3PH 120V circuits.
Shared Neutral (3H, 1N, 1G)
The Ground was shared between 3 sets of wires (all in one conduit)
All boxes were grounded through the EMT and with ground screws.
If you are feeding 3 different circuits of receptacles and are using a shared neutral, sounds like a "classic" harmonics problem. All of the new equipment use switching power supplies. The non linear current spikes are NOT canceling on the shared neutral. That explains the voltage difference between the neutral and ground at the receptacles.
The fix for that is to run dedicated neutrals for each circuit.
The hum bars come from AC current running on the video cable shields. Dedicated neutrals might fix that. Verify that the source for the video and audio feeds comes from the SAME subpanel that feeds the TVs. If there is a small difference in voltage between the grounds at the TVs and the Video source, current will flow on the video shields and give you the bars. Also verify the Audio and Video cables are not laying on top of the lighting fixtures or tied to conduits carrying power.
Larry C
