Electrical power transformer on local distribution lines in Mexico  (C) Daniel Friedman Effects of Power Line or other Source Distance on EMF Strength Measurements

  • EMF MEASUREMENT DISTANCE AFFECTS STRENGTH - CONTENTS: 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? Distances from the source of an EMF or RF field as sources of error and variability in electromagnetic field strength measurement surveys. HOW TIME of DAY & SEASON IMPACT EMF STRENGTH
  • POST a QUESTION or READ FAQs about the effect of distance from power lines or other EMF (or RF) sources on field strength & exposure
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Effects of EMF exposure as a function of distance from the power transmission line or other source of electromagnetic fields: an easy-to-understand summary.

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.

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EMF strength effects and distance from the electromagnetic field source

Power substation (C) Daniel FriedmanThe 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 single 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.

Electrical power transmission lines at the US Canadian border (C) Daniel Friedman

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 or RF Radio Frequency 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 signal transmission for RF sources such as radio, TV, or cell towers, 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 or RF 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

This article describes several low-cost and reasonably accurate EMF measurement devices that are readily available. See Radio Frequency RF Detection Meters This article describes several low-cost and accurate radio frequency or RF detection and measurement devices suitable for radio, TV, cellphone, microwave, and similar signals.

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.


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