Could anyone please inform me, as to how a public telephone system, actually works, I have been brought up on the old system, with 3wire equipment, but now we have Category 5 Enhanced, cabling, for normal phone circuits. Could anyone (Paul), please help. I have heard a lot about tip and ring, but I do not know what these terms mean. HELP, Please.
Take a MONO (two conductor) earphone plug (the same kind used by telephone co. plug boards to switch between lines). The tip would be the physical tip of the plug. Next is a little plastic insulating ring. Then you have the ring (which is the second conductor) - actually a sleeve behind the insulator. So the thing looks like this:
If you have a stereo (three conductor) phone plug, you wind up with tip-ring-sleeve:
If you're looking at basic operating voltages on the line, in very general terms, the central office battery is a nominal 48V (U.S.) or 50V (Britain). The battery banks have the positive pole grounded.
Power is applied to the line through two coils, one on each side, so as to keep the circuit balanced. Usually, the tip side of the circuit connects to ground, and the ring side connects to -48 or -50V, each through its coil. The resistance of the coils plus that of the line to your house is why you read 48-50V on an idle (on-hook) line, but a much lower voltage when off-hook, typically 10-15V if close to the exchange, as little as 5V or less on a very long line.
I said that the polarity is as above usually. On just about all modern exchanges, it is always that way round, but on some older systems the polarity would reverse to indicate answer supervision (i.e. when the called party answered). Most telephone equipment will work fine whichever way it is connected, although some of the early TouchTone phones wouldn't work on reversed polarity. (Correct connection was also very important on party lines.)
Ringing voltage applied to the line is generally 90 to 120V AC at a frequency of about 20Hz, superimposed on the 48 or 50V DC.
Back to the plugs. The tip and ring parts of the plug would connect the two sides of the circuit through the operators board. The sleeve connection was not extended to the subscriber's line, but was just used within the exchange to control the seizure and release of switching equipment. In other words, the subscriber's loop is just two wires.
In America, the terms tip and ring are still the accepted designations for the two sides of the line. With automatic switching equipment, British GPO terminology favored calling them the "A wire" and the "B wire" respectively.
[This message has been edited by pauluk (edited 09-06-2002).]
Re: Telephone Systems#133655 09/07/0208:02 AM09/07/0208:02 AM
In Austria these wires are called A and B, white and brown in phone cords, yellow and brown for fixed wiring. Typical cable for fixed work is 3 (on newer work 4) conductor solid SVT-like with the colors yellow, brown, green and blue, the latter sometimes used for 2nd line. Blue exists only on newer cables, green was used (I believe) for extra bell. One I have seen requires a 200V DC supply. Wonder where to get that in a country with 220V AC supply? Old color code for fixed wiring (maybe 1960ies and earlier) was white for a, red for b and green. The letters for the lines must be pretty old here, as I found them on a phone from 1949. They're used in Germany as well.
Re: Telephone Systems#133656 09/07/0211:23 AM09/07/0211:23 AM
The A and B designations in the British system also go back many, many years, at least to the 1930s. I've mentioned the color code in another thread, but the flexible cords on GPO phones also used white & red for A & B respectively (and still do), although fixed wiring colors are different.
The subscriber loop to the exchange is only two wires, but many systems developed with a third line internal to the house for interconnecting the bells.
In the old U.K. hardwired system, all extension phones had their internal bell capacitors disconnected and the blue wire in the cord was used to link them all back to the capacitor in the main instrument. This insured that the bells were shunted when any phone was taken off-hook so as to prevent the bells on the other phones tapping, or tinkling during dialing.
The green wire in GPO phones was most often the bell return, and on a one-phone installation would just be linked to the white in the wall junction box. It was done that way so that an extension bell could be connected in series by just opening the box. Some phones were fitted with only a 3-wire cord (red, white, green).
One of the old "extension plans" used a 5-way plug and jack and extra wires so that when a phone was plugged in its bell would be added in series with all the others. The jacks had shorting contacts to maintain the bell circuit through unused jacks.
The new-style wiring with modular-type jacks is wired somewhat differently as the bell capacitor is located in the jack unit rather than in the phone itself.
Many other places used similar arrangements for bell wiring, e.g. in America the yellow wire was often the bell return and would be connected to the green (or maybe to a local ground on party lines). I have an old 1962 Western Electric 500 set which has only a 3-wire cord (red, green, yellow).
Thanks Sven and Paul, I think, Paul,that our telephone system in New Zealand here was designed by English Telecommunications people,who had moved over here, to get out of Britain. The systems that you have explained, sound very familiar. Just a short note on the Party Line set-up, in Temuka, a small town where I grew up, we never had an automatic exchange until 1987, until then, we were all on party lines, we didn't even have a Manual exchange, with respect to the 746 GPO telephones that we had, they did not even need the dials that they came out with, as we could not use them anyway.
Re: Telephone Systems#133658 09/08/0208:24 AM09/08/0208:24 AM
I assume that 746 phones were shipped with a New Zealand style dial. A side note: When STD was introduced in the U.K., London was allocated the code 01 as being the shortest to dial. I always wondered why on earth the NZ phone system allocated 09 to Auckland until I learned about the "backward" dials!
Do you know how the party line system was se up for automatic operation in New Zealand? Did you use earth testing as was common in America, or did you copy the British scheme where you had to press a button to get dial tone? (The button would earth A-wire for one subscriber, B-wire for the other, and ringing would be applied between one wire and earth rather than across the pair.)
Re: Telephone Systems#133659 09/11/0203:11 AM09/11/0203:11 AM
Paul, At the time, I was only an apprentice Electrician, but, what I do know, was,that at the time (when the then Post Office Telecoms), ran all new wiring through houses, buildings, in Temuka. But, we still had the old 746 phone, the only difference being that we could actually use the dial on our telephones, to dial out directly. Now that I live in Ashburton, here, we still have the same technology, except we are trying to pump a certain number of Megabytes of Data down it now.
The phone system has changed drastically in the U.K. in the last 20 years.
Now we have digital exchanges practically throughout the network, but in the early 1980s much of the system was still Step-by-Step, with just some electronic and a small number of crossbar systems (a very small percentage by American standards).
Paul, Is it possible, to get an Electric Shock, from the Ring Voltage, if you are working on a Telephone system, inside a customers house and somebody rings the house where you are working?. Just interested.