Document Type:Technical Results
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Distribution lines constitute one of the major sources of the public's
time-weighted average exposure to magnetic fields. This report
discusses the three main sources of magnetic fields along with
statistically significant data for each source, methods of calculating
distribution line magnetic fields, and general guidelines for
performing distribution line magnetic field measurements. Technologies
for magnetic field exposure reduction are presented and discussed, with
emphasis on the most promising techniques.
BackgroundThe general public has become increasingly aware of possible health
effects from exposure to electric and magnetic fields (EMF). This has
led to controversy, delay, and cost increases in the construction of
some utility lines and facilities. EPRI has initiated research to
identify field sources, characterize field levels, and provide
management options and strategies that could be used to reduce fields
ObjectiveTo provide data on typical distribution system current levels, magnetic
field levels near the lines, residential exposures to the public, and
magnetic field management options.
ApproachInvestigators gathered available data on representative current and
magnetic field levels. Using a set of simplified expressions for
calculating distribution line magnetic fields, they identified the
principal parameters governing magnetic field levels and the optimal
strategy for magnetic field exposure reduction.
ResultsThis report provides utility engineers with insight into representative
data on magnetic field levels inside residences and in proximity to
distribution lines. Typical current levels (average current, current
unbalance, and net current) of three-phase distribution lines in the
United States are provided for both the fundamental (60 Hz) and
harmonic frequencies. The report describes commonly used methods for
calculating distribution line magnetic fields based on typical current
levels. In addition, it offers guidelines for performing distribution
line magnetic field measurements, including spot measurements, lateral
profiles, and temporal variations. Finally, it identifies options for
managing magnetic fields from distribution lines. Following are the
most promising field reduction strategies:
o Balanced primary lines can be reduced by compacting the phase
conductors, reducing the line current by increasing voltage, or
increasing the distance to the subject by relocating the line or
raising the conductors.
o Net current primary line magnetic fields can be reduced by balancing
the phase currents, increasing the size of the neutral conductor,
isolating the primary and secondary neutrals, or implementing a
o Secondary line magnetic fields can be reduced by using triplex cable
in lieu of an open-wire secondary configuration.
o Residential grounding system magnetic fields can be reduced by
properly locating the distribution transformer, repairing broken
service neutrals, or inserting a net current control device on the
EPRI PerspectiveEPRI's goal is to provide tools, concepts, and guidelines for reducing
magnetic fields from distribution lines. Magnetic field
characterization and management for distribution lines is more
complicated than for transmission lines due to the inherent variability
of loads and the relatively high proportion of unbalanced current and
harmonic content. Complicating the issue further, distribution lines
have gone through a tremendous evolution over the past several decades,
yet many of the older distribution line designs are still in operation.
While wholesale changes in the design of existing distribution lines
may not be practical or necessary, construction of new overhead lines
with balanced loads may reduce enhance system protection, and result in
low magnetic fields.
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