Telephones - POTS
Plain Ordinary Telephone System
The basic UK telephone wiring is what you will find on an ordinary domestic phone line or "2 wire" extension from a PBX (Private Branch Exchange). Unlike the American system where 2 wires are simply daisy-chained from jack to jack, the UK system uses 2 wires for the incoming line and 3 wires between jacks (4 in the case of a PBX). This is because the components to ring the phones are located in the "Master" jack where the line first enters the building. Subsequent extensions are connected to "Secondary" jacks.
The Master contains 3 additional components:-
  • C1  A 1.8uF Capacitor - to pass the ringing current to the phones.
  • R1  A 470Kohm Resistor - for testing purposes.
  • S1  A Surge Suppressor - to protect the line from high voltage transients.
The basic circuit is shown below.

Master Socket Circuit

In newer installations the Master will be in the form of an "NTE5" which is a standard UK size plate (86x86mm) with 2 parts. The rear part of the socket is fitted to the box and has 3 connections A,B & Earth. The removeable front part has 6 connections for your extension wiring to connect to. Details of connections and designated wire colours are shown below.

pots wiring

A The positive wire of the telephone line. 0 Volts with respect to earth
B The negative wire of the telephone line. -50 Volts with respect to earth
Earth Protective/Signalling Earth. Not normaly provided on domestic lines
Wire colours for the above depend on the type of cable feeding the premises.

Pin MAIN/Stripe Colour Function
1 GREEN/White Not normally used
2 BLUE/White B wire -50 Volts with respect to earth
3 ORANGE/White Bell wire
4 WHITE/Orange Earth (Not normally used except on PBX's)
5 WHITE/Blue A wire 0 Volts with respect to earth
6 WHITE/Green Not normally used

Wire terminations will be either by screw terminals or "Punchdown" IDC connectors. With the punchdown connectors it is essential to use the proper tool to terminate the wire for a reliable connection. Disposable plastic tools are available very cheaply or even free when you buy your sockets. Long life professional tools are available for around £17. These are well worth the investment. The professional tool terminates and trims the wire in a single action.

Rules for Reliable IDC Connections

  1. Do NOT strip the wire.
  2. Do NOT terminate wires of different diameters in the same IDC.
  3. Do NOT terminate more than 2 wires in the same IDC.
  4. Do NOT push the wire in with a screwdriver.
  5. DO anchor the cable sheath with a "tywrap" to prevent breaking the connections.

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This page last updated : 5th October 2000