RF interference to monitors

[bdarnell]

I have a customer in a new building we just constructed. In a room there are 6 PC workstations, three down one wall, three down another. Directly behind one of the walls is the warehouse wall against which sits the 3000 amp main switchgear. All of the screens on these 3 PC's are experiencing waving and RF interference. Yes, there is a dry-type transformer, but we have shut it off and the problem remains. If we change out to LCD monitors, the problem goes away. So it's apparent that the problem is airborne. The monitors work just fine elsewhere in the building. The switchgear isn't anything out of the ordinary - just breakers and wires. Any thoughts ?

 

[catchtwentytwo]

Re: RF interference to monitors

The problem is common near switchgear and doesn't have to be caused by transformers. There are strong fields caused by the flow of current in conductors and/or bus bars.

The LCD monitors will be the most cost-effective way to solve this, you'd spend a fortune to mitigate it any other way.

 

[petersonra]

Re: RF interference to monitors

Ditto what he said.

 

[rcwilson]

Re: RF interference to monitors

The 3000 amp switchgear bus and cable magnetic fields are causing the problem.

You can try curing it using a magnetic shield of 1/4" steel plates over the wall. Make sure there are no holes or seams in the plate like ones for receptacles, ducts, switchplates, windows or doors. If you have to use more than one plate, fully weld the seams, don't tack weld it. The magnetic field gets concentrated at the plate's edges and holes, so run the plate a few feet past the monitors in each direction. Ground or bond the plate to buiding steel.

Depending on the bus configuration, eddy currents induced in the plate may cause losses and heating that cook the wallpaper adhesive and/or paint used to make the plate look less ugly.

Or you can build a magnetic shield box of 16 gage steel for each of the monitors.

The above solutions were actually tried by our field crews on a couple of my projects. The suggestions are based on problems they encountered. The solutions were not pretty or cheap, but they eventually worked...kind of, sort of..

I agree - Use LCD monitors.

 

[karl riley]

Re: RF interference to monitors

There are two kinds of magnetic fields causing the monitor jittering (this is not RF). One is from Tfrmers and busbars operating normally, and this can be shielded against as described.

But the second kind is called net current from circuits, including those in busbars, which are not balanced due to the shunting of some of the neutral to grounding paths in the building, or to other neutral circuits. One common cause is the grounding of a neutral bus in a subpanel. Net current cannot be shielded against using shielding materials: no effect. So before spending a lot of money on shielding you have to determine how much if any is net current.

If you have a flexible probe clamp-on ammeter (like AEMC models) you can clamp around all the busbars. Should read zero; if not, you are measuring the net current component. Fix problem.

Absent a flex probe, clamp on each phase and neutral and calculate.

This is a bare bones explanation; there can be many variations of neutral shunting from many sources.

Karl

 

[EEPeder]

Re: RF interference to monitors

I agree that this problem is often an indication of a net current problem causing EMI with the CRT, and that this should be checked for since it is a serious problem for other reasons. However, given the proximity to the panel this is probably just the natural result of bus bar phase spacing. I have not had consistent results with attempting mathematical addition of phase and neutral currents where there is a wiring problem -- you have to make an angle assumption and that is quite possibly off in this situation. You can probably find someone (salesman or friendly contractor)to borrow the wraparound CT from as a professional courtesy.

 

[karl riley]

Re: RF interference to monitors

If you don't have the flex probe you can also determine net current with a gaussmeter (if you have one). Net current magnetic field weakens directly with distance. Regular balanced current magnetic field from busbars weakens with the square of the distance from the source.

So you take two measurements, say 4 feet from source and 8 feet from source. If the field weakens to 1/4 then you do not have net current. If it is just cut in half, it is pure net current. If it is in between you have both.

Karl