There are two basic 60-Hz magnetic field mitigation (reduction) methods: passive and active. Passive magnetic field mitigation includes rigid magnetic shielding with ferromagnetic and highly conductive materials, and the use of passive shield wires installed near transmission lines that generate opposing cancellation fields from electromagnetic induction (beyond the scope of this paper). Active magnetic field mitigation uses electronic feedback to sense a varying 60-Hz magnetic field, then generates a proportionally opposing (nulling) cancellation field within a defined area (room or building) surrounded by cancellation coils. Ideally, when the two opposing 180-degree out-of-phase magnetic fields of equal magnitude intersect, the resultant magnetic field is completely canceled (nullified). This technology has been successfully applied in both residential and commercial environments to mitigate magnetic fields from overhead transmission and distribution lines, and underground residential distribution (URD) lines.
There are two basic types of 60-Hz magnetic shields:
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. The relative permeability (μr) of mumetal ranges between 350,000-500,000 depending on the composition and annealing process. Unfortunately, mumetal sheets are extremely expensive: a single fully annealed 30 x 120 inch sheet (0.062-inches thick) costs around $1,800 (prices are very volatile due to fluctuation in nickel costs).
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.
Shielding factor (SF) is the ratio between the unperturbed
magnetic field Bo and the shielded magnetic field Bi as expressed in:
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.
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
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.
The AC ELF lossy shielding factor (SF) due to circulating eddy currents in a single thick conductive plate (assume infinite cylinder in transverse magnetic field) is calculated with the following formula (use meters for radius and thickness and substitute the parameters were noted):
Parameters include permeability of free space µo = 4π x 10-7 H/m, the conductivity of aluminum AA1100 plate σ = 3.31 x 107 1/ohm-m, aluminum shield plate thickness (t) 0.25 inches (6.35 x 10-3 m) and frequency (f) of operation 60 Hz.
Calculating the flux entrapment shielding factor (SF) for the DC magnetic shield composed of ferromagnetic layers is an iterative process. Although there are several DC shielding formulas that can be used (all rather confusing), VitaTech prefers to use a more simplified method based upon our experience (yes, this is a black art).
First, the magnetic induction (BI) must be determined for each applied layer using the following formula and the relative permeability µr determined from the appropriate permeability chart for each specific ferromagnetic shielding material:
To calculate the shielding factor (SF) for the ferromagnetic shielding, the following formula is applied:
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.
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 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.
So, if you are a design engineer
first experiment with small shield designs, various ferromagnetic
and conductive materials
and call VitaTech Engineering @ (540) 286-1984. My professional
advice is do not attempt any large-scale room shield designs, only experienced
60-Hz magnetic shielding design companies with professional engineers on staff have the technical expertise
to design and successfully install complex shielding systems for offices
and apartments. |