[rescue] Anybody have knowledge of Telco 48V wiring???

James Fogg James at jdfogg.com
Mon Apr 14 12:25:05 CDT 2008

> James Fogg wrote:
> > The reason we have -48VDC power systems is because telco facilities
> > it. They use it because it's a standard for "wet" (battery) power.
> > reason it's negative is to prevent corrosion on the lines.
> >
> I have wondered about that for a while.  Care to elaborate on that?
> do you have a link that explains it?
>     -Dan Sikorski

I really don't have a good explanation, it's been about 20 years since
I've studied that part of the phone system. Now it's just in my memory
as "this is how it is".

I know most underground metal storage tanks have a negative potential
applied to prevent corrosion too.

There's another concept in telco lines called "sealing current". It's an
overlayed 10ma DC current that is used to keep splices and junctions
from building an oxide film. It's usually a -24VDC signal that "rides"
over the other signals. If you measured the voltage on a phone line with
reference to ground you'd find the -48VDC plus the sealing
voltage/current. The sealing current "injector" is current regulated, so
if oxides begin to build the injector will compensate to keep the
current steady.

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