Shielding Your Office from Magnetic Fields?

jonfromspace asks: "We just recently moved into our shiny new office, and low and behold, we have a problem. It seems that there are 3 50HP "Chillers" beneath our office, and whenever they fire up, an entire 1/4 of our office gets filled with this crazy magnetic feild! We tested this theory by placing a box and a monitor on a chair, and moving it around the office. As we entered our Bermuda Triangle, the tell tale signs of magnetic interference appeared on the monitor (Rainbows, Moirre patters, etc.) This field has caused 2 RAM Dimms, 1 HD, and 1 Mainboard to fail in less than a week. We currently have our entire staff on one side of the office, and we need to solve this quick! Does anyone know of a way to shield our office? The Electrician we asked suggested placing a large steel plate on the floor of our office and grounding it to the pipes. Is this a viable solution? Are there companyies that deal with this kind of problem?"

Comments

Better to shield at the source

[Tau Zero]

If you have any access to the chiller room at all, the best thing to do would be to shield the chiller compressors and any other motors in that room. A solid steel cage is good, mu metal is better (but very expensive). Shielding the problem right at the source means you will be covering a lot less area, and it will probably cost you a lot less money.

I can think of ways to do this with active cancellation, but it's not something that I've ever heard of being available off the shelf and I doubt you want to get into the magnetic flux abatement R&D business.
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Mesh won't do.

[Tau Zero]

I'm not sure... would a steel mesh also work for this?

No. What you are trying to do is short-circuit the magnetic fields emitted by the motors (which is mostly at 60 Hz). This requires a path of high magnetic permeability. The permeability of steel isn't that much higher than air, and a lot of flux will go right through a steel mesh. You absolutely need something like plate. At high frequencies you can use the image currents induced by the applied time-varying field to shield the field with only a mesh, but the image currents decay exponentially with time and the resistance of steel is too high to give good cancellation at such low frequencies with mesh.

New thought: make a hollow form which goes around the motor, out of plywood or whatever. Fill it with steel shot. This allows you to create a shield of almost arbitrary shape (how many ways can you glue thin plywood?) and it is certainly cheap.

(Don't bother modding me up unless you think this really rates a +3 for visibility, I'm above the karma cap. Post instead.)
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Re:Further clarifications.

[Tau Zero]

Not that a 60 Hz magnetic field will penetrate through that electronic route.

Isn't that what I said?
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Further clarifications.

[Tau Zero]

I hate to pick nits so much, but this is a very different class of problem from the one your techniques are suitable for.

The problem is that as soon as you have wires leading into and out of the shell, your shell becomes almost completely ineffective. You'd have great trouble avoiding this, as you need things like power cables entering the computer.

That's only for RF leakage. The problem in this case appears to be 60 Hz magnetic field interference (which causes monitors to shake at the difference between the line frequency and the scan rate, among other things). A penetrating power cable will do a good job of carrying 300 MHz RF interference, but it won't carry any 60 Hz magnetic fields at all because it's made of copper and not ferromagnetic.

There are a couple of effects that work in your favour for this. Firstly, the high frequency components of the noise won't diffract around *too* much.

They won't diffract at all, they are in the "near field" (wavelength at 60 Hz is five thousand kilometers!). Second, there probably aren't significant high-frequency components beyond 300 Hz or so (magnetic non-linearities create them from 60 Hz, but not all that much). What really works in your favor is that you can put something between you and the compressor motors which allows the magnetic fields to flow back without going any further into space. The emissions from a 2-pole motor drop off as 1/r^3, emissions from a 4-pole drop off at 1/r^4. If you put a ferromagnetic cage around the motor the induced field will set up opposite image fields and raise the exponent even higher. Get it to 1/r^6 and you probably won't have problems with monitors shaking any more. You'll still have trouble with switching transients from the compressors turning on and off, so use a surge protector, line filter, UPS or ferroresonant transformer to eat them.
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Mu Metal

[Xenu]

Mu metal is often used in electronic test equipment to shield components from magnetic fields.

Re:UPS, power factor correction hardware

[tzanger]

Next, try putting "power factor correctors" and/or "soft starters" on the power lines to the chiller motors (I think that's what they're called, anyway). The basic idea is that they help correct the EMF problem at the source (and can sometimes save electricity bills too!).

