Re: Why -48V for telephone lines, and not positive?

From: Peter O. Brackett (none_at_no-such-domain.nul)
Date: 12/07/04 

Date: Tue, 07 Dec 2004 02:40:29 GMT

All:

Cathodic protection has been used to protect metallic conductors in the
environment for well over a hundred years!

Negative voltage applied to exposed metals provides "cathodic protection"
for the metal conductors.

If conductors are positively charged and emersed in a solution (the ground
surrounding them) then metallic ion migration will cause complete erosion
and the total decay and breakdown of the metal in the conductors.

If kept at a postitive potential of 48 volts with respect to ground, number
AWG 24 copper conductors would completely erode due to ionic migration and
dissappear within about a month. That is why telco plant always is supplied
with -48VDC with respect to ground, and never with a positive potential.

This is also why ships at sea are equipped with sacrificial zinc anodes
which, due to the postion of zinc on the electrochemical potental scale with
respect to salt water, preferentially corrode thus protecting a ship's metal
hull from erosion.

Such sacrificial zinc anodes are a passive form of cathodic protection,
whilst applying a negative voltage to exposed metals is an active form of
protection.

All Navies and most merchant marine vessels use such active cathodic
protection systems to prevent metal hull erosion and the telephone company
copper plant applies -48 volts with respect to the environment to their
copper cables for the same reason. 

--
Pete
Professional Consultant
Indialantic By-the-Sea, FL
"Ken" <___ken3@telia.com> wrote in message
news:qbr3r0t9vr838gf20t2l3p5j0j5rhsrcjl@4ax.com...
> On 1 Dec 2004 07:48:48 -0800, shoppa@trailing-edge.com (Tim Shoppa)
> wrote:
>
> >>>> when this stuff was invented
> >>>
> >>> Yup.  Stroger Switches were invented in 1884--by an undertaker.
> >>
> >>  Do you think they used -48V back then?
> >
> > It was less standardized.  Short loops were as little as -12V, long
> > loops could be more than -48V.  It depended on who was selling the
> > switch and equipment too.
> >
> > Large parts of the civilized world use -24V as the default
> > on telco loops.
>
> In Sweden -48V for POTS, and -96V for ISDN.
>

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