"Ron Hardin" wrote in message
...
David Ballinger wrote:
"Ron Hardin" wrote in message
..
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.
The answer to the unsolvable hum problem is "The Balanced Pair".
the hum exists on both sides of the line as long as the resistance each
side
of the line is the same and the capacitance to ground equal; the hum
cancels
itself out between the two sides of the line. If you listen from one side
or
the other to ground they both hum. The signal you want to hear is between
the two sides of the pair. The only place that the circuit is actually
connected to ground is in the central office at the positive side of the
battery 48 volts or in these modern times,out in your neighborhood Demux
point. That little box on the back of the house or office that is
grounded
only has a spark gap and a fuse in it for your protection, sometimes they
fail and cause the imbalance on the pair, usually the first thing the
telephone repairman checks after they hear the hum. You wouldn't believe
the
stray voltages that can be induced on a pair of wires that run parallel
to
HT power lines, both static and dynamic, real current flows!
Anyway, that is why telephone lines Tip & Ring are balanced instead of
unbalanced, if they could have done it on one wire $$$ they would have,
half the copper.
Now Fiber-Optics, that is another story.
Hope that helps,
Dave___________
Thanks. That's fairly scary, that my computer modem is at the end of a 5
mile
wire grounded only at the far end. Quite an antenna, and its own chassis
ground
rignt nearby.
I'd assumed they'd have to be grounded at my end too, hence the ground
loop.
--
Ron Hardin
Yes it is, now if you think about it, out on those poles going down the
road, the power wires are on top -- usually a ground wire with the hot wire
and or three phase underneath, then a little closer to earth the telephone
then the cable TV and if you live in town maybe traffic signal wires or
intersection video wiring. If lightening does hit the top wire, it is
usually a large spike you get at your house on the power side -- but your
modem line does make a nice return to ground. I believe in surge
protectors! Now, if you do get a close strike, figure where you are in the
voltage divider circuit.
Really close to home, nothing survives, very close to nuclear, distance is
the key to equipment survival. Best bet unplug until after the storm
passes, Bet #2 Insurance, Bet #3 save the guarantee that comes with a really
good surge protector, the one with the $250,000 policy -- read the fine
print & save the receipts for everything, back up your data, at least the
important stuff, live a good life and pray. On the other hand, you may be
ready for a new computer anyway, bigger, better, faster, cheaper,,, well
they don't get cheaper, but you do get a lot more features and speed.
You don't know what you'll miss, till it's gone - - - -
Dave____________