SAFETY COMES FIRST!
"While flying a control line model airplane, a 28-year-old man was electrocuted when the metal control wires contacted one or more uninsulated power lines traversing the field where the plane was being flown."
The above statement is the synopsis included in an accident report on file at the Consumer Product Safety Commission. It is one of eight similar reports involving contact with power lines by a control-line (CL) model, all of which occurred in the last six years. In seven of these incidents the modeler died.
It is hard to believe that control-line fliers would deliberately fly their models close to power lines. However, we observed such a practice taking place just last year at the Nationals in Lincoln, Nebraska. Several fellows were fun-flying in a ball field just outside the dormitory where we were accommodated. Surrounding this field were power lines for the lighting system. Fortunately, nobody had the misfortune to contact these lines.
A near miss in England
Many years ago I witnessed what could easily have been a tragic accident caused by contact with overhead power lines. At that time I was living in England and was a member of the Vickers-Armstrongs model club. We had the use of several playing fields for control-line flying whenever they were not in use for soccer or cricket. Traversing the edge of one of these fields was a canal and following the canal were power lines which were part of the National Grid. I believe these carried a voltage of 67,000 volts.
On the day of the incident a stiff breeze was blowing (it's always windy in England) toward the lines. A modeler took off with a rich engine setting and had trouble keeping control lines taut when the model was upwind. Each time the lines started to go slack he took several paces downwind. After a dozen or so laps it was obvious to the rest of us that if this practice continued he would be in danger of contacting the power lines. We began shouting at him to alert him of the peril but the sound of the engine drowned out our voices. The end of the flight came when the model's outboard wing was cut off by one of the power lines. Six inches closer and we would have lost one of our friends.
We theorize that many of the incidents of electrocution that have occurred did so because the modeler moved his position during the flight, probably because the wind was causing his lines to go slack.
Electrocution and high-voltage arcing
In all of the accident reports involving electrocution of control-line fliers the power lines that were contacted carried several thousand volts. At these voltages contact with the wires is not necessary to receive a shock. Close proximity to a high-voltage source by a grounded body can result in an arc jumping the gap.
Generally speaking, it takes about 70,000 volts for an arc to bridge a one-inch gap. Fortunately, most overhead power transmission lines are at lower voltages than this figure, but bear in mind that the maximum voltage that is used in this country on transmission lines is 765,000 volts.
How electric shock works
Lest this discussion of electrocution by sources of many thousands of volts give you a false sense of security when dealing with the mere 120-volt outlets in your home, let's take a quick look at the mechanism of electric shock.
Most of us have probably contacted an electrically live wire in the home and have felt either a tingling sensation or a distinct jolt depending on how much current passed through our bodies. Notice that I used the word current rather than voltage. While it is the voltage of the electrical source that, to a large extent, determines its hazard, it is the amount of current that flows through the body and its duration that will determine the end result.
For example, suppose we are standing on a carpeted floor and touch a finger to an electrical source of 120 volts (the voltage of most household electrical appliances: soldering irons, power drills and saws, battery chargers, and other electrical items used in the pursuit of our hobby). If we have dry hands in such a situation we will most often feel merely a tingling sensation at the point of contact. The nerve endings in the finger are being stimulated to the sensation of touch and warmth. For some people this sensation can be produced by a current flow of 0.5 milliamps. This current is commonly known as the threshold of perception.
Since the workshops of most of the modelers that we know are located in the basement of their houses, it is more than likely that if they are going to contact a live wire at the household voltage of 120 volts they will be standing on a concrete floor, perhaps even with bare feet. Now we have what could be a serious situation. Our body is grounded and provides a more ready path for the flow of electricity. That tingling sensation now changes to a distinct jolt in which our muscles involuntarily contