Human Radio Frequency Exposure from SCADA Remote Sites – Technical Note 125

by Gordon Schlesinger, Ph.D

The public, workers, and risk managers often raise questions concerning human exposure to radio frequency (non-ionizing RF) energy that might be expected from proximity to antennas, such as at a remote SCADA site.  This brief note will provide calculated exposure values and will relate them to U. S. Federal government maximum permitted exposure limits.

The current human exposure radio frequency energy limits were adopted jointly in 1996 by the Federal Communications Commission (FCC) and the Occupational Safety and Health Administration (OSHA).  In FCC Bulletin OET-65, Evaluating Compliance With FCC Guidelines for Human Exposure to Radiofrequency Electromagnetic Fields, the FCC provides both its maximum accepted exposure limits to radio frequency energy for humans, and equations for calculating exposure from transmitters and antennas of various power levels and operating frequencies.

For purposes of example, let us consider a SCADA remote station operating at 950 MHz, with 5 watts of radio frequency power (equivalent to 5,000 milliwatts, the value used below).  The antennas used with the remote provide 10 decibels of power gain relative to a half-wave dipole, or 12.15 decibels relative to an isotropic radiator.  This is equivalent to a numerical gain of 16.4 times compared to an isotropic radiator, yielding an isotropic effective radiated power in front of the antenna of 82 watts.

The maximum permitted human exposure for non-occupational workers at 950 MHz is 0.63 milliwatts/cm² for a continuous period of 30 minutes.  The boundary of this power density is usually considered an “action level” for safety considerations. This is determined from the FCC-supplied equation for energy density:

where   S is the power density in milliwatts/cm² 

            P is the transmitter power in milliwatts

            G is the power gain of the antenna in the direction of interest

            R is the distance of the human subject to the antenna in cm.

Using the maximum exposure value 0.63 mw/cm² for S, and solving for R, we obtain a value of 101 cm; this is equivalent to slightly over 40 inches.

Applying what we have just calculated, and noting that the SCADA antennas are typically mounted atop a 20’ high pole (or higher), we can interpret this result.  In order for a non-occupational person to exceed the maximum allowable dose of radiation from this typical SCADA antenna, he/she would have to position himself/herself directly in front of the SCADA antenna, about twenty feet in the air, and within three feet of the front of that antenna.  AND that individual would have to remain immobile in this position for a minimum of 30 minutes.

Even then, biological damage would be unlikely, as the FCC/OSHA exposure limits also contain generous safety factors far below the laboratory-determined tissue damage exposure level.

As a subsidiary case, consider the situation of a human standing on the ground twenty feet in front of the 20’ high SCADA pole.  What would be the exposure at street level?

Application of both simple trigonometry and the above equation yield a maximum radio frequency energy level on the street of 0.00887 mw/cm².   This is 71 times lower than the maximum permitted level, and it is an overestimate.  The antenna concentrates its energy toward the horizon, and not at the street.

Thus it can be concluded that the probability of dangerous human exposure from the RF of a similar SCADA remote unit under practical conditions is very small indeed. However, every radiating situation and workplace is unique and should be fully investigated and documented.

Further, it should be noted that all FCC licensees are responsible to certify compliance with the Commission’s emissions requirements that include the contributions from any other antennas in the vicinity. In addition, it is important that appropriate signage and/or restrictions be in place for “action level” areas. In complex situations, sophisticated modeling software in the hands of an RF specialist can provide a relatively painless definition of occupational and non-occupational considerations. One such expert resource is Lawrence Behr Associates, Inc.

Note: Gordon is a member of the LBA engineering staff with many years of experience in the utilities industry. He has been responsible for both RF hazards compliance and major SCADA programs.