Effects of Power Line or other Source Distance on EMF Measurements InspectAPedia® -
What is the effect of the distance to a power line or other EMF source when making EMF measurements?
What is the effect of season & time of day on accuracy of EMF field strength measurements?
How far away from a power transmission line does a building need to be to measure no effects?
Sources of error and variability in electromagnetic field strength measurement surveys
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This paper discusses the effect of distance from a power transmission line (or other EMF sources) when performing electromagnetic field (EMF) or
electro-magnetic radiation EMR measurements to measure EMF exposure levels in gauss or milligauss. We discusses
sources of error and variation in EMF measurements and we review and make suggestions for using several low-cost EMF
measurement devices to determine the instantaneous electromagnetic field exposure.
See ENVIRO-SCARE, EMF & Property Values if you don't know what EMF, ELF, or electromagnetic fields are or if you want a summary of the possible health effects of EMF exposure and the more likely effect on the property value of homes located very close to power transmission lines. Readers who intend to make their own EMF measurements should be sure to also see WORKSHEET for EMF MEASUREMENTS and also WORKSHEET for EMF MEASUREMENTS - Example.
The information provided here is for research and study purposes. The author makes no representation of unique
expertise on this topic, other than having field experience in EMF measurement, having studied technical literature and
having conversed with other experts and authors in the field for a number of years.
EMF strength effects and distance from the electromagnetic field source
The strength of an electromagnetic field diminishes as the square of the distance one is from the power line or field
source. So in general, walking closer to a power transmission line will give a higher reading.
But near a large power
transmission line, since the electrical field has a "shape" and since a building owner's property line may not be exactly
parallel to the power transmission lines themselves, I've found that field strengths along a line parallel to the edge of
a property may vary widely.
At our little decimal point demonstration at see Comparing Gauss versus Milligauss Field Strength Measurements where we explain the greater accuracy of low-level EMF readings when using the milligauss scale on an ELF EMF meter, you can see a measurement of about 25 milligauss obtained when the instrument was touching a 120VAC to 12VDC power converter that draws about 1Amp. But if we move the instrument just a foot away we will probably not detect this field whatsoever. That's why, unless you sleep with your electric clock right next to your head, you are probably not going to be affected by its electrical field.
The electromagnetic field produced by a step-down electrical power transformer on a local electrical system distribution utility pole such as the one shown in our photo at left can be measured easily from the street below the pole.
If the power transformer is just a meter or two outside of a bedroom window, it's likely that one can measure this electrical field inside. At greater distances it's not likely that you can detect this effect.
Power transmission facility design also affects the strength of the EMF generated.
Load or phase balancing for
overhead transmission lines, distance between electrical conductor wires, and the choice to use buried transmission lines (which are much more costly
to install) can make a very significant difference in the measurable EMF.
Distances from power lines and EMF field strength: The distances for common field strengths and power lines are available in a number of EPA documents, such as
"Evaluation of the Potential Carcinogenicity of Electromagnetic Fields," EPA/600/6-90-005B October 1990 (DRAFT review
copy), page 2-21.
For example, the field strength of a 500 KV Transmission line begins to fall off measurably at 50
meters, but does not fall off below 1 mG until distances nearing 1000 meters.
Keep in mind that independent of proximity to power transmission facilities, a careful survey of conditions in any
building may reveal other devices like home appliances or electrical service entry cabling: areas where strong EMF can be measured from distances of a few inches to several feet away.
Also keep in mind that based on our own field experience, we note that it would be unusual in any urban or suburban environment to find ambient EMF levels below one or two milligauss.
EMF strength variation and seasonal or time of day variations in electrical power usage levels
But distance from the power transmission line, while important, is not the only important factor. The strength of the
electromagnetic field varies dramatically as the current passing through the power line varies.
