16-12-2016, 07:46 PM
I don't think it matters, Mike, because of the way the ringer circuit is configured.
In GPO days, when the GPO owned everthing and nobody was supposed to touch the installation in any way, the bells had an impedance of 500Ω. Extension bells were added in series up to a maximum of 4, making a total of 2,000Ω.
Obviously the interwiring to do this was quite complicated. One extension was simple enough - if you knew which links to remove - as the master phone had its ringing capacitor in series with bell as usual, but the link from the bell to the other side of the incoming line was removed. The link in the extension between the capacitor and the bell was removed and a wire joined the floating ends of the bells together, putting them in series.
Adding the third and fourth extensions was a bit more complicated as both sides of the bells needed to be disconnected so that they could be arranged in series and inserted into the bell circuit of the first two phones!
Then there was the bell tinkle suppression which had to be linked between all of the phones. If this wasn't done, every time somebody made a call, all the other phones would 'tinkle' as the dial pulses were generated.
Obviously, such a complicated system was out of the question when the service was opened up and anybody could add extensions themselves.
The most important change was to change the ringer impedance to 2,000Ω. Extension bells could then be wired in parallel, again up to a maximum of 4, when the figure of 500Ω would be reached.
This is where the REN - Ringer Equivalent Number - came in. The maximum REN is 4 - usually 4 phones - but if anything greedy is connected, that has to be taken into consideration. I would guess that a heavy duty external bell is quite a bit greedier that a normal phone, so assume REN of 2. The maximum number of phones you now have is 2, because 1 + 1 + 2 = 4.
Now for the master socket. Why only one socket equipped with a ringer capacitor? It's our old friend bell tinkle again except that in the new system it doesn't have a separate wire. Quite simply, a short is placed across the ringer whilst dialling. By only have one capacitor and the bells connected in parallel, the bells on all the phones are shorted at the same time. If every socket had its own capacitor and all the bells are connected in parallel to stop bell tinkle, there would now be 4 ringer capacitors in parallel across the line which might cause some problems to the shape of the dialling pulses, particularly on long lines!
Now, of course, everybody uses DTMF dialling (unless they've got a vintage installation!) so no more bell tinkle. This means the third wire is no longer absolutely necessary if master sockets are installed everywhere. In which case only a 2-wire connection should be used so that, again, the capacitors are not connected in parallel.
Such a system might be frowned upon, officially, but there is no reason that it not work as well as one using the correct wiring layout.
In GPO days, when the GPO owned everthing and nobody was supposed to touch the installation in any way, the bells had an impedance of 500Ω. Extension bells were added in series up to a maximum of 4, making a total of 2,000Ω.
Obviously the interwiring to do this was quite complicated. One extension was simple enough - if you knew which links to remove - as the master phone had its ringing capacitor in series with bell as usual, but the link from the bell to the other side of the incoming line was removed. The link in the extension between the capacitor and the bell was removed and a wire joined the floating ends of the bells together, putting them in series.
Adding the third and fourth extensions was a bit more complicated as both sides of the bells needed to be disconnected so that they could be arranged in series and inserted into the bell circuit of the first two phones!
Then there was the bell tinkle suppression which had to be linked between all of the phones. If this wasn't done, every time somebody made a call, all the other phones would 'tinkle' as the dial pulses were generated.
Obviously, such a complicated system was out of the question when the service was opened up and anybody could add extensions themselves.
The most important change was to change the ringer impedance to 2,000Ω. Extension bells could then be wired in parallel, again up to a maximum of 4, when the figure of 500Ω would be reached.
This is where the REN - Ringer Equivalent Number - came in. The maximum REN is 4 - usually 4 phones - but if anything greedy is connected, that has to be taken into consideration. I would guess that a heavy duty external bell is quite a bit greedier that a normal phone, so assume REN of 2. The maximum number of phones you now have is 2, because 1 + 1 + 2 = 4.
Now for the master socket. Why only one socket equipped with a ringer capacitor? It's our old friend bell tinkle again except that in the new system it doesn't have a separate wire. Quite simply, a short is placed across the ringer whilst dialling. By only have one capacitor and the bells connected in parallel, the bells on all the phones are shorted at the same time. If every socket had its own capacitor and all the bells are connected in parallel to stop bell tinkle, there would now be 4 ringer capacitors in parallel across the line which might cause some problems to the shape of the dialling pulses, particularly on long lines!
Now, of course, everybody uses DTMF dialling (unless they've got a vintage installation!) so no more bell tinkle. This means the third wire is no longer absolutely necessary if master sockets are installed everywhere. In which case only a 2-wire connection should be used so that, again, the capacitors are not connected in parallel.
Such a system might be frowned upon, officially, but there is no reason that it not work as well as one using the correct wiring layout.
