Magnetic Shielding - Often a Poor Strategy




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It is natural to assume that some kind of shielding will be the best way to eliminate or reduce an AC magnetic field. Unfortunately, EMF shielding is generally the least effective, hardest, and most expensive way to handle a power-frequency AC magnetic field, if it works at all.

Yes, shielding does work easily and well for static or low-frequency electric fields - using any sheet of metal, metal screen, or metal fencing, provided the metal is grounded.

And yes, EMF shielding works well for both the electric and magnetic components of higher-frequency RF and microwave fields. This is because at those frequencies if you can reduce or stop the electric field, you will also reduce or stop the magnetic field, since the two are inseparably linked in most situations. (That is why we use the word "electromagnetic radiation" to describe microwave or high-RF (radio frequency) fields, while at power frequencies we prefer to talk about "AC electric fields" and/or "AC magnetic fields" - as separate entities - since we can often have one without the other.)

And we can shield even higher frequency electromagnetic fields, still more easily. For instance, anything opaque will stop visible light (which is an electromagnetic radiation) or ultraviolet radiation. Even X-radiation can be shielded, though it begins to get harder again in that range, requiring lead or thick metal.

But we can't shield power-frequency AC magnetic fields.

Or rather, we may be able to do so, but the cost is typically prohibitive, compared with some better approach. Partly this is due to the fact that the best magnetic shielding materials contain copious amounts of nickel, and they must be used in fairly thick layers for any large job. For instance, you can shield a match box for a few dollars, or a shoe box for tens of dollars, but to shield a whole room, you will need thicker metal (the reason is explained in Silencing the Fields) as well as more square feet of it, and it will set you back tens of thousands of dollars. Don't even ask about shielding a house. Generally large-scale magnetic shielding is a rational choice only in certain commercial properties having very high real estate value - if a strong field source is nearby and cannot be moved or corrected in some better way.

But if not shielding, what then?

Magnetic shielding is a last resort. It never makes good sense (either for a homeowner with health concerns or for a business owner with shimmying-computer-picture problems) to start out by thinking of shielding.

Almost every high-AC-magnetic-field problem in a house or business building can be solved more economically (often much more economically) in some other way - by eliminating the field's source or by moving further away from it. Generally, either the high-field zone is fairly small, which is typical when the field source is some appliance or piece of equipment - or if the zone is a big one, it is caused by some big loop of one-way (or "net") current, which can be eliminated, often without too much trouble, and sometimes with improved electrical safety as a bonus.

The book Silencing the Fields discusses these various remedies. In certain cases, they may require expert help (though frequently just the help of any capable electrician). In other cases, the remedies may require the more expensive or difficult-to-obtain help of your power company. Often, however, the remedies are feasible for, or can be supervised by, a knowledgeable homeowner using the techniques and tricks described in Silencing the Fields - at a reasonable cost.

If you decide that professional help is needed, EMF Services is a good starting point. In addition to a full range of testing and mitigation services, they have a technology called active shielding that can sometimes be an effective solution for power line magnetic fields.

Silencing the Fields - A Practical Guide to Reducing AC Magnetic Fields

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