EMF shielding is a broad term with diverse meanings,
primarily because there are many different types of EMF (electric and magnetic fields, RF electromagnetic fields,
different frequencies, different sources).
It is important to realize, however, that shielding is not applicable to all problems, nor is it always the most
cost-effective solution. A large proportion of those who are looking for EMF shielding would be better
served by an alternative mitigation approach, such as field cancellation (active EMF shielding),
or correction of net current problems in electrical wiring systems.
This page will describe different types of EM field, and the EMF mitigation techniques used for control.
It will also explain circumstances where shielding is ineffective, and discuss alternative approaches.
The concepts are the same whether the purpose is limiting human exposure (protection for people), or preventing
electromagnetic interference (protection for sensitive equipment).
|Type of Field
High Current Feeders
Utility Electric Meters
|Passive Magnetic Shielding with Bulk Materials - often a combination of high conductivity and high permeability metal plates. |
Unbalanced Feeders and
Branch Circuit Wiring
Current on Water Pipes
|Magnetic Field Cancellation|
Restore Circuit Balance
Type NM Wiring
|Easily shielded with any grounded conductive barrier, from metal screen to a row of trees. |
|RF (radio frequency)
(100 KHz - 100 GHz)
Cellular and PCS Systems
Mobile Two-Way Radio
RF Heat Sealers
|Mitigation method varies widely depending upon frequency and required field attenuation. Generally involves the complete enclosure of a space by metallic or conductive materials, with special attention to shield penetrations such as doors, vents, and cables. |
(3 KHz - 100 KHz)
|Induction Heating Systems
Retail Anti-Theft Systems
|Passive Shielding, usually with high permeability materials - less material thickness required than for equivalent ELF magnetic field. |
(3 KHz - 100 KHz)
|Corona Treating Systems
|Easily shielded with any conductive barrier, such as metal foil. Industrial sources can exceed established safety guidelines. |
(0 Hz, non-time-varying)
|Passive Shielding with Bulk Materials - often steel plate or high permeability layered metal sheet. |
Power frequency magnetic shielding is what many people have in mind when they are looking for an
EMF shield. It has become common in areas of commercial buildings near the power control and distribution
equipment. Interference can be experienced with sensitive equipment. Once the problem is recognized, human exposure concerns can become
an issue as well. This type of shielding is moderately expensive for large areas, but if space is at a premium,
the cost can often be justified. It is essential to ensure that the problem is not the result of an electrical net current condition
or wiring error, since shielding is not effective for these conditions, and correction of wiring problems is
a far less expensive mitigation path than shielding. In many cases, however, magnetic shielding is the only viable solution.
Radio frequency shielding was the first fully developed EMF shielding application, and is widely used in many
different forms. Two broad categories of utilization are recognized: (1) protection of sensitive equipment (or people) from
high intensity electromagnetic fields, and (2) prevention of signal escape from secure facilities where secret or classified information is
processed. The primary design considerations are the frequencies to be shielded and the required attenuation, or degree of shielding
effectiveness. Types of material utilized range from conductive coatings and fabrics to a fully welded steel enclosure. Modular EMF
shielding systems with proven performance characteristics are also available.
Shielding effectiveness depends upon matching the proper mitigation technique and materials with the problematic field source.
It is important to realize that a shield can be completely effective against one type of field and not at all effective against another.
For instance, a well performing RF shield of copper foil or screen would be minimally effective against power frequency or DC magnetic fields.
Likewise, a power frequency magnetic shield could provide excellent reduction of 60 Hz fields, but offer little reduction of radio frequency fields.
The same is true for different RF frequencies. A simple large-mesh screen shield would work well for lower frequencies, but would be ineffective
for microwaves. Nevertheless, it is possible to create a shield that would be concurrently effective against most types of field, although it
would be impractical for all but the most demanding, high-end applications.
EMF Services can evaluate your problem and propose an appropriate and cost-effective solution, which may or may not involve shielding.
We handle many types of mitigation projects directly, but if your application would be best served by a shielding specialty contractor, we can
provide a referral to reputable companies with a track record of successful jobs. The starting point in most cases is a site survey.
Feel free to call for a discussion of your needs.
Magnetic Field Mitigation Article
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