I recently moved and connected my new Cox Contour DVR to an audio receiver and TV. (A Blu-ray player is also connected to the receiver and TV; this may
or may not have any relevance to my issues.) 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 the voltage between the outside of the cable connector and safety ground with a cheap VOM and found 0.2VAC and 1-9 mVDC.
I have a number of questions that I hope someone can answer for me.

Is this voltage difference the cause of the hum?

It seems entirely possible and likely. It could easily result in 200
millivolts of AC ripple appearing on the audio signal. Since audio
these days is usually 2 volts or so, peak-to-peak, you'd end up with a
serious hum.

Common problem with cable installs, and the solution you've used is a
common one.

Was my measurement approach appropriate?

What I'd suggest doing, is disconnect the DVR from all of your other
A/V equipment, and reconnect it directly to the cable. Then, measure
the AC voltage between its A/V ground (e.g. the shell of the RCA
connectors for the audio-out) and the corresponding AC ground on
something else in your A/V setup.

I strongly suspect you'll get roughly the same voltage rating.


Do the measured voltages indicate that the cable is improperly 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?

DO NOT BOND IT TO NEUTRAL. Neutral, by definition, is a
current-carrying wire, and it can be pulled several volts away from
ground by voltage-drop in the wiring. The only place this isn't the
case is back at the service panel, where neutral and ground are bonded
together.

The only place you should bond grounds to, is other grounds (and
ideally do so at a single point).

You don't need 3/0 to comply with NEC - 14/0 or heavier is apparently
adequate.

Whether it would eliminate the hum or not, should I have it done?

There's no harm to doing so, it's a common solution, and since it
fixed your problem, Be Happy!

Grounding the coax at the point of entry (per NEC) would provide you a
bit of additional protection against something like a near-strike by
lightning onto the cable wiring.

Does the cable company have any obligation to do it in order to comply with regulations?

My recollection is that in most jurisdictions, cable-TV drops are
supposed to have their braids bonded to building ground at the point
of entry. A lot of installers skip this step, as there's often no
"good" ground at the point of entry, and they don't want to run a
heavy ground wire back to the panel and do a proper job of bonding at
both ends.

National Electric Code, article 820, seems to be the relevant one.
I don't have the full text here (my old salvaged copy of the NEC is at
home) but the summary I see on-line does indicate that this is
necessary:

http://ecmweb.com/code-basics/articl...bution-systems

Sidebar: 820 Tips

Determine point of entrance.

Ground the incoming cable as close as practicable to the point of
entrance.

If you run cables above a suspended ceiling, route and support them to
allow access via ceiling panel removal.

If you use a separate grounding electrode, bond it to the power
grounding system.

Use the correct cable type and raceway for application the general,
plenum, or riser.