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EMF Shielding & Mitigation


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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 approach, such as field cancellation (active EMF shielding), or correction of net current problems in electrical wiring systems.

This page will describe some different types of 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
(Frequency)
Typical Sources Shield or
Other Solution
ELF Magnetic
(50/60 Hz)
Electrical Panels
Electrical Switchgear
High Current Feeders
Transformers
Utility Electric Meters
Passive Magnetic Shielding with Bulk Materials - often a combination of high conductivity and high permeability metal plates.
Power Lines
Unbalanced Feeders and
  Branch Circuit Wiring
Current on Water Pipes
Unbalanced Underground
  Distribution Lines
Magnetic Field Cancellation
  (Active Shielding)

Restore Circuit Balance
ELF Electric
(50/60 Hz)
Power Lines
Type NM Wiring
Fluorescent Lighting
Easily shielded with any grounded conductive barrier, from metal screen to a row of trees.
RF (radio frequency)
electric, magnetic,
plane wave
(100 KHz - 100 GHz)
Broadcast Transmitters
Cellular and PCS Systems
Mobile Two-Way Radio
Radar
Microwave Systems
RF Heat Sealers
Medical MRIs
Electrosurgery Equipment
Diathermy Equipment
Navigational Systems
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.
VLF Magnetic
(3 KHz - 100 KHz)
Induction Heating Systems
Computer Monitors
Retail Anti-Theft Systems
Electrical Transients
Passive Shielding, usually with high permeability materials - less material thickness required than for equivalent ELF magnetic field.
VLF Electric
(3 KHz - 100 KHz)
Corona Treating Systems
(user modified)
Easily shielded with any conductive barrier, such as metal foil. Industrial sources can exceed established safety guidelines.
DC Magnetic
(0 Hz, non-time-varying)
MRIs
Aluminum Refining
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 a net current condition or wiring error, since shielding is not effective for these conditions, and correction of wiring problems is far less expensive 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 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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