Reduction of ELF Magnetic Fields Emanating from Circuits with Net-Current Conditions by Cancellation Techniques INTRODUCTION:
OBJECTIVE:
METHOD:
Elevated levels of power frequency magnetic fields in commercial
buildings are often caused by "net-current conditions present on
conduits, bus ducts and other distribution circuits. In multi-grounded,
four-wire industrial and commercial building wiring systems, some
portion of the neutral current may return to the source transformer via
building grounds. In other cases, neutrals of different circuits may be
tied together thus allowing current from one neutral to return to the
transformer via a
lternate neutral conductors. In any of these
instances, the vector sum of the currents for any given circuit may not
add up to zero. When the vector sum of the phase, neutral, and
parallel ground wire (if present) for a given circuit does not equal zero,
a net-current condition is present. This circuit condition creates net-
current magnetic fields that decay at a 1/D rate versus the 1/D rate
normally associated with magnetic fields emanating from such
distribution circuits.
Reconfiguration of con
ductors is usually not effective at reducing net-
current magnetic fields; however some reduction may be possible by
arranging conductors in such a way to cause the displacement
magnetic field to partially cancel the net-current field. The best
mitigation measure for magnetic fields caused by a net-current
condition is to correct wiring problems (improper neutral connections,
missing neutrals, improper grounding, etc.) so as to minimize or
eliminate the alternative current paths that are creating
the net-current
circuit conditions.
Most often, it is logistically and economically prohibitive to consider
reduction of elevated magnetic field from net-current circuits with
magnetic field shielding. Elevated magnetic fields from net-current
circuits can best be reduced by corrections or changes to wiring
systems. However, in some commercial and industrial buildings, such
corrections to wiring systems are not practical.
This study was intended to investigate the effectiveness and
commercial vi
ability of using magnetic field cancellation techniques to
reduce magnetic fields in commercial buildings with net-current
magnetic field conditions present.
The objective of this study was to (1) develop and evaluate magnetic
field cancellation schemes, in a laboratory setting, utilizing both
passive and active cancellation techniques to effectively reduce
magnetic fields produced by net-current circuit conditions, and (2) to
investigate the viability and practicality of using such techniques i
n
real world commercial and industrial buildings that have net-current
magnetic field conditions present.
2

Utilizing three 10-foot-high by 10-foot-wide by 10-foot-long non-
metallic test structures, a series of single-phase and three-phase
conductors in a variety of conduits typical of commercial buildings,
were suspended above an x and y measurement grid on the floor of a
laboratory to simulate an office beneath the test circuits and
conduit(s). Three variable transformers were connected to a 208 volt,
three-phase, four-wire commercial electric service. The output of the
three variable transformers p
rovided an adjustable 0 to 140 volt
source to three loading transformers, each configured for 120 volt
input and 5 volt output. The output of each loading transformer was
connected directly to one of the phase conductors in the test circuit
conduit. The three individual phase conductors plus neutral conductor
were connected together utilizing a short length of copper bus located
at the exit of the conduit. This test power network thus allowed either
three-phase or single-phase current to circula
te in values ranging
from 100 to 800 amps in the test conductors. Real world circuit
imbalance conditions were controlled by different phase current
settings and use of different wire sizes for the neutral conductor. Net-
current conditions were controlled by connecting a separate external
wire loop from the neutral point of the loading transformers to the
copper collection bus at the end of the conduit. This external by-pass
loop was fastened to the perimeter walls of the laboratory to create
di stance from the magnetic field measurement grid. An adjustable
choke used in the by-pass circuit allowed for further adjustment of
circulating current. This arrangement allowed net-current conditions
from 1 to 50 amps to be created in the tescircuit conduit. Metallic
(non-ferromagnetic and ferromagnetic) and non-metallic conduits
where included in the test program.
t
Reduction of ELF Magnetic Fields Emanating from Circuits with Net-Current
Conditions by Cancellation Techniques
Jon W. Munderloh, Kenneth L. Griffing, Michael L. Hiles, Kirby C. Holte.
1112
12
Field Management Services Corp., Los Angeles, CA 90048, Grid Technology Associates, Walnut, CA 91789
120/208 V
Test Power Network
Test Circuit & Conduits
Copper Bus
Adjustable
Choke
By Pass Current Path
(to create net-current condition)
Variable Transformers
0 to 140 Volt
Loading Transformers
120 to 5 Volt
A
B
N
3
f
C
N
X
Y
Z
Copper Collection
Bus
By Pass Conductor
(net-current)
Passive Induction Cancellation Loop Test Set Up
To
Power
Network
A
B
C
N
Shunt Loop
Test Conduit(s)
Inductive Cancellation Test Loop
Active Cancellation Loop Test Set Up
To
Power
Network
A
B
C
N
Shunt Loop
Test Conduit(s)
Active Cancellation Test Loop
Sensor
Phase & Amplitude
Processor
Cancellation Loop
Driver
Copper Collection
Bus
By Pass Conductor
(net-current)