Magnetic fields around low voltage panels

[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.

 

[dpc]

Interesting question. Check this:

http://www.emf-data.org/related-projects.html

You might also search EPRI website.

1 milligauss seems pretty low. Your local electric utility may have instruments available for doing field measurements. Make sure you take measurements when panels would be heavily loaded. Fluorescent lights could also be a problem.

You may need to think about moving locations, or applying some additional shielding.

 

[nbucska]

Can't you use the microscope itself ? the best test of the pudding is eating...

<nbucska@pcperipherals.com>

 

[busbar]

Two condensed online references are

http://www.ce-mag.com/archive/1999/sepoct/Casamento.html

http://www.microwavenews.com/EMF.html

You might contact Don Deno at Electrical Field Measurements Co in Stockbridge, MA for paper articles and basic instrumentation for Magnetic Field surveys. {DenoD@RNetworx—no web site}

 

[jbartos]

Suggestion: Try to possess a gaussmeter. Visit

http://www.thomasregister.com

and type Gaussmeters under Product or Service, which will return 54 companies to approach to for the gaussmeter. Electromagnetic field around panels changes with the load, harmonics, voltage spikes, etc. over a period of time.

 

[alehman]

This can be a real problem. Recently one of my clients experienced problems with EM fields severely distrupting the display on computer monitors due to panelboards on the opposite side of a wall.

Shielding can be used as an expensive last resort, but relocation is the usually easier. See

http://www.amuneal.com/

 

[busbar]

Wholesale replacement of CRTs with LCD monitors were a very good solution for one office with excessive power-frequency magnetic fields. The building owner compensated a tenant group for a significant portion of replacemnt expense.

 

[dbaird]

Check out:

http://home.t-online.de/home/electrosmog/emf-exposure-limits.htm

David Baird
mrbaird@hotmail.com

Sr Controls Engineer

EET degree.

Journeyman Electrician.

 

[alehman]

For something as complex and costly as an electron microscope, I would suggest getting an architectural EM shielding specialist or knowlegeable EE in the same room with the microscope supplier. Typically equipment suppliers can provide recommendations for shielding.

For some basic info, see

http://www.magnetic-shield.com/start.html

http://vitatech.net/emfshielding.html

 

[DanDel]

Chuk1t, 1 mGauss is very low, as already stated by dpc. The actual measurement around a panel will be directly proportional to the current flow in the panel circuits. Also, you will find a great increase in EMF readings for a circuit that does not have all conductors in the same conduit or cable, such as a circuit which may be using the ground as the neutral return.
Obviously, the largest offenders will be high-current carrying circuits and switchboards/panelboards, but, after many EMF surveys from DC to 60Hz, I have found that one of the most common but overlooked sources are CRTs themselves. Some of the older units can have over 100mGauss at a distance of approximately 1'.

 

[alehman]

As suggested by Dandel, sometimes improperly wired equipment or appliances can result in neutral current returning to the panel via the conduit system. This results in high magnetic fields in proximity to the panel and conduits.

 

[umrpwr]

In response to your first question about measuring the EMF level around the panel. It is best to use a triple axis gauss meter and take measures at fixed intervals in a grid pattern. This way you can create a plot showing the levels at various points and the max levels.

You may want to consider hiring someone to do a survey. Some electrical testing companies can do it and almost all shielding designers and engineers offer this service. I have had experience working with one of the the aforementioned engineers mentioned in the above posts. Intereference from can defintiely be a problem with sensitive electronic equipment. In my experience though it was costly, but attainable to shield to the 1mg level. Optimal design might include relocating the panel or other equipment causing the interference as opposed to shielding. It may be more cost effective. It may also be feasible to put only a small shield around the equipment. In my particular case I was invloved in a project in which entire utility vaults a building were shielded. It certainly an after thought in the design and not something that was considered. Strategic placement of the equipment is the optimal solution.