[Ron Hardin]

My telephone line developed an AC hum, which, thinking about it, left
me curious how ground loops are avoided.

If you drive two ground stakes into the ground about 20 feet apart,
you can measure about a quarter volt of AC between them. There's
AC floating around everywhere, and no two pieces of ground are the
same potential.

The telephone company would then seem to have an unsolveable AC hum
problem, and I wondered how they got around it.

They do ground the phone line boxes every couple hundred feet or so,
though I don't know if that affects the twisted pair directly; but
certainly they have the trouble at the miles-distant termination of
the line and your house, as to what potential the ground is at the
two places.

It can't be just isolation transformers, because they need the DC
circuit to detect off-hook, not to mention powering the phone on
your desk, if it's got any IC's.
--
Ron Hardin


On the internet, nobody knows you're a jerk.