GPO 312 experiments - UK Vintage Radio Repair and Restoration Discussion Forum

els1967 · 3rd Jul 2017, 10:14 pm

With a shared service, the n diagram shows a connection to earth so the bells ring. When I try this (as in a earth back to a local plug socket) nothing happens. An ohms reading between the phone line and the "house" earth is around 90k so I guess there is some sort of connection.

Maybe it's something to do with the isolating transforming in my small PBX?

Any idea's anyone?

http://www.samhallas.co.uk/repositor.../0000/N412.pdf

Thanks

Graham G3ZVT · 4th Jul 2017, 12:02 am

It depends on what you are trying to achieve.
The ringing current on your exchange line will be across the pair, and not from one leg wrt earth.
The idea that a PBX extension might support a shared service is an interesting one, It's not something I have ever seen.

20 odd years ago I encountered earth-start exchange lines on a PABX, but the functional earth was just needed to seize the line, the ringing was across the pair.
I have no idea why they were ground-start, they were a PITA and I nearly condemned them as dead lines, until one of them burst into life with an incoming call.

Have a thought for the "holes and poles" guys when experimenting near power sockets.

AndiiT · 4th Jul 2017, 9:35 am

Hi, I'm not too clued up on shared service or earth start lines but there is some information here on different ways of seizing a phone line http://www.britishtelephones.com/exchline.htm

I am aware that a standard phone line pair does not have either the A or B legs connected to true earth, the B leg should sit at around -42 to -48 volts above earth potential and the A leg is around +5 Volts above earth which I suspect is derived from a common current source at the exchange end.

Earth calling, and ringing current return, was used on shared service (party) lines where one station was rung on the A leg and earth and the other on the B leg and earth; The exchange being seized by earthing either the A or B leg depending on which subscriber wished to use the line.

Looking at the information in Bobs Telephone file (link above) earth calling was used on PBX lines to prevent call collision i.e. where a line could be seized at the same moment as an incoming call arrived.

Although not 100% certain I think that you can still connect a telephone bell across the B leg of the line and earth and it will ring on an incoming call, don't forget that there should be a capacitor in series with the bell, it's not a good idea to directly earth a telephone line.

By earth I mean a physical earth in to the ground and not the house mains earth which is effectively bonded to neutral once it gets back out in to the real world.

Regards

Andrew

Pellseinydd · 4th Jul 2017, 1:35 pm

You'll find the specification for what is on a single BT PSTN line at http://www.sinet.bt.com/sinet/SINs/pdf/351v4p7.pdf

I'm sure I remember some years ago, a notice going out that the 'ringing' was no longer 'earthed' . It is many years ago since the old 'shared service' system was in use - replaced by WB900 1+1 analogue carrier system that was later replaced by 'DACS' Digital Access Carrier System. Hence the 'earthed ringing' of the old 'shared service' facility is no longer needed.

Pellseinydd · 4th Jul 2017, 1:41 pm

Quote:

Originally Posted by rambo1152

<SNIP>
The idea that a PBX extension might support a shared service is an interesting one, It's not something I have ever seen.
<SNIP>

Never come across 'shared service' on a PABX in 60 years but it certainly existed on PAXs. I've got the circuitry somewhere.. Virtually the same as the old GPO 'common metering' shared service which didn't have separate meters for the two subs. Hence no 'Call Exchange' button on the phone.

els1967 · 4th Jul 2017, 8:11 pm

Thanks for the input everyone. I tried disconnected the earth and bonding at my main earth terminal and connecting a wire to the gas bonding clamp to see if I could get a decent "earth rod" type of earth but alas it still doesn't work. I connected the legs, each in turn, to the T8 and my "earth" to T2 and no bells on either. I'm a qualified sparky so don't worry, I know what I'm doing (I think) lol.

I was, more than anything, curious to see if I could get the shared service working on one of the lines on my PBX, just out of curiosity really. Shame it doesn't seem to work. I wonder what the GPO had done differently.