Thus in the middle of
summer in the Northeastern United States, when many people are running air conditioners and thus the load on the
electrical grid is high, a lot of current is passing through the power transmission lines, and the EMF strength will be
quite high - thus extending further from the power lines and being measurable at higher levels than it will during times
of low electricity usage. Therefore no single instantaneous EMF measurement at a particular spot may be quite
repeatable.
A power transmission company can tell you the kilovoltage that a given transmission line is designed to carry.
This is not enough data to calculate exposure or risk. One would need to know the actual minute-to-minute
load on the transmission line to be able to predict the probable EMF strength during those intervals.
The absence
of this data has plagued attempts to correlate proximity to power transmission lines, EMF exposure, and health risks.
The "Swedish study" mentioned herein was able to overcome this difficulty and obtain actual usage data and thus
was able to calculate the actual EMF exposure levels.
Often but not always, the relative strength of
such fields falls off in much shorter distance than that from power transmission facilities. However in some instances
where occupants wish to maintain prudent avoidance, it is possible to make a significant reduction in exposure by small
changes in arrangement of devices or locations of working or sleeping areas.
Power companies in the US have been singularly uncooperative in providing actual load data, making it difficult to
establish a dose-exposure relationship between exposure to EMF and occurrence of disease. This is why the Swedish
studies are so important. There the government cooperated with researchers in providing load data, permitting clear
establishment of exposure to occurrence relationships.
Instead of contacting us with a request to perform EMF Electromagnetic Field Strength measurements, in most cases it is more economical and convenient for a property owner to purchase their own instrument, making measurements under varying conditions. In this series of articles we describe how to make measurements using a consistent approach and using good documentation. See Recommended EMF Measurement Procedure for details of how to collect EMF measurement data.
Following good procedure and using instruments properly are two steps towards making accurate, repeatable EMF measurements. But because the load on a power transmission line is not under control of an individual property owner, and because the EMF strength varies as the power transmission line load varies, it is important to have an idea of that condition as well when attempting to characterize EMF exposure at a specific location. In contrast, EMF measurements are quite accurate and repeatable at other EMF sources such as close to electrical appliances and service entry cables.
Please do not contact us with a request buy EMF measuring equipment. We do not sell anything. To do so would be a conflict of interest for this website. These devices are readily available from many electrical equipment and home inspection equipment suppliers. See Evaluation of Low-Cost EMF Instruments where we describe several low-cost and reasonably accurate EMF measurement devices that are readily available.
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Additional technical contributors & reference sources for this article are listed below.
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US Environmental Protection Agency, Office of Pesticides
and Toxic Substances, TSCA Assistance Office (TS-799), 800-424-9065
or 202-554-1404.
"Evaluation of Potential Carcinogenicity of Electromagnetic Fields,"
EPA Report #EPA/600/6-90/005B October 1990. EPA: 513/569-7562.
"Biological Effects of Power Frequency Electric and Magnetic Fields"
background paper, prepared as part of OTA's assessment of "Electric Power
Wheeling and Dealing: Technological Considerations for Increasing Competition,"
prepared for OTA by Indira Nair, M. Granger Morgan, H. Keith Florig, Department
of Engineering and Public Policy, Carnegie Mellon University, Pittsburgh, PA
15213
"Biological Effects of Power Line Fields," New York State Powerline
Project. Scientific Advisory Board Final Report, July 1, 1987.
"Extremely Low Frequency (ELF) Fields," Environmental Health
Criteria 35. World Health Organization, Geneva, 1984.
"Electric and Magnetic Fields at Extremely Low Frequencies:
Interactions with Biological Systems. In: Non ionizing Radiation Protection,
World Health Organization, Regional Office for Europe, Copenhagen, 1987.
"Electric and Magnetic Fields from 60 Hertz Electric Power: What do
we know about possible health risks?," Department of Engineering and Public
Policy, Carnegie Mellon University, Pittsburgh, PA 15213 1989.
"Electromagnetic Fields Are Being Scrutinized for Linkage to
Cancer," Sandra Blakeslee, New York Times, Medical Science section, April
2, 1991
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