB - White with Blue banding
WO - White with Orange banding
OW - Orange with White banding
NW - Blue with White banding
GW - Green with White banding

Note 1: The wiring to the socket is of a single core type which is secured into the socket terminals using an insulation displacement tool.

Note 2: The cable most often used to connect apparatus to the wall socket is a multi-cored tinsel type wire manufactured for it's flexibility.

With tinsel cable it's important that the correct tools are used when making terminations as the individual stands of wire are very difficult to solder satisfactorily.

From the diagram it is clear that the wiring arrangement is quite different to that of most countries.

Some countries, such as Ireland and New Zealand have a similar wiring arrangement but use a different type of socket.

The principal differences between the UK and other countries is the incorporation of a voltage arrestor device and components (470kohm resistor and a 1.8uF capacitor) into the wall socket.

The voltage arrestor device is a Gas Discharge Tube (GDT) component intended to short circuit the A-Wire to the B-Wire in the event of voltages exceeding approximately 250V becoming present on the telephone line.

This type of device is relatively slow acting and has been superceded by the installation of a polyswitch type of device in the line interface of most newly designed products.

The 470kohm resistor and 1.8uF capacitor are installed in the wall socket to allow testing of the telephone line from the telephone exchange.

The wall socket also contains a connection to the telecoms apparatus intended to suppress inductive spikes which are generated when loop-disconnect dialling into electro-mechanical exchanges which terminate the phone circuit with a relay coil.

Note:These exchanges are presently being phased out of operation, which, coupled with MF4 detectors being a design feature of the replacement exchanges is resulting in the diminishing use of loop-disconnect dialling.


Electron Parametrics Ltd 2002. This document may be reproduced in whole or in part provided that this copyright notice is reproduced on each copy made.

All trade marks recognised.

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