Telephone wiring

[Taylortwocities]

Pedant

All the OP needs to know is that there should be a potential difference of 48v between the A and B legs.

 

[SUNRAY]

Shouldn't that be +48 volt ?

The exchange battery positive is grounded at the exchange. At the exchange the A wire is connected indirectly to the battery postive and the B wire is connected indirectly to the battery negative

The A wire is positive relative to the B wire

The Orange is normally the B wire

http://www.wppltd.demon.co.uk/WPP/Wiring/UK_telephone/uk_telephone.html

Good heavens do you really think that the guys who jumper these things in the cab bother very much about which colour goes where?

 

[SUNRAY]

Whose being the Pedant?

Actually it's 50.5V or it always used to be up until last year when I last got involved

 

[Taylortwocities]

Yes, 48v in the US, 50v in the UK (nominal). Memory slipping now, its 50 years since I was a telephone apprentice....

In my day it was very important that A&B were the correct way round, as reversal on some legacy system (eg shared service) could provide some interesting results!

 

[securespark]

I have a plug in telephone socket tester that shows REV if A & B are the "wrong" way round.

Used to matter with computer installs but does not bother equipment now.

 

[AndyPRK]

Does it still matter for those phones that require the 3rd ring input wire?

I'm sure it does matter for extension bells! For the few that have them!

 

[Taylortwocities]

Only needed for mechanical bells, etc. These days it can be left off.
Testing has shown that broadband speeds can improve if pin 3 is left empty!

 

[SUNRAY]

Yes, 48v in the US, 50v in the UK (nominal). Memory slipping now, its 50 years since I was a telephone apprentice....

In my day it was very important that A&B were the correct way round, as reversal on some legacy system (eg shared service) could provide some interesting results!

Indeed polarity can be relevant but even for SS lines the polarity correction would be applied anywhere on the system. As a youngster our phone was supplied by an open wire pole route and when this was changed to UG feed to the DP us and the neighbours had the wrong service, the engineer came indoors and swapped the wires over, I tried to convince him the problem was elsewhere as no one had been in prior to him.


As an apprentice I was taught to join the brightest colours when jumpering in cabs or dis cases. So a brown/white pair could be easily be jumpered to black/orange pair with blue/yellow jumper.

We had some contractors in to help with an exchange changeover and an American guy commented our 48V was very low at 51V, apparently their nominal 48V was more like 52.5 to 53V whereas our 50V was supposed to be 50.5 to 51.5V.

 

[rsgaz]

Shouldn't that be +48 volt ?

Thanks, yes, my mistake. Edited now.

 

[JohnW2]

Thanks, yes, my mistake. Edited now.

Thinking of how/when it all started, given the voltage drops in those very long and tiny conductors when an electro-mechanical bell rang, I would imagine that a 'couple of volts between friends' one way or the other would be of very little consequence!

Kind Regards, John

 

[rsgaz]

Actually it's 50.5V

Yes, 48v in the US, 50v in the UK (nominal)

That's not what I'm seeing. BTNR 2511.pdf states that BT have been conforming to European standard ETSI EN 300 132-2 since Jan 2003.

That standard states...

Interface "A" is where the voltage is fed into the equipment at the exchange, rather than measured at it's output, or at the end user's NTE.

It's unlikely to read 48v at home, I agree, just like you never see 230v AC at home either.

 

[rsgaz]

couple of volts between friends

Yeah, but I was 96 volts off!!

 

[ban-all-sheds]

Thinking of how/when it all started, given the voltage drops in those very long and tiny conductors when an electro-mechanical bell rang, I would imagine that a 'couple of volts between friends' one way or the other would be of very little consequence!

It would as long as currents are very small.

Try connecting a lamp across that 48V.

 

[JohnW2]

Yeah, but I was 96 volts off!!

Well, I'm even less worried about "a difference in polarity between friends"

Kind Regards, John

 

[JohnW2]

As I said, in the days when electro-mechanical bells (and, indeed, carbon microphones and wirewound ear-pieces) were ubiquitous, the currents would not have been all that small.