Richard Strickland’s article in the Jan. 13 edition, concerning de-energized towers that are part of an AM array that is otherwise energized (“May I Climb This De-Energized Tower?”), was very good and timely.
I have a quibble that is more academic or theoretical in nature than practical.
Mr. Strickland states, “Induced currents are a result of forming a branch circuit with the tower. Most of the current flows through the highly conductive steel, but some of it flows through the climber …”
Experimental evidence indicates that the current driven into the climber’s hands and feet is a result of the radial electrical field surrounding the tower. In areas near the tower, the electrical field decreases radially as you get farther from the tower. The fact that the climber’s body hangs away from the tower means that the difference in the electric field across his body will drive a current through it.
There are a couple of experimental observations that demonstrate this.
The first is, the farther your butt hangs out, the higher the measured current in your wrists and ankles will be. The second is that the wrist/ankle current is highest in locations where the electric field is highest on the tower. These locations correspond to where the current in the tower is the least. If the body current were a result of forming a branch, or parallel circuit, with the tower, then it would be expected that the body current would be highest where the tower current is highest.
For radiators that are approximately 90 degrees tall, the highest electrical field occurs at the top. For radiators approaching 180 degrees, this occurs at the top and bottom.
The current (and power) density is highest in the wrist and ankle because these are the areas with the smallest cross section. In most circumstances, the wrist is the controlling area. Typical climber’s boots are not very conductive and are thick enough to reduce capacitive currents through the ankles.
It has been determined that double-gloving with a leather glove over a thick rubber glove will greatly reduce wrist currents.
Much of this information was developed by Richard Tell in the early 1990s as part of an investigation done for the Federal Communications Commission. Reports about this work prepared for its Office of Engineering and Technology include “Induced Body Currents and Hot AM Tower Climbing: Assessing Human Exposure in Relation to the ANSI Radiofrequency Protection Guide” (1991) and “Current Reduction Provided by Work Gloves at AM Radio Broadcast Frequencies” (1993). Copies may be ordered at www.ntis.gov, the National Technical Information Service.
Gray Frierson Haertig
Gray Frierson Haertig & Assoc.
Author Richard Strickland of RF Safety Solutions replies:
Mr. Haertig is correct that the primary source for the current flowing in the wrists and ankles is from the body acting as an E-field antenna, which is the reason that body’s distance from the tower and the field levels at that elevation on the tower are the primary factors in determining the amount of current flow. There are secondary sources, as well, including the body acting as a loop and the body acting as a loaded branch circuit.
I tend to simplify things when I teach as long as it does not impact getting the primary point across, which I also did in the relatively short format of the RF Safety column in this case. I appreciate Mr. Haertig’s comments, which are correct and further explain the phenomena. The practical guidance offered in the article remains the same.