Phone Line Voltage?

Discussion in 'Electrical Forum discussion & Blog' started by molo, Aug 19, 2007.

  1. molo

    molo New Member

    Messages:
    840
    Location:
    cold new york
    I would like to know how much voltage is flowing through phone lines? I had the cover off of a phone jack, and was told it had alot of volts running through it. Does anyone know how much voltage they have?

    TIA,
    Molo
  2. jimbo

    jimbo Plumber

    Messages:
    8,997
    Location:
    San Diego
    Historically, the telephone battery voltage has been around 48 volts. Yes, they run the phones on batteries, which is why they ALWAYS work. Your home cordless phone or answering machine of course does not work without power, but any POT (plain old telephone) will always work.

    More modern switchboards can be as low as 37 volts.

    The ringing voltage can be anywhere between 40 and 90 volts.

    In all cases, because of the amount of line resistance, the amount of current you could get through your body is not enough to hurt you, although if you put you fingers across the line just as it rang, you could get a tingle.
  3. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,804
    Location:
    New England
    Ring voltage can really hurt...from personal experience! The static DC voltage you probably woundn't notice unless the conditions were just right, but watch out for it when it is ringing!
  4. molo

    molo New Member

    Messages:
    840
    Location:
    cold new york
    am i missing something?

    The guy found 55 volts DC on my phone jack with his volt meter. Am I overreacting or does it seem strange that this much voltage is allowed to run through a box that is often very flimsily mounted on walls. I mean look at the strict code related to residential 120 wiring and the boxes. Phone lines are often housed in boxes that are attached to the wall with sticky tape. Am I missing something here?

    TIA,
    Molo
  5. CHH

    CHH New Member

    Messages:
    225
    Location:
    Denver, CO
    Don't know if you're missing something or not but teleco voltage is not a safety issue.

    Voltage in and of itself is not an indication of safety once the system is above a given minimum level.

    Voltage is just the driving potential for current to flow through a circuit. If a large current is unavailable, no harm can be done by the system. The descriptive equation is V=I*R so if R is big, I can't be big no matter how big V gets (for practical values of V). V is voltage, I is current , R is resistance

    Either the source of the voltage or the wiring system can restrict the available current. In the teleco case, the wiring system resistance ensures that large currents won't develop. A Van Der Graaff generator is an example of a source that can't develop much current even though it develops very large potentials (>50,000 volts). A VDG generator can produce artificial lightning but it is safe to have the spark discharge into your body because there is very little real current available.
  6. snafflekid

    snafflekid Electrical Engineer

    Messages:
    45
    The 48 Volts will cause a tingle as well! I found out while working on a line inside an attic, hot and sweaty.
  7. jimbo

    jimbo Plumber

    Messages:
    8,997
    Location:
    San Diego
    The voltage at your house comes from a battery which is anywhere from a few thousand to 20,000 feet away at the central office. Due to several hundred ohms of resistance, it will not drive enough amps through even a dead short to hurt you, and your body resistance is usually quite high.

    As a comparison, in a dry winter , you may have experienced a spark from your finger when touching a doorknob,, etc. The charge on your body generally exceeds 20,000 volts, hence the arc, but resistance limits the current to an amount which is not harmful.
  8. Mr_Pike

    Mr_Pike New Member

    Messages:
    136
    Location:
    Nebraska

    Jimbo is right on with his #'s.

    And yes, you can get a shock if the phone rings while you have your fingers on the wires. I can't recall the exact sequnce of events that lead to me finding that out personally, but I do recall being upset with the person calling.
  9. molo

    molo New Member

    Messages:
    840
    Location:
    cold new york
    Any ideas on how bad the "worst shock" from a phone line would hurt a little kid?
  10. cwhyu2

    cwhyu2 Consultant

    Messages:
    1,331
    Location:
    Cincinnati OH
    It might teach him not to touch again.
  11. jimbo

    jimbo Plumber

    Messages:
    8,997
    Location:
    San Diego
    How would a kid have access to any live phone wires?? If he does, that is a the issue to deal with. If he has a tendency to stick paper clips into things like phone jacks and electrical outlets, the parents need to figure that out, or else he will be a candidate for a Darwin award ( given to people who do such stupid things that they are removed from the human gene pool)_
  12. CHH

    CHH New Member

    Messages:
    225
    Location:
    Denver, CO
    Probably not as bad as an electric fence. I've never gotten the ringer shock from a phone but I do remember a strong "nip" from a fence charger 40 years ago...
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