Grounding noise??

[Bill Peters]

What would cause classroom ceiling projectors to have a while faint line go up and down the screen? We have several locations with this problem different parts of the county. Each projecter is on a dedicated circuit. Each panelboard has been checked for grounding issues etc. We have turned off all circuit breakers in the panel except for the projector feed. No help. The projector mounts to a metal square pan. The square pan is mounted to the metal ceiling grid. A metal box is secured to the metal pan with MC cable. The only thing that works is to remove the outlet from the metal box and disconnect the ground. The white line will go away. We also tried isolated grounding recpts. Any ideas?

 

[brian john]

Number one culprit a neutral conductor grounded downstream of the main, be it a service or SDS.

Do a visual inspection of all distribution equipment looking for obvious improper grounding, equipment grounds on the grounded conductor/neutrals termination bar or grounded conductors on the equipment grounding termination bar.

Then on to the grounding electrode conductor and measure for current there should be none.

You can go to panels and complete zero sequence measurements the measurement should be "0" amps.

You can also do zero sequence on each branch circuit (often hard to do) find a circuit (all conductors) with current and you have the culprit.

Arrange for an outage and megger feeders and branch circuit for a neutral short, clear the shorts clear the problem.

 

[LarryFine]

I believe you're seeing typical ground-loop issues. Opening the EGC on the projector shows that. The video-cabling shielding is paralleled with the EGC, but very indirectly.

I'd start by temporarily powering the projector from the same receptacle as the equipment feeding it. If that helps, hard-wire the projector receptacle from the same outlet.

If not, you'll need to find a 'ground-breaker' for the video feed, what is usually a 1:1 transformer that has no DC connection between the input and the output terminals.

I wouldn't be surprised if the equipment has a cable-TV feed. If it does, see if disconnecting the cable also corrects the noise bars. Cable groundbreakers are easy to find.

 

[dbuckley]

Larry covered it pretty comprehensively, I'd just like to note:

Each panelboard has been checked for grounding issues etc.

Whereas from the perspective of an electrician and the NEC the grounds might be fine, it does not mean that the ground configuration is acceptable for AV.

The first red flag is "Each panelboard" which strongly suggests that multiple panelboards are involved, and if that is the case, and if you have AV equipment powered from two separate panelboards, the chances of a quiet ground are dramatically reduced.