View Full Version : RF shielding against plasma

11-17-2004, 03:10 PM
I rearranged "the furniture" in my machine shop lately. I moved a mill with Sony DRO approximately 15 to 20 feet closer to the welding room. Now, I notice whenever someone uses the plasma cutting outfit in the welding area, that it blows the AC line fuse into the Sony.

Also, a Woods VFD that is located about 10 feet from the welding room, will display a fault code spurred by the use of the plasma torch.

So, obviously the plasma is bad to have near electronic stuff. So I'm wondering how effective it would be to line the intervening wall of the welding room with steel "siding" to act as an RF shield? Will the interference just diffract around the panel, or go right on through it anyways?

I toured a cycltron once, and they had the control room walls lined with copper sheath for RF control. Does copper have special properties over steel paneling?

11-17-2004, 03:42 PM
HU. I suspect it might not be airborne but either something has changed regarding grounding or there is alot of noise going back into the line from the plasma, if you use the plasma on a fixed table, make sure it is well grounded and you could also sink a ground rod next to the table. An inline choke on the plasma ac supply could be an option also.
Usually, air borne interference does not blow fuses in adjacent equipment.

11-17-2004, 08:55 PM
Thanks, Al. I'll look into the choke.

One reason I thought the problem was airborne, was because the Sony DRO's power supply comes from a 110v tap off of a 550 volt transformer in the mill, plus, the 550 volts is itself, coming from a seperate 220 to 550 volt transformer. So its quite isolated.
The blown fuse is only a 500 milliamp, and it blew twice in short order.

The plasma is just a hand held unit, so the ground clamp gets placed in various places.

Alan T.
11-18-2004, 08:51 AM
If it does turn out to be a RF problem here might be a cheaper solution for you. My RF room for the MRI is actually quite simple. 24ga galvanized steel sheet pressed on both sides of 3/4 mdf. Works quite well, just make sure and cover both sides of the seams with 18-20 ga.

Alan T.

11-18-2004, 10:18 AM
Thanks Alan. Does the protective metal completely encase the entire room (I bet the floor isn't shielded :) ) around the machine or is it only one wall between it and your control room?

11-18-2004, 10:34 AM
Well, I suspect that it is noise entering the power line and not RF (airborne stuff). If you had a line monitor or oscilloscope you would probably notice some KV spikes of short duration. Was the mill being used at the time the fuse blew? Because it might also have been a brown out condition...kinda depends on the type of fuse and it's rating....anyway a line monitor or oscilloscope would help you pin it down.

11-18-2004, 10:36 AM
Also...for RF...copper is an excellent conductor....normally it isn't sheet copper, but copper screen (like window screen)....depends on the RF energy.