Telephone wiring in UK

It is common practice in England to ring a telephone by signaling extra voltage across one side of the two-wire circuit and ground (earth in England). When the subscriber answers the phone, it switches to the two-wire circuit for the conversation. This method allows two parties on the same line to be signalled without disturbing each other.

British phones which use BT plug and socket system have three wires form the telephone to wall jack. There are only two wires A & B (Tip & Ring) coming into a house. In the primary jack in the house is a 2 uF capacitor. On the end of this cap is the third wire. The AC ringing signal is fed to the phone on this wire and its DC counterpart. See diagram:

                  ----| |------O (3)
                  |
                  |
    (B) O----------------------O (2)



    (A) O----------------------O (5)

Note: The Numbers in the diagram are the numbers engraved on the jack terminals.

To avoid the situation of several capacitors shunting the speech path, BT decided to locate a single capacitor in the master socket, with none in the phones. That's why a phone designed for the UK market won't ring elsewhere. Phones designed for other countries will ring on typical UK wiring, because the ring signal is also left between A and B wires.

You could try and obtain a 'master' type of UK 'phone socket - they are available from some companies in the UK. Or you could just add the 1.8uF capacitor yourself. Here is a schematic of the master socket:

Master Socket

  2o------+------------------+--------------------(2
          |                  |
          |                 ---
          |                 --- 1.8uF 250V
          |                  |
          V                  +--------------------(3
            Overvoltage      |
          ^ Spark gap        \
          |                  / 470k
          |                  \
          |                  /
          |                  |
  5o------+------------------+--------------------(5
And there is the wiring of the phone cable in UK phone wire:
      / Locking Clip
     /
 6  +-------------------
    ====--------------------- [Orange]
    ====--------------------- Red
    ====--------------------- Blue
    ====--------------------- Green
    ====--------------------- White
    ====--------------------- [Black]
 1  +-------------------

Why a third wire for the bell?

Three wire telephoen wiring was designed at a time when pulse (loop disconnect) dialling was common, and telephones still had real bells. During pulse dialling, there is a tendency for the bells on the associated telephones to tinkle. One theory on that is that using a separate wire for the bell, it was easiser possible for the telephone that's doing the dialling to shunt out the bells, preventing them from tinkling. Other reason might have been the idea to put the propably bulky large capacitor (1.8 uF capacitor which is not electrolytic and must withstand some voltage might be a bit large when built using old technology).

There are some types of telephone approved for UK use in which the ringer is connected between the two lines, just as it would be elsewhere. Many modems will also detect ringing current from the line pair and disregarding the bell wire. If the modem also offers a socket for connecting a telephone, UK modems can be expected to supply the telephone with bell current correctly; but foreign modems might not do so, with the result that the phone, if it uses the "British" bell arrangement, will not ring.


Tomi Engdahl <Tomi.Engdahl@iki.fi>