In October 2002, VitaTech Engineering conducted
an AC extremely low frequency (ELF) site survey within the leased
tenant spaces in a bank located in Dallas, Texas. Subsequently, recommendations
were made to install a 3,072 ft2 magnetic shield inside the tenant
space to provide attenuation from the magnetic fields emanating from
the “ONCOR” transformer vault located below. Additionally,
in late June 2003, VitaTech was contracted to design, install, test
and certify a AC ELF EMF dual-substrate magnetic shield. On July
10th-16th 2003, VitaTech installed the 3,072 ft2 (surface area) shield.
AC ELF Site Assessment
A detailed
AC ELF magnetic field site survey of the building (basement level
switchgear room, transformer vault, electrical rooms, and 1st floor
tenant
office space) was performed on Monday & Tuesday, October 14th-15th,
2002. AC ELF magnetic flux density data was recorded at the floor and one
meter
level inside the office/cubicle spaces, switchgear room, and surrounding
areas located on the basement and first floor. During the initial recording
period, it was determined that the magnetic fields recorded in the office/cubicle
areas did not decay at a normal nonlinear rate ( 1/r2 meter) indicating
a potential circulating ground/net/zero sequence current problem. After
preliminary
data was recorded in the primary areas of concern, a check of the electrical
system was conducted and recommendations to correct electrical faults were
made. Data was recorded on the floor and at the 1-meter level to determine
the rate of field decay of the magnetic fields emanating from the transformer
vault and collector bus ways below. This is extremely important because
slow decay rates (greater than 25% for floor to 1-meter) are indicative
of net,
ground, or zero sequence currents which emanate magnetic fields which cannot
be effectively shielded. Calculating the peak decay rate (dividing the
1-meter peak by floor peak) one obtains the percentage field decay from
the floor
to 1-meter. Magnetic fields generated by an electrical system which is
properly wired and grounded, decay quickly at no greater than 25%, i.e.
the lower
the percentage, the faster the fields drop off. Calculated peak decay rates
we well above 25% in every surveyed office and cubicle space. This is indicative
of egregious net, ground, and zero sequence currents which would have to
be corrected before other magnetic field mitigation solutions (i.e. shielding)
could be implemented. Since the data was recorded on a mild October (high
60's) day and building under partial (holiday) load (approximately
60 %), it can be assumed that the magnetic field levels can be multiplied
by a factor of two (2) during peak summer loads. It is common for the
magnetic field levels to increase/decrease due to the loads placed
upon the electrical system. VitaTech Engineering recommends correcting
any and all N.E.C wiring/grounding errors throughout the entire facility
to include main electrical service and secondary systems. Additionally,
VitaTech highly recommends installing a magnetic shielding solution
to attenuate (reduce) the magnetic field levels caused from the transformer
vault with collector busses located below the tenant space.
Project Data
Before
Shield Site Survey
3-D Contour Plot of Site Before AC ELF Magnetic Shield Installed
After
Shield Site Survey
3-D Contour Plot of Site After AC ELF Magnetic Shield Installed
 Slideshow
Magnetic Shield Images
Before Shield
3,072 square-foot area. Peak emissions were 401mG at floor level.
After Shield
3,072 square-foot shielded area. Peak emissions were 11.6mG at floor level. |
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