As concern over exposure to low frequency AC magnetic fields has grown
over the past several years, our knowledge of field characterization and effective
control strategies has grown concurrently. It is now possible, through a
comprehensive application of this knowledge, to create living and working
environments that are essentially free of magnetic fields of the magnitudes that
are sometimes associated with adverse health effects, or with disruption to sensitive equipment.
This article will discuss the reduction of fields from sources interior to, and
forming a part of, existing structures, as well as fields from external sources.
It is not intended as a tutorial in the application of these techniques, but rather
a presentation of the wide range of possibilities currently available. It is
especially important that individuals who conduct EMF surveys be aware
of these options for managing the situations that they encounter.
In many cases the magnetic fields present within a building
are a combination of fields produced by power lines on the outside, and electrical
wiring, grounding systems, and equipment on the inside. In regard to power line fields,
an array of alternative transmission and distribution line designs, which reduce magnetic
field levels on adjacent property to a fraction of their previous levels, have been developed
by EPRI (the Electric Power Research Institute) and other organizations. Some of these
designs are routinely incorporated into new construction, but their use as a field
reduction modality for existing systems (retrofit) is extremely rare, and among most
utilities, simply not done. The primary approach available to non-utility service providers
and private individuals is active magnetic shielding (active cancellation). The application
of this technology to large areas such as a whole building has advanced dramatically in recent
years. Nevertheless, for all practical purposes, there are times when we can do nothing. For
those instances in which elevated field levels can be effectively reduced, which comprise the
majority of cases, we will examine a range of viable options.
The most common source of elevated field levels in residential
situations is the phenomenon of "ground currents" or "plumbing currents." This
can also be a problem in commercial buildings. In a perfect world all of
the current flowing into a building on a particular side (phase) of the
electrical service line will return by way of the neutral conductor in the same
service drop. Since these conductors are very closely spaced, most of the
magnetic field will be cancelled. In other words, there will be minimal net current
on the service drop. However, the type of power distribution system
in use throughout the U.S., in combination with important National Electrical Code
requirements, results in a number of alternative neutral return paths. The most
notable being through the neutral-ground bond, into the plumbing system, and from
there into the community water main. For this reason, that which is actually a neutral current
diversion problem is often referred to as "plumbing current".
In a residential or small commercial building, this
flow of current can be blocked very effectively, and in a manner that
is code compliant, by the installation of a dielectric coupler (insulating coupling)
in the water supply line. Several strong cautions are necessary here!
If this action is taken without an analysis of the nature of the problem, and if
the integrity of existing electrical facilities is not verified, extremely hazardous
conditions can be created. In addition, a large sum of money will have been
spent on an ineffective solution. In large commercial buildings, the presence of grounded
building steel, on-site power generation facilities, transfer switches, and
uninterruptible power supplies can create a considerably more complex net current situation.
Nevertheless, a resolution is generally possible. It should be noted that in a few
cases an adequate remediation of ground current fields can be attained by a simple
relocation of grounding conductors.
Interior Electrical Wiring
A number of common wiring errors and irregularities have been identified
which create significantly elevated field levels that have a widespread spatial
distribution throughout a building. Some of the most common are: cross
connected neutrals from separate branch circuits, neutral-ground shorts (in
sub-panels or anywhere else), unusual three-way switch wiring, and half-switched
duplex outlets. The variety of troublesome configurations seems limited only by
the resourcefulness of the installing electrician. These problems are neither
unusual nor infrequent. The author recently surveyed a small commercial building and
found not only a significant ground current problem, but three neutral crosses and
one receptacle circuit that was fed from two separate breakers (in different panels).
There were high magnetic fields throughout the structure, and a serious lack of overcurrent
protection on the double fed receptacle circuit. This building had just passed inspection by
local code enforcement authorities. These types of
problems are equally common in a residential setting. They are
usually resolved by locating the error and rewiring, a task that is greatly
facilitated by good troubleshooting methodology and basic wire tracing equipment.
The old knob and tube wiring used extensively in the early part of this
century and occasionally seen in old homes is often in reasonably good condition
and fully functional. The wide spacing of conductors, however, produces high
magnetic fields. This system uses white porcelain insulators to support single
insulated wires and prevent them from contacting wood structural members. The
solution is replacement with modern wiring.
Another obsolete system of more recent vintage is radiant ceiling heat. In
addition to consuming vast amounts of electricity, it can produce magnetic fields
as high as 10 to 15 mG in the rooms where it is installed. Again, the solution is
replacement with a different system.
Prior to the last decade, it was a commonly held belief that low frequency
magnetic fields could not be effectively shielded. This is clearly not the case.
High permeability alloys, as well as less exotic metals used in combination, have
proven both effective and practical in controlling ELF magnetic fields.
Shielding is being installed in more and more commercial buildings, as tenants and
building owners become aware of the extremely high magnetic fields that often exist
near the electrical power control and distribution systems of the building.
Two basic modes of shielding utilization are recognized:
containment and exclusion. The enclosure of an electrical switchgear room by shielding material
in order to protect people or equipment in adjacent offices from high field intensities
is a common application, and an example of containment shielding. Contrast this with the
enclosure of a room containing sensitive monitoring, diagnostic, or research
instrumentation (and people) in order to provide exclusion of externally produced
Despite advancements in our knowledge of shielding applications,
it is still impractical to consider protecting a home or office from power line fields using conventional
metallic shielding. The sheer volume of material required, and our need for windows, makes this job
impractical. A far better approach would be active field cancellation.
Active Magnetic Field Cancellation
These systems work to negate the presence of an off-site
magnetic field from a power line or other source by creating, within a defined area,
a field that is equal in magnitude and opposite in phase. A loop of wire is
placed around the protected area and a control unit automatically adjusts the current in this
loop to just null out the field within the area. To the extent that an equal and opposite field
can be created, both fields will cease to exist. Magnetic field cancellation may initially appear as a universal solution for power line fields,
but there are drawbacks which limit its applicability. Adequate free space must exist
around the buiding or area where correction is desied, and an overhead cable is required
at some sites. There are, nevertheless, many circumstances in which these systems
can work effectively to produce a favorable result.
In the application of technical mitigation strategies,
it is often easy to neglect the simple, common sense steps which can provide completely
effective exposure reduction. Increasing the distance between people and equipment is
sometimes all that is required in the office or work environment. Converting the
classroom nearest the power line to a storage area or placing file cabinets along
the wall next to the electrical closet accomplish this same result. In other cases,
space is so limited or so valuable that remediation is fully justified. Although we
have focused on existing structures here, it should be noted that the
incorporation of low field configurations at the architectural design stage is far
more cost-effective than coming back later to change things around. The range
of options is also far greater.