Do you really need magnetic shielding? We’ll give you all the options.
When electromagnetic field levels are elevated, Vitatech offers EMI consultant services as an option to reduce EMF exposure and potential EMI problems from Magnetic Field Interference. Elevated electromagnetic fields are produced by various sources: transmission and high current distribution lines, substations, transformers, secondary feeders, switchgears, busways and risers, electrical panels, grounding systems, plumbing currents, wiring loops and subpanel feeders (NEC violations), office equipment, shop tools, and machinery. Magnetic field management begins with an ELF/EMF Contour Survey of the site that includes a detailed assessment of the electrical sources and the resultant magnetic field characteristics (magnitude, polarization, harmonic content, temporal and spatial variations).
After the assessment, Vitatech considers the various magnetic field management strategies available to attenuate the Magnetic Field Interference (see below) and prepares a final recommendation with estimated mitigation costs:
- Magnetic Shielding – reduces emissions from electrical sources and equipment with aluminum/copper, low carbon steel, silicon-iron & nickel-iron (mumetal) enclosures and barriers.
MAGNETIC SHIELDING – AN OVERVIEW
There are two basic types of 60-Hz magnetic shields: magnetic flux-entrapment shields and lossy shields. A flux-entrapment shield is constructed with ferromagnetic, highly permeable (µ-mu), 80% nickel-20% iron alloy (i.e., Hipernom Alloy, CO-NETIC AA, Aumetal, AD-MU-80, etc.) which either surrounds (cylinder or rectangular box) or separates (“U” shaped or flat-plate) the area from the magnetic source. Ideally, magnetic flux lines incident upon the flux entrapment shield prefers to enter the highly permeable (µ-mu) material, traveling inside the material via the path of least magnetic reluctance (R), rather than passing into the protected (shielded) space.
Lenz’s Law and Lossy Magnetic Shielding
Lossy magnetic shielding depends on the eddy-current losses that occur within highly conductive materials (i.e., copper, aluminum, iron, steel, silicon-iron, etc.). When a conductive material is subjected to a time-varying (60 hertz) magnetic field, currents are induced within the material that flow in closed circular paths – perpendicular to the inducing field. According to Lenz’s Law, these eddy-currents oppose the changes in the inducing field, so the magnetic fields produced by the circulating eddy- currents attempt to cancel the larger external inducing magnetic fields near the conductive surface, thereby generating a shielding effect.
What’s Your Shielding Factor?
Shielding Factor (SF) is the ratio between the unperturbed magnetic field Bo and the shielded magnetic field Bi as expressed in: SF = Bi/Bo The final shielding design depends on several critical factors: maximum predicted worst-case 60-Hz magnetic field intensity (magnitude and polarization) and the geomagnetic (DC static) field at that location- whichever is greater; shield geometry and volumetric area; type of materials, permeability, induction & saturation; and, number of shield layers.
Equal Parts Art and Science
Small fully enclosed shields (conduits, video display terminals, etc.) follow simple formulas that guide the design engineer through the process to a functional, but not necessarily optimal design. After assembling a prototype, the design engineer measures the shielding factor (SF) and modifies the design (adds materials, additional layers, anneals bends, etc.) to achieve the optional shielding requirements. This is a very interactive design process, from concept to final product. Unfortunately, magnetic field shielding is more of an art than a science, especially when shielding very large areas from multiple, high level, magnetic field sources.
At this time there are no reliable design formulas or current EMF simulation programs that offer design engineers practical guidelines for shielding large exposed areas from multiple, high level, magnetic field sources.
When People Are at Risk
People are typically exposed to very high 60-Hz magnetic field levels ranging between 10-1,000 mG (milligauss) when their offices and apartments are next to, over or under transformer vaults, network protectors, secondary feeders, switchgears, distribution busways and electrical rooms. Usually employees and tenants are not aware of this potential hazard unless the magnetic field source compromises audio/video equipment, electronic instruments, magnetic storage media, VDT’s, computers, and networks. Once detected it ultimately becomes the responsibility of the building owner/manager to remedy, otherwise the employee and/or tenant may seek legal action. Unfortunately, there are only three practical solutions to mitigate magnetic field exposure produced from electrical systems within buildings: move the victims (people and equipment) away from the source, shield the source or shield the victims from the source.
To Shield or Not to Shield?
It is usually not desirable, especially if office or living space is limited, to evacuate an entire room or several rooms exposed to very high magnetic field levels. So, when space is at a premium the only other alternative is magnetic field shielding. To shield or not to shield the source? That is the question! Generally, when physically practical, source shielding is the most effective and least expensive alternative. However, if there are multiple magnetic field sources (i.e., parallel transformer vaults, network protectors, secondary feeders, etc.) it may not be economically feasible to separately shield each source. In that case shielding the room, and consequently the victims, is the preferred solution.
Thinking of Designing Your Own Shield? Think Again.
If you are a design engineer, first experiment with small shield designs, various ferromagnetic and conductive materials, and call Vitatech Electromagnetics at (540) 286-1984. Do not attempt any large-scale room shield designs, only experienced 60-Hz magnetic shielding design companies with electrical engineers on staff have the technical expertise to design and successfully install complex shielding systems for offices and apartments.
The Other Essentials of EMF Mitigation
- Active & Passive Magnetic Field Cancellation Systems – protect rooms, whole buildings (including homes) and exposed property (playgrounds) from outside distribution and transmission lines.
- Simulations & Profiles – define safe limits and distances from EMF sources, also known as prudent avoidance.
- Detection, Control & Mitigation – EMI/RFI sources, NEC violations & potentially lethal electrical shorts, plumbing currents and net and ground currents.
We’ll provide you a clear, comprehensive, written report. And manage everything after that.
Upon client approval of our magnetic field management recommendation and quote for services, Vitatech prepares a detailed, engineered design and project management schedule, implements corrective measures, manages subcontractors, and provides a final ELF EMF survey to verify the results.
RFI Mitigation: The other invisible threat we know all about
Radio Frequency Interference (RFI), is increasingly an issue for siting sensitive instruments, communications system design and operation, and human RF exposure safety.
Decades of experience
Vitatech applies our decades of EMI consultant experience in assessing radio-frequency environments against your facility needs and functions so that your systems perform as expected. The assessment, design and installation of low-RFI environments is a specialty of Vitatech, and a requirement for controlling the electromagnetic environment in critical, sensitive, or high RF areas.
Specifically, RF interference assessment is a requirement for:
· Sensitive medical devices (e.g. MRI’s, NMR’s, EEG’s, MEG’s)
· Sensitive research equipment (e.g. STMs, some AFM’s, certain cryogenic
amplifiers, CCD’s, and other applications)
· Commercial Grade recording/production studios
· Systems employing RFID (e.g. manufacturing, medical)
· High-Rise buildings with first-responder/cellular repeaters
· Safety Systems – Elevator Controllers, Environmental controls
· Dense RF transmitter environments (rooftops, etc.)
It is important to perform an assessment if you have sensitive facilities or functions that are:
· Nanoscale/high-resolution in character
· In proximity to high-power broadcast or communications transmitter
· Installation sites for multiple RF transmitters (rooftops)
· Utilizing RF Spectrum
· Classified, Sensitive, Secure
· Utilize Radio Frequency Identification systems
· Highly automated with sophisticated control systems
Sources for RFI are not always obvious
· TV/AM/FM Broadcast transmitters
· WiFi Nodes
· Cellular Phones
· Variable Frequency Drive (VFD) Pumps and motors
· Fluorescent Light Ballasts
· RFID scanners
· Heat Sealers
Give us a call. We’re happy to walk you through it.