Magnetic Shielding for Motor Control Center

[Aaron William]

Hey guys and gals,

I'm not an electrical-anything so please forgive my lay-person speak.

I work in a lab. One of our rooms has a high magnetic field. It is causing interference with some analytical and computer equipment. Of course there is some health concern as well...

Half of this room shares a wall (8-in concrete) with a Motor Control Center (Square D model 4) for our fume hoods, air handlers, compressors, what-not. It is clearly the source. I think the magnetic field is emanating from the solenoid relays...

We use a cheapo EM meter (Trifield 100XE) to determine that the magnetic field is 100+ mGau at the wall and drops to @0 when I back up about 5 feet away from the wall. The field is only present on the half of the room that shares a wall with the MCC.

Even if the meter isn't all that accurate it is clear that we can't put analytical equipment or people in that space (health effect studies are inconclusive - the worst kind for a management-type like me!). Cordoning that area and using it for storage would not be our first choice.

Is there any way to shield against this in a cost effective way? Could the excessive field be caused by improper grounding or is it a necessary evil for MCC operation?

 

[vk6kro]

The magnetic field of the Earth where I live is given in Wolfram Alpha as 583 mGauss, so 100 mGauss would not be detectable.

If you are worried, you should get this evaluated locally, but it is unlikely that a few relays would generate much stray magnetic field. They are designed to use power efficiently and not leak magnetic field.

The effect of any magnetic field on humans is still being studied and any health warnings would probably involve much stronger fields than this. Nobody can say it isn't harmful, but it seems unlikely.

However working in a place where relays are continually clicking and humming might be very unpleasant and possibly harmful psychologically, so maybe you could check this.

Shielding can be done with iron sheet material, so a simple cabinet may be enough.

 

[Aaron William]

The 100 mGau is above background. Also it is alternating at (I guess) 60 Hz. It is the alternating part that has people concerned that it is a health hazard. Or if it is the solenoid relays I guess it would be direct. I don't know enough about this thing.

But health hazard aside...

It is clear that the source of the magnetic field is the Motor Control Center. It isn't just a few relays. It is a bank of 20-30 for high amperage devices. If iron shielding were enough I would think that the cabinet enclosure for the MCC would stop the field. Could I just put a big piece of sheet iron on the wall?

 

[vk6kro]

Have a look at the following table:

http://en.wikipedia.org/wiki/Orders_of_magnitude_(magnetic_field)

This gives the strength of various magnetic field sources.

It is given in Teslas so you need to know that 100 mGauss = 0.00001Tesla or 10 μT.

The suggested limit for pacemakers is 500 μT.

A Neodymium magnet has a field strength of 1250000 μT.

I wouldn't presume to give you safety advice, but I think the field you are talking about is miniscule and not worth worrying about from a health viewpoint.Could I just put a big piece of sheet iron on the wall?
Yes. It might help.

 

[vk6kro]

Interference with computer equipment is likely to be caused by electrical pulses traveling on the power lines.

If this is the case, you may be able to get suitable filtering installed.

 

[jim hardy]

hmmm..

i have successfully shielded loudspeakers using a tin can around the magnet. but that's DC

try a piece of , say, cheap corrugated iron roofing, or flashing. i'll wager it will attenuate your field.

probably the field emanates from the large conductors carrying substantial current, where they run separately inside the MCC. you might concentrate yiou shielding efforts between them and the room of interest.

if you get curious, make a coil of wire with say ten turns on 1/pi meter^2 and see what fraction of a volt you pick up before and after shielding.

old jim