I design soft starters and inverters for a living; the former won't help you here unless the magnetic field is only occurring when the chillers/compressors start, and not while they run. You're absolutely correct about the saving of electricity bills, though. The inrush currents caused by any decently sized squirrel-cage motor are incredible (we're talking 1200-1400A on your typical 250HP) which causes all kinds of bad things, but mostly just problems with the mechanical couplings and mechanical stress on the compressors and motors themselves. The electrical stress is easily overcome with oversized conductors and switchgear. That still doesn't mean the electricity provider will like you. (low power factor, high surge currents, etc., these guys make a living off charging you premiums for this stuff)

Remember, most conductors are run through conduit which is then grounded. I don't think any of this grounded steel plating is going to get you anywhere unless you're missing it already.

I'm anxious to find the solution to your problem -- I can run a computer beside a loaded 300HP dyno without any flicker at all -- 3 phase electricity is magnetically coupled very well and there is not much for a magnetic field if the conductors are close to each other. The motor is a huge chunk of steel which is grounded. I'm not 100% certain on this but I'll check tomorrow: I am almost certain that I can hold a compass near a running motor and not get much deflection, grounded or no. If I run it near a single conductor it'll deflect like crazy but that's what is supposed to happen. :-)

and that all their hardware (starting capacitors, etc.) are functioning properly

You won't have starting capacitors on motors large enough to cause these kinds of problems. Perhaps a weak line is causing your poor magnetic coupling? I am sure you'd hear that though... Motors do not like to be single-phased.

To the story poster/question asker: email me. As I said, I work with this kind of stuff for a living. I may not be able to solve your problem, but I can at least help get you to a solution.

Re:some corrections to bad info

[Tower]

Ugh... 60Hz... for any real work, I can see flicker under 72Hz (with incandescent light) - up to 75Hz with fluorescent... 60Hz drives me insane... even if it is the only light source in the room I get a headache...

Try telling them they are imagining it??? Only if these people have never heard of refresh rate and are easily swayed...

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UPS, power factor correction hardware

[bgat]

Try putting your computers on UPS's. It may be that what's killing them is actually EMF on the electric lines caused by the motors in the chillers. A UPS would both spot that, and correct for it.

Next, try putting "power factor correctors" and/or "soft starters" on the power lines to the chiller motors (I think that's what they're called, anyway). The basic idea is that they help correct the EMF problem at the source (and can sometimes save electricity bills too!).

Make sure the chiller motors are properly grounded, and that all their hardware (starting capacitors, etc.) are functioning properly. Also make sure that the wiring in the office is up to code--- if you don't have proper grounding, *everything* in your office will be sensitive to electric noise of all types. My office used to be next to some large electric motors (also in chillers, but also in some seriously freeeeeky RF equipment my employer was into), and it was no problem, even though the building was nearly 100 years old (extensive remodeling/rewiring, though).

Get on a different electrical circuit than the chiller motors.

Finally, it may be easier to put a grounded steel box around the chiller motors, rather than on your whole floor.

Good luck...

b.g.

Re:I'd go with the steel plate too...

[TheTomcat]

Wouldn't it be easier to shield the ceiling of the room where the 'chillers' are? That would be the underside of the floor -- less ugly, and in a place where people would expect such a thing.

Might be an OSHA or some such violation

[scotpurl]

There's limits on what strengths of magnetic fields you can be exposed to. Call OSHA (occupational and health folks), or even one of the local labour unions. Someone will be able to look it up in a manual, and track down the correct inspector who can gauge field strenghts with an expensive gadget.

Meantime, be afraid. Or at least cautious.

Cautious beats dead/cancerous any day.

Some URLs....

[scotpurl]

http://www.niehs.nih.gov/oc/factsheets/emf/emf.htm
from National Institute of Environmental Health Sciences.

http://www.osha-slc.gov/SLTC/elfradiation/index. html OSHA's page on ELF radiation.

NOTE, one of the links is titled, "Possible Association of EMF and Suicide. News Release No. 147, University of North Carolina, (2000, March 15), 1 page. Press release from a large and detailed positive study of the possible link between exposure to low frequency electromagnetic fields (EMFs) and suicide among electric utility workers. "

Clarifications.

[Christopher Thomas]

Here's a question for you: If all it takes is a wire mesh or steel plate on the floor of their office/ceiling of the other office to block the magnetic flux, why isn't a computer already encased in metal with a ground (via the power cable) protected?

The problem is that as soon as you have wires leading into and out of the shell, your shell becomes almost completely ineffective. You'd have great trouble avoiding this, as you need things like power cables entering the computer.

