[SI-LIST] Re: -48v

Santhosh;
This is what I had come across on one of the web resources when I was
searching to get an answer to the same question back when I was in the
college. I have no idea of where I had got this writeup from...would like to
acknowledge the source though.....
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The -48V voltage was selected because it was enough to get through
kilometers of thin telephone
wire and still low enough to be safe (electrical safety regulations in many
countries consider DC
voltages lower than 50V to be safe low voltage circuits). 48V voltage is
also easy to generate from
normal lead acid batteries (4 x 12V car battery in series). Batteries are
needed in telephone central
to make sure that it operates also when mains voltage is cut and they also
give very stable output
voltage which is needed for reliable operation of all the circuit in the
central office. Typically the CO
actually runs off of the battery chargers with the batteries in parallel
getting a floating charge.

The line feeding voltage was selected to be negative to make the
electrochemical reactions on the
wet telephone wiring to be less harmful. When the wires are at negative
potential compared to the
ground the metal ions go form the ground to the wire instead of the
situation where positive voltage
would cause metal from the wire to leave which causes quick corrosion.

Some countries use other voltages in typically 36V to 60V range. PBXes may
use as low as 24
Volts and can possibly use positive feeding voltage instead of the negative
one used in normal
telephone network. Positive voltage is more commonly used in many
electronics circuits, so it is
easier to generate and electrolysis in telecommunications wiring is not a
problem in typical
environment inside office buildings.

Some older offices employ battery reversal (swap DC feed to tip and ring) to
signal off-hook at the
remote end.
--------------------------------------------------------------------------------------------------

Regards,
Ravi

Santhosh E P wrote:

> hi all...
> is there any specific requirement of having -48v for telephone lines. if
> then whats the advantage? why we can't have other voltages to carry the
> signal, not even +ve?
> regards
> santhosh
>
>
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