Ground Loop
Bob Simon wrote:
I recently moved and connected my new Cox Contour DVR to an audio receiver =
and TV. There was a pretty bad hum so I inserted an isolation transformer =
between the cable feed and the DVR, which eliminated the hum. I measured t=
he voltage between the outside of the cable connector and safety ground wit=
h a cheap VOM and found 200 mVAC. =20
This is pretty good.
Does this voltage indicate that the cable is improperly grounded?
No, it indicates that it _is_ properly grounded.
If the cable were to be bonded to the house neutral with 3/0AWG, would this=
likely eliminate the need for the isolation transformer?
No, because you still have a loop. You have 30 feet of ground wire between
the cable entrance and the panel, then you have 100 feet of ground wire
between the panel and the TV set... and you have 130 feet of loop antenna
picking up hum. You can make it the largest diameter wire possible and it
won't do a damn thing because it's still just as long.
The only thing that matters is the diameter of the loop. The solution is
to break the loop.
Whether it would eliminate the hum or not, should I have it done?
No, that would make an even worse ground loop.
The rule is: everything should have one and only one ground path to everything
else. Never zero, never two. Adding more ground connections makes ground loop
problems worse.
Does the cable company have any obligation to do it in order to comply with=
regulations?
No. They provide a connection which is in compliance with the FCC
Report and Order. It's your job to connect up to that properly, and
often that involves a transformer.
Two back-to-back 75-300 ohm transformers make a perfectly good isolation
transformer, or you can buy one for a couple bucks from the cable TV
company usually.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."
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