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    why -48volt dc is used in telecommunications?

    why -48volt dc is used in telecommunication?

    •   Alt4th July 2016, 17:37

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    Re: why -48volt dc is used in telecommunications?

    Are you referring to telephone lines on land? In the US the live wire has a negative voltage, normally between -40 and -60 V. The reason is so that telephones can work during a blackout. The power comes from batteries at the telephone center. The high voltage can work for a long distance, because current is small.

    That's how it was years ago anyway. It's hard to be sure anymore, with fiber optics, cell towers, etc.

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    Re: why -48volt dc is used in telecommunications?

    Quote Originally Posted by BradtheRad View Post
    The reason is so that telephones can work during a blackout. The power comes from batteries at the telephone center. The high voltage can work for a long distance, because current is small..
    I still do not understand - why the chosen voltage is negative? What is wrong with a positive voltage?

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    Re: why -48volt dc is used in telecommunications?

    A little googling would answer this.

    http://electronics.stackexchange.com...stems-use-48-v
    http://archive.digivation.net/2006/0...ight-volts-dc/

    Basically to prevent corrosion. :)
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    •   Alt4th July 2016, 20:03

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    Re: why -48volt dc is used in telecommunications?

    -48V was chosen to prevent corrosion and is the highest voltage that can be used that will not pass a fatal level of current though the human heart.

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    Re: why -48volt dc is used in telecommunications?

    Quote Originally Posted by pjmelect View Post
    -48V was chosen to prevent corrosion and is the highest voltage that can be used that will not pass a fatal level of current though the human heart.
    This is the right answer.
    With a negative voltage {with respect to ground) any electrolytic action (ion migration) will be from the surroundings to the wire.

    In other words any resultant electroplating from leakage current will eat away the surroundings and deposit it on the wire, not eat away the wire.

    This is important where most of your wiring is either outside fully exposed to the weather, or buried in often very wet ground, and which may be expected to last for many tens of years without failure.
    Cheers, Tony.

    •   Alt5th July 2016, 00:56

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    Re: why -48volt dc is used in telecommunications?

    Quote Originally Posted by Warpspeed View Post
    This is the right answer.
    With a negative voltage {with respect to ground) any electrolytic action (ion migration) will be from the surroundings to the wire.
    Thanks; it sounds convincing. But telephone lines never use earth as the return path in any circuit. All telephone connections are minimum two wire devices and they will work even if you lift them from the ground. And this is true even at the exchange level, I believe.

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    Re: why -48volt dc is used in telecommunications?

    No they never do use a return path through the earth, it would be far too noisy.
    There is always a return wire associated with every signal wire.
    But one side of the system (positive side) is always at or very near ground potential.

    You simply cannot float all the buried communications wiring of hundreds of thousands of telephones in an entire city.
    Think about it....

    There will always be "some" electrical leakage down to ground practically everywhere.

    Its rather like the mains power distribution system, where the neutral wire is supposed to be at or near ground potential, and is actually deliberately grounded at many points around the entire system.
    Cheers, Tony.

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    Re: why -48volt dc is used in telecommunications?

    Quote Originally Posted by Warpspeed View Post
    But one side of the system (positive side) is always at or very near ground potential.
    True; but then the corrosion argument falls flat; one wire will always be vulnerable to corrosion. Electrolysis will take place between the two wires and one of them will get eaten up, sooner or later.

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    Re: why -48volt dc is used in telecommunications?

    Yes that would certainly be true, if there is direct leakage between both wires.
    But in that case, your fully floating system would be no better off.

    Most dc systems use -ve as ground, its become the convention.

    But 50 volt dc based telecoms systems have always used positive ground because of the known corrosion problem.
    As an old telecoms trained technician, that is what we were taught.
    I can assure you that is the reason.
    Cheers, Tony.

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