Sean Breheny wrote:
> Electrical distribution substations often have (wired) telephones which u=
se
> super high-voltage isolation transformers between the phone and the phone
> line. This is to protect the user of the phone in the event that there is
> some kind of fault at the substation which produces a large difference
> between the local ground where the user is standing and the phone line. T=
his
> could be either contact between a power line and the phone line OR it cou=
ld
> be a large ground potential difference between the ground reference of th=
e
> phone line and the local ground where the user is standing. The transform=
er
> (at least the one I saw) is about the size of a large desktop PC so it
> dwarfs the phone itself!

It would have to be more than just a transformer, as POTS uses DC for both
power and signaling, in addition to the AC voice and ringing. It would also
need a relay or two, and possibly a local battery charged by some sort of
high-isolation DC-DC converter.

-- Dave Tweed
--=20
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
.