Also, with the metal plate/mesh solution, how do you deal with flux wrapping around the plate?

There are a couple of effects that work in your favour for this. Firstly, the high frequency components of the noise won't diffract around *too* much. They're the ones that will couple the most strongly to electronics, and so the ones that would otherwise cause the most damage. Secondly, even if the low-frequency components _do_ diffract around the edge and reach your equipment, they've still been spreading out the whole time (diffracting over a wide angle range), so your equipment is still "farther away" from the source of the interference.

If you used metal mesh, aren't you then just creating a huge inductor on the floor? Inductors work by the change in magnetic flux going through loops. This induces a current acting opposite to the magnetic flux. The bigger the change in flux, the stronger the reactant current will be.

However, the reaction currents act to _cancel_ the imposed field.

A mesh (or a sheet with holes in it) can block most noise components with wavelengths substantially larger than the hole size. You can consider a solid sheet to be the limiting case as hole size goes to zero.

Even in a sheet, you'll have currents around the edge, and other circular currents within the sheet if the imposed magnetic field isn't uniform. Using a mesh just imposes a minimum size on these current circles.

The _lower_ limit to frequency blocked is governed both by diffraction (as you pointed out) and by the fact that your plate isn't an ideal superconductor. When the wavelength of the noise is substantially larger than the size of the plate, diffraction effects will become severe (though you still have _some_ benefit, as noted above). When the frequency drops to the point where the magnitude of the resistance of the current paths within the plate is substantially greater than that of the inductive reactance, the plate (or mesh) will similarly be ineffective.

This is actually a fascinating topic to think about.

Can you pick up a paperclip through copper?

[Russ Nelson]

Put a magnet on one side of a copper plate. A penny is a useful approximation. Now attack a paperclip to the other side of the penny. Notice how readily the magnetism goes through the penny? Copper screening isn't going to do shit to keep any magnetism out.
-russ

Two words..

[technos]

Tinfoil hats.

Not only do they protect the most sensitive organ in your body, the brain, they protect you from the alien mind-control satellites and they muffle the transmission from the secret spy transmitter the NSA had installed in one of your molars.

I suggest lining a baseball cap with the tinfoil. I do, and it saves me a lot of stares and comments. I think the aliens have caught on to the source of immunity, the hat, and have programmed their drone minions to ostracize me into taking it off.

Re:In my office...

[The Iconoclast]

I used to work at a theme park in their computer repair division. We had handheld so we could be dispatched to fix any problems in the park or the offices. In the offices there was the server room which conatined all of the Netware, POS, and mainframe servers for the entire park. Everywhere in the room there were BIG SIGNS saying "DO NOT KEY RADIOS IN THIS ROOM!!!" Keying radios sends out a EMP that computers tend not to like.

A wealthy eccentric who marches to the beat of a different drum. But you may call me "Noodle Noggin."

Same Problem Here -- Way Expensive to Fix

[InitZero]

For the last 24 years, we've had green-screen mainframe terminals and all was right with the world. About a year ago, we phased out the mainframe and installed PCs with lovely 21" NEC monitors. Everyone in one area of the building complained about horrible flicker and the investigation started.

It turns out that the department was over the electrical vault. Several times the acceptable limit was coming up through the eight-inch thick concrete floor. Needless to say, many jokes were made about low sperm count. Then people started to get worried. (First the people in that area then our corporate lawyers.)

Many ideas were floated. The easiest way would to have been to place a three-quarter-inch thick piece of lead on all the vault walls. (I say all because the folks on either side of the vault on the first floor were now also seeing problems.) This seemed like a good idea in theory.

While lead did fix the problems in the neighboring offices, the EMF level in the room skyrocketed. There was no place the the EMF to go so it just bounced around in the room. We went from slightly over the legal level to three-headed babies. We had to take down the lead.

Since the department in question refused to wear lead jock straps, we had to move the electrical vault to outside the building. (As is the standard now, I'm told.)

In the end, it cost us several thousand dollars to move the vault and, more costly, two power outtages and a few weekends of load testing.

If I were you, I'd talk to the building's property management. Ultimately, they will be the ones to fix the problem. Further, if they won't or can't fix it, I'd be looking for another building before you get too settled in.

Even if the EMF is within legal limits, a flickering screen is a hazzard to worker productivity. If a worker comes down with a bad headache, you're looking at a law suite. If a woman has a child with any birth defects even ones that could not possibly be caused be EMF, the company will be sued out of existance.

InitZero