3rd Jul 2017, 10:14 pm	#1
els1967 Pentode Join Date: Aug 2016 Location: Dunstable, Bedfordshire, UK. Posts: 140	GPO 312 experiments With a shared service, the n diagram shows a connection to earth so the bells ring. When I try this (as in a earth back to a local plug socket) nothing happens. An ohms reading between the phone line and the "house" earth is around 90k so I guess there is some sort of connection. Maybe it's something to do with the isolating transforming in my small PBX? Any idea's anyone? http://www.samhallas.co.uk/repositor.../0000/N412.pdf Thanks

4th Jul 2017, 12:02 am	#2
Graham G3ZVT Dekatron Join Date: May 2010 Location: Greater Manchester, UK. Posts: 18,715	Re: GPO 312 experiments It depends on what you are trying to achieve. The ringing current on your exchange line will be across the pair, and not from one leg wrt earth. The idea that a PBX extension might support a shared service is an interesting one, It's not something I have ever seen. 20 odd years ago I encountered earth-start exchange lines on a PABX, but the functional earth was just needed to seize the line, the ringing was across the pair. I have no idea why they were ground-start, they were a PITA and I nearly condemned them as dead lines, until one of them burst into life with an incoming call. Have a thought for the "holes and poles" guys when experimenting near power sockets.

4th Jul 2017, 9:35 am	#3
AndiiT Octode Join Date: Aug 2005 Location: Saltburn-East, Cleveland, UK. Posts: 1,786	Re: GPO 312 experiments Hi, I'm not too clued up on shared service or earth start lines but there is some information here on different ways of seizing a phone line http://www.britishtelephones.com/exchline.htm I am aware that a standard phone line pair *does not have either the A or B legs connected to true earth, the B leg should sit at around -42 to -48 volts above earth potential and the A leg is around +5 Volts above earth which I suspect is derived from a common current source at the exchange end. Earth calling, and ringing current return, was used on shared service (party) lines where one station was rung on the A leg and earth and the other on the B leg and earth; The exchange being seized by earthing either the A or B leg depending on which subscriber wished to use the line. Looking at the information in Bobs Telephone file (link above) earth calling was used on PBX lines to prevent call collision i.e. where a line could be seized at the same moment as an incoming call arrived. Although not 100% certain I think that you can still connect a telephone bell across the B leg of the line and earth and it will ring on an incoming call, don't forget that there should be a capacitor in series with the bell, it's not a good idea to directly earth a telephone line. By earth I mean a physical earth in to the ground and not the house mains earth which is effectively bonded to neutral once it gets back out in to the real world. Regards Andrew Last edited by AndiiT; 4th Jul 2017 at 9:42 am.*

4th Jul 2017, 1:35 pm	#4
Pellseinydd Heptode Join Date: Jul 2007 Location: Flintshire, UK. Posts: 707	Re: GPO 312 experiments You'll find the specification for what is on a single BT PSTN line at http://www.sinet.bt.com/sinet/SINs/pdf/351v4p7.pdf I'm sure I remember some years ago, a notice going out that the 'ringing' was no longer 'earthed' . It is many years ago since the old 'shared service' system was in use - replaced by WB900 1+1 analogue carrier system that was later replaced by 'DACS' Digital Access Carrier System. Hence the 'earthed ringing' of the old 'shared service' facility is no longer needed.

4th Jul 2017, 8:11 pm	#6
els1967 Pentode Join Date: Aug 2016 Location: Dunstable, Bedfordshire, UK. Posts: 140	Re: GPO 312 experiments Thanks for the input everyone. I tried disconnected the earth and bonding at my main earth terminal and connecting a wire to the gas bonding clamp to see if I could get a decent "earth rod" type of earth but alas it still doesn't work. I connected the legs, each in turn, to the T8 and my "earth" to T2 and no bells on either. I'm a qualified sparky so don't worry, I know what I'm doing (I think) lol. I was, more than anything, curious to see if I could get the shared service working on one of the lines on my PBX, just out of curiosity really. Shame it doesn't seem to work. I wonder what the GPO had done differently.