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)

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

      / Locking Clip
     /
 6  +-------------------
    ====--------------------- [Orange]
    ====--------------------- Red
    ====--------------------- Blue
    ====--------------------- Green
    ====--------------------- White
    ====--------------------- [Black]
 1  +-------------------

Why a third wire for the bell?

(My own interpretation what I hav read)

Three wire telephone 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.

Other view on ring wire

Alan Trevennor who worked eariler UK phone company gave me another explanation of what's the reason for third wire (text combined and edited from the original material send to me):

Essentially in the 1970s if you had more than one phone in your house they were linked inside the house with four wires. These were colour coded Blue, Orange, Green and Brown.

Blue was the "A" wire of the incoming telephone line, Orange was the "B" wire of the incoming line, Green was the "ring" wire (see below), and Brown was the bell-tinkle suppression circuit

Bell-tinkle problem occurred on electromechanical exchanges known as Strowger exchanges (named after their inventor Mr Strowger) For each line on the exchange there was a relay known as the "A" relay. This relay was operated by the loop current when you picked up the phone and was required to pulse each time the dial contacts momentarily broke the connection. Unfortunately this resulted in a back-EMF with a frequency of up to 10Hz which was easily capable of making the telephone bell and (in those days) hard for the necessarily cheap electronics in the phone to differentiate from the 25Hz ring tone.

Therefore an extra set of contacts on the mechanical dial were used to shunt the bell out of circuit whenever the dial was off it's normal position (thus these contacts were called Dial-Off-Normal or DON contacts). Phone doing the dialling had to activate this shunt across its own bell and also all the other bells. This was the function of the brown wire, to link the shunt points in each phone - such that any phone could activate it.

The Green "Ring" wire was required because all the bells were connected in series. Each bell in a 700 series telephone (705, 746, 711 etc) had a resistance of about 1K ohms and connecting them in parallel would have meant there would have been appreciable amounts of current required to ring them all (in those days a typical line was expected to be able to provide up to 6 ringing phones). Thus, there has to be a "ring" wire. The incoming AC ringing current (75Vac at 25Hz) was fed into the series-wired bells from the "main" phone (now called the master) and all other phone were jumpered to connect their bells to the common ring wire.

This scheme did work well, for it's time. Of course now we have low current electronic ringers none of this is really necessary and things are comparatively simple.