Magnetic Field measurement 1

[Chuk1t]

I have an electron microscope installation I'm working on and one of the requirements is that any magnetic fields in the vicinity of the unit be less than 1.0 milliGauss at 60Hz. There are four 208/120 Volt 3-ph panelboards located on the corridor side of the room partition. The panels are steel and all raceways are steel. Distance to the microscope from the panels is about 10 feet.

If anyone has experience with measuring the magnitude of magnetic fields around panels or knows of IEEE papers on the subject I would appreciate your input.

 

[VEBill]

With the recent interest in human safety, there are a lot of measurement instruments available.

Calculation would probably require too many details about the exact layout of the conductors. Practically, you would probably have to prepare a 3-D model and run a computer simulation.

Or just phone the application engineer - he/she would probably know the answer from practical experience.

 

[logbook]

Magnetic fields in the milligauss region can be measured with a battery powered hand-held meter such as a TriField meter (that is a manufacturer’s trade name). However, if you just wish to measure 60Hz fields then a coil of wire fed into a scope will do the job more cheaply.

You have Faraday’s law of induction, E = N * d(phi)/dt

E the EMF (voltage)
N the number of turns
phi the total flux passing through the coil
d(phi)/dt the rate of change of flux

You will have a coil of a particular area A (metres squared) and a magnetic flux density B (tesla)

E= N*A* dB/dt

The rest is lest as an exercise!

 

[mvio]

We do this quite frequently using two methods. One cheap way is to get a gaussmeter from Newark electronics or similar electronics house; we use them for rough surveys and they work well. The brand we have is an Extech 480822; it's the size of a multimeter. The meters usually read milligauss up to 200 mG. Other suppliers include FW Bell who sell magnetometers that use Hall-effect sensors for higher fields.

We also use a loop antenna and spectrum analyzer, but this is overkill for your measurement.

1 mG may be tough to meet; you may be in a 1 mG field close to appliances and normal house current.

If the raceways include bus ducts and carry significant current, I wouldn't be suprised to see 30 or 40 mG at 10 feet from the panels, depending on how the conductors run in your space. If the raceways are cabled, there is magnetic cancellation that reduces the net mag field. Shielding is complex and very costly. Often, the cheapest method of reducing mag fields is to move the equipment!

A mapping of the room is usually performed to give an idea of the hot spots; this may help you locate the equipment in a quiet zone.

 

[logbook]

http://www.extech.com/instrument/products/451_499/480823.html

 

[VEBill]

mvio: "...you may be in a 1 mG field close to appliances and normal house current..."

As a side note - it can be quite informative to hold one of those small but powerful magnets in your hand, and then hold it near(*) a just-turned-on electric stove top 'burner' element. The magnet will vibrate quite vigorously in your hand.

(*Obviously, be very careful not to burn yourself.)

This demo can be a 'real eye-opener' for those that worry about the purported negative health effects of power lines and such.

 

[Chuk1t]

I want to thank all those who have responded to my original posting RE: Electron Microscope installation and magnetic fields. From the many responses I received it looks like I'll use the empirical approach and do some local measurements.

Going back to fundamentals, a coil of wire used with a portable scope such as suggested by LOGBOOK sounds interesting. :-D