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 occupants (people and equipment) away
from the source, shield the source with magnetic
shielding or shield the occupants from the source.
VDTs - The Canary Birds Of 60-Hz Magnetic Fields
How
does one tell if the magnetic field levels are greater than 10 mG without
a gaussmeter? Use the EMF equivalent of a coal miner's
Canary Bird - a color video display terminal (VDT). Place a computer
with a color VDT in the
area under investigation, select a full-screen display, and rotate the
display 360 degrees - if the screen appears to jitter or distort, then
the magnetic
fields exceeds 10 mG. A better alternative is to purchase an inexpensive,
but highly accurate, single-axis EMF 131 gaussmeter ($115) from
EMF Company (413-637-1929)
or a triple-axis Model 4080 gaussmeter ($199) from F.W. Bell. (order from Less
EMF, catalog # F171). Considering the potential health risks
and perilous electromagnetic interference (EMI) that usually emanatesfrom
transformer
vaults, network protectors, secondary feeders, switchgears, distribution
busways and electrical rooms, all offices and apartments in proximity should
be magnetically
surveyed by an experienced EMF engineer.
To Shield or Not To Shield 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 occupants,
is the
preferred solution. Call a professional 60-Hz magnetic shielding company
such as VitaTech
Engineering, LLC at 703-440-9400 for an estimate.
60-Hz Magnetic Shielding Fundamentals
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 occupants 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.
Lossy 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:
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.
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.
60-Hz Magnetic Shielding Information
In
1994 the Electrical Power Research Institute (EPRI) at 510-934-4212
published a two volume set Handbook of Shielding Principles for
Power System Magnetic
Fields, April 1994, EPRI TR- 103630-V1 & V2 for a mere $50,000 (discounted
to EPRI members only). These two huge volumes provide an encyclopedic
treatise on 60-Hz magnetic shielding and materials; however,there are
very few practical
shielding design equations and useful examples. So, if you are a design
engineer first experiment with small shield designs, various ferromagnetic
and conductive
materials, read every page in the EPRI Handbook, 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
have the technical expertise to design and successfully install complex
shielding systems for offices and apartments. |