telephone wires

Discussion in 'General Electronics Chat' started by milee, Oct 30, 2007.

  1. milee

    Thread Starter Member

    Sep 20, 2007
    15
    0
    I was repairing the telephone lines and came to know that there is no electric flow while workin but as soon as i touched the wire with my mouth i got a kind of sudden shock....
    therefore i would like to know what exactly had happened when i dealed with the telephone wires...
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Phone lines still carry 48 VDC. This is a legacy from the old days. They also carry up around 100 volts AC when ringing. Be careful.
     
  3. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    As an apprentice, I was once leaning against a 66 block in a high-rise. Somebody's phone rang. I've not leaned against a 66 block since.

    Never assume that "low volts" equals "no volts." Never assume that "low volts" equals "low danger" either. Some of my Union brothers have died because of that mistake. 48 volts may not sound like a lot, but when a screwdriver falls across a pair of 48 volt busses with 120 Amps able to flow, the screwdriver literally explodes. It's not cool, and it's not fun.

    I suggest the human mouth is not a good place for conductors.
     
  4. JohnBoy

    New Member

    Oct 30, 2007
    7
    0
    If you measure each wire to earth ground, one will measure nearly ground potential (tip) and the other will measure approximately -48VDC (ring). When you go "off hook", you complete the circuit through the telephone back to your telco. The telco's switch "sees" the current and returns dial tone to you. Ringing voltage is an interupted 20Hz voltage which can be nearly 100VDC ring to tip. It's less if you're a considerable distance from your telco. You're not likely to get a fatal shock from telco battery. However, considering the possibility of an high induced voltage on the pair it is never a good idea to put the wires in your mouth or otherwise provide a current path with your body.

    John
     
  5. milee

    Thread Starter Member

    Sep 20, 2007
    15
    0
    it was nice knowing the important facts...thnks to all of u!!!!
     
  6. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
    10
    Happens the same with 12V. Never touch the wires with your mouth. This is the worse way to get a shock, because the inside of your mouth is pretty conductive and there is the possibility of electricity flowing through the brain.
     
  7. h.d

    Active Member

    Oct 22, 2007
    150
    0
    i know that the voltage of telephone lines is 12-24V but i dont know thats when calling it will be 100 V
    thanks for that facts....
     
  8. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
    10
    In my country, phone line voltage is 50VDC, and 20VDC when calling. Tested it myself.
     
  9. studiot

    AAC Fanatic!

    Nov 9, 2007
    5,005
    513
    The international standard is -48 Volts, relative to the second wire which is 'grounded' at the exchange.
    You can often find slightly more than this (around the 50 mark) in cities, near to exchanges.
    Conversely, especially in large countries like the USA, it can drop considerably below this, where the test point is remote from an exchange or repeater.

    If you make measurements relative to local ground (earth) you can measure quite different results.
     
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