Lighting Power vs Video Hum - Isolation

[[Director] drew ryan]

I ran this past our lighting guy/resident theatrical electrician. Here are his comments.
OK, here's what my brain kicked out.

Hum and buzz and all those nasty irregularities in audio and video are caused by voltage on the ground. This is particularly notable when the voltage is modulated, either as the generic 60Hz buzz of main AC or a more complex buzz of this and various harmonics generated by modern dimmers. The root is still the same: inductive current (proximity buzz) or crossed neutral and ground.

If I had to take a guess, somewhere in the installation of the new dimmer pack, someone crossed neutral and ground - perhaps many times. It still works fine, but the result is a very dirty current getting dumped on the ground. This shows up as the video ground-loop roll.

Trouble-shooting steps:
1. Ensure the new pack does not bond neutral and ground.
NEC requires this only happen at the main service panel.

2. Check the new branch circuits from the pack out.
Make sure no wires are swapped - hots are always hots, neutrals are consistently neutrals, grounds are always grounds.

3. Check the connectors on the PAR cans.
The can doesn't care which conductor is hot or neutral, but neither one of those can be grounded.

4. Check the lamp leads inside the PAR can to ensure the insulation is intact.
This will also reduce the potential for shocks while focusing the cans.

If this does not resolve the problem, try it again. If after two attempts it does not work, consider installing an isolated grounding system for video and audio. I doubt this will be necessary.

Hope that helps.
Yours,
C.Perkins

[prochlea]

ooh - drew's got a point. I bet someone crossed the neutral and ground. Pose this to your lighting contractor - they should be obligated to at least check it. They may think you're wrong, but insist that they check it...

[bobbyv4HIM]

We just completed our new church building with 3 phase power. We kept motors off lighting and electronics transformers. We still get a hum in monitors with the dimmers at less than 100%. We are still working with that item.

[D.R.HADDOCK]

Originally Posted by bobbyv4HIM

We just completed our new church building with 3 phase power. We kept motors off lighting and electronics transformers. We still get a hum in monitors with the dimmers at less than 100%. We are still working with that item.

do you have any lighting equipment on transformers ? what type of transformers? what are electronic transformers? this could cause problems! I am not a great fan of isolation transformers,they might be useful in isolating what equipment is causing problems, but they only mask a problem rather than fixing it. if you can not solve the problems ask the local electricity authority to check your buildings earth and nuetral system. if everything is correctly earthed, noise problems should not happen, unfortunitly we use a lot of unearthed, double insulated euipment now days and this is causing a lot of noise problems. it will be interesting to see what finally fixes your problems, i hope you will let us know.

[tohmit]

Thanks Drew! That information really makes sense. I will go through and check all those things again. I do know the dimmer is properly wired, as I did that myself and double checked it multiple times. I also went through all of the circuits run off the dimmers before the electrical contractor left, and those are all wired right....but I did not think of the lighting fixtures. I bet it is something in there! Some of these lights are very old.

Thanks again! I'll try and follow up once I find the fix.

[Steve Vanciel]

Originally Posted by tohmit

..but I did not think of the lighting fixtures.

Check the duplex outlets as well. It is all too common that wires get crossed on these, sometimes resulting in worse conditions than a hum. And it only takes one miswired outlet to mess up the whole line.

SteveV
Orlando, FL