People are typically exposed to very high 60-Hz magnetic
field levels ranging between 10-1,000 mG (milligauss) when their offices
and work areas are next to, over or under transformer vaults, network
protectors, secondary feeders, switchgears, distribution busways and
electrical rooms. Usually the occupants are not aware of this potential
hazard unless the magnetic field source compromises VDT's, computers,
networks, electronic instruments, audio/video equipment, and magnetic
media. Once detected it is the responsibility of the building owner or
manager to remedy.
This article will examine an actual ELF magnetic shield
design and installation project for a New York City community college.
Very high magnetic field levels
between 35-150 mG were measured (waist height) by the College's OSHA Officer
in the fitness center and adjoining areas (wrestling room and offices). The
fitness center is located directly above four transformers, four network protectors,
and the main electrical room. Fortunately, the college decided to mitigate
the problem by installing a ELF magnetic shield in the fitness center and wrestling
room; however, the offices were converted into storage rooms. VitaTech surveyed
the site, designed the ELF magnetic shield, prepared the design/build documents,
and managed the installation work.
Contour ELF EMF Survey-Site Evaluation
A
contour ELF EMF survey was recorded at the site to evaluate the magnetic
field profile for the shielding design. Map
Plot - Figure #1, shows the recorded survey path superimposed
over a scaled architectural drawing of the transformers, network
protectors, and electrical equipment room under the fitness center.
Data (in milligauss-mG)
was recorded at one-foot intervals, one-meter above the floor, along
the
path. A peak spot of 111 mG was recorded that day over network protector #4.
Resultant (Rrms) magnetic field levels as a function
of path distance are presented in Profile
Plot - Figure #2, with positional markers referenced to the
map plot on Figure #1. This is a very informative plot that vividly
shows how the magnetic fields from each electrical source penetrates
through
the floor.
A visually
informative three-dimensional graphic, 3-D
Contour Plot - Figure #3, shows the overall magnetic field
profile throughout the fitness center. Additional data was necessary
to correlate
the varying worst-case peak spot floor levels with the building load.
Therefore, peak spot timed (dosimetric) data was recorded at one-minute
intervals
for
two days (48 hours) as shown in Dosimetric
Plot - Figure #4.
Estimated School-In-Session & Worst-Case
Seasonal Peak Load (WCSP) Since the survey was performed during the
August vacation,
it was
necessary to estimate
the school-in-session and worst case seasonal peak load (WCSP) from
1-meter above the floor data - then extrapolate floor levels from
these estimates.
At the peak spot in the fitness center, the estimated WCSP school-in-session
floor and 1-meter levels were 850-mG and 185-mG, respectively. The
highest predicted levels were 195-mG(floor) and 58-mG(1-meter)
in the wrestling
room next to the common wall adjoining the fitness center, then
diminished to
under 3-mG approximately 25 feet away.
3-mG Shielding Objective & Selected Shield
Design
In each room the college selected a 3-mG (±1 mG) maximum
performance criteria measured one-meter above the floor. Obviously,
a
very technically challenging
objective that required a multilayer, three-substrate, magnetic
shield composed of silicon-iron sheets, aluminum plates, and specially
annealed
AD-MU-80
mumetal sheets by Ad-Vance Magnetics, Inc. The magnetic
shield would be installed on
the floors and walls in the fitness center and half of the
wrestling room. VitaTech also prepared a complete design/build bid
package
with detailed
engineering drawings and installation specifications for
the college.
Magnetic Shield Installation
A local
New York City environmental company, AquaX Conservation, Inc. (212-226-6189),
was awarded the shielding installation
contract by competitive
bid. AquaX had
the project management experience and skilled craftsman
necessary to comply with the technically demanding installation
requirements. After each substrate
was carefully installed (silicon-iron, aluminum, and
mumetal), VitaTech inspected the workmanship and recorded the magnetic
field profile
to monitor shielding
performance. When the installation work was completed
an
on-site
final performance test was executed by VitaTech to
validate the 3-mG design
objective.
Final Performance Test-Contour ELF EMF Survey
The
shielded fitness center superimposed contour survey path is shown in Map
Plot - Figure #5. Data was collected
one-meter above the floor at one-foot intervals.
The peak
spot over network
protector #4 was now only 4.28 mG
- it was 111 mG in Figure #1. In Profile
Plot - Figure #6 the shielded mean (average)
level was only 2.96 mG compared to the unshielded
34.5 mG level
in Figure #2. Furthermore, the
shielded peak spot on the floor was merely 5.2
mG compared to the 720 mG unshielded peak spot
in Figure #2. Remarkably, the peak
spot shielding factor (SF) was -43 dB on the
shielded floor.
Slightly elevated
levels between
3.2 -
4.2 mG emanated from the shielded walls (denoted
with marker letters A-O) around
the perimeter of the room.
A three-dimensional
graphic of the shielded fitness center, 3-D
Contour Plot - Figure #7, shows the center
region was only 2.5 mG compared to the unshielded
(mountainous) area with
peaks
between 80
- 111
mG in Figure #3.
The 3-D
Contour Plot - Figure #8 presents a three-
dimensional graphic of the shielded wrestling
room where the
mean (average)
level was
only
1.41 mG compared
to the previously
unshielded wrestling room 9.25 mG level. Also,
the shielded peak spot was 3.36 mG (4.1 mG on
floor)
compared to the
unshielded
peak spot of 37.6
mG (93.2
mG on floor). Finally, the shielding factor (SF)
was -29 dB on the peak spot shielded wrestling
room floor.
Conclusion
The final design objective
of 3-mG (±1 mG) was
verified in both the shielded fitness center
(MEAN of 2.96 mG)
and the
shielded wrestling
room (MEAN of
1.41 mG). Although the 3-mG objective
was achieved, it should be noted this project
was difficult and expensive to implement.
Furthermore, it demanded the utmost cooperation between the
college, VitaTech
Engineering,
and AquaX
Conservation.
It is more practical and significantly
less expensive (nearly half the cost) shielding any room to
5mG or 10mG, rather
than 3-mG, which
is
the ultimate
challenge. |