How can I be sure there's a connection between the telephone line and the power line? (Answer: they should not be connected.)

Question

I noticed there I can feel the electricity when I touch one of my telephone wires. I thought that we couldn't feel the 48 V that is usual for this kind of wire.

Maybe one power line 127 V is submerged with this wire and that's why I can feel it.

I have a multimeter. To verify it should I measure the voltage in relation to the nearest ground or what?

Here are my results

ACPhone 1Phone 20
DCPhone 1Phone 245

ACPhone 1GND0.4
DCPhone 1GND0.1
ACPhone 1Neutral0
DCPhone 1Neutral47.8
ACPhone 1Phase77
DCPhone 1Phase47.8

ACPhone 2GND0
DCPhone 2GND0
ACPhone 2Neutral17.3
DCPhone 2Neutral2.8
ACPhone 2Phase129
DCPhone 2Phase2.8

48 volts is feelable, try touching a twisted pair to your tongue once (no, don't, was done on a dare in a dumber, younger time) and you really feel it. — Fiasco Labs, Aug 10 '13 at 19:35

score 3 · Answer 1 · answered Jul 28 '13 at 00:11

Many people can feel the 48V on a phone line pair. The big issue with 48V is that it is relatively safe and should not cause electrocution.

There should be no direct electrical connection between the phone line and the power line. If there is you have a serious safety issue to get cleared up right away.

The phone line itself operates as a current loop from one wire to the other. If one side of the phone line or the other is connected to GND or some other wiring circuit it will unbalance the phone pair and make the phone not work correctly.

score 3 · Accepted Answer · answered Jul 29 '13 at 14:17

Telephone line voltage is nominally 48V DC (varies though, depending on your distance to the CO and what voltage they are set at), and ring voltage is nominally 90V AC at 20 Hz (as opposed to 60Hz mains power). You should be able to test these between the tip and ring (red/green, or center pair) wires on your phone line. When not ringing, you should measure near 0V AC.

It seems like between 50V DC is about the threshold of when people can feel DC power: your line may be higher or you may be more sensitive, or have had sweaty hands at the time, etc. While phone lines are normally safe, the ring voltage will DEFINITELY get your attention (as I discovered as an early teen just starting to mess with electronics). I generally won't touch live phone wires without disconnecting at the demarc anymore.

I edited the question to contain my results, are they within the normally expected for telephone lines? — Jader Dias, Aug 10 '13 at 17:49

How can I be sure there's a connection between the telephone line and the power line? (Answer: they should not be connected.)

2 Answers2