Telephone line voltage.

[troutybrown]

Hi All
My mum has a phone line that was installed late 70s early 80s so does not have a test point on the Master socket.
Her phone has stopped working no dialling tone. I put a meter across 2 and 5 on socket and getting 22v is this correct?
Trying to figure out if this is a internal or external problem before calling an engineer out.
TIA
Colin

 

[Mottie]

I thought they ran at 50v? I’ve had a shock off of them in the past. My dad said don't be silly, he touched them and got a belt too. Mind you, that was about 50 years ago!

 

[Harry Bloomfield]

Hi All
My mum has a phone line that was installed late 70s early 80s so does not have a test point on the Master socket.
Her phone has stopped working no dialling tone. I put a meter across 2 and 5 on socket and getting 22v is this correct?
Trying to figure out if this is a internal or external problem before calling an engineer out.
TIA
Colin

No, it should be around 50v DC. They put a form of ac on the line, to make it ring..

 

[Harry Bloomfield]

I thought they ran at 50v? I’ve had a shock off of them in the past. My dad said don't be silly, he touched them and got a belt too. Mind you, that was about 50 years ago!

You can get higher voltages when it rings..

 

[troutybrown]

That's what i thought and goes higher when ringing but this being the old GPO setup i'm not sure if it's the same.

 

[troutybrown]

No, it should be around 50v DC. They put a form of ac on the line, to make it ring..

So therefore there is a problem externally

 

[Harry Bloomfield]

That's what i thought and goes higher when ringing but this being the old GPO setup i'm not sure if it's the same.

Well, what you feel is a result of the inductance on the line. Same idea as your ignition coil on a car.

 

[troutybrown]

Openreach came this morning and Master socket was the problem new socket fitted and all working again.

 

[SimonH2]

It's a nominal 48V DC, positive earth, when idle - i.e. one wire is roughly at earth potential, the other is at -48V relative to earth. But since it's run from lead-acid cells float charged, in practice it's a bit higher.
When ringing, an AC signal is superimposed on top of the DC - i.e. the line is at -48V +/- the peak ringing voltage.
When you pick up a phone, it loads the line and the combination of resistances (line and relays at the exchange) means the voltage will drop significantly. In the old days, when off-hook, the current drawn would operate a relay, not sure if there's still relays on the line cards or if it's all solid state these days.