[SI-LIST] Re: A general question of -48V power supply

Yes, I tracked the -48V "Office Battery" issue down time ago.  It goes back

to the turn of the last century in New York City.  Telephone cables were
already being placed underground.  The insulation was paper (called pulp
cables).  The rats attacked the cables so a lead wrap was added.

The cables occupied space in the streets with Edison's DC lighting systems
and the electric trolleys using 600VDC. A problem was encountered with the
lead sheath electroplating from the cables to the trolley rails. The fix
was to reverse the polarity on the telephone office battery. This was an
early form of what is called galvanic protection today.  I don't know if
Edison had a hand in the fix but it sounds like something he would do.

Fred Townsend



ÒË·« wrote:

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> Is there anybody know why using -48V instead of +48V  power supply on =
> backplane in telecommunication field? Except traditional style, any =
> other sepcial consideration?
> =20
>
> Regards
>
> Beck =20
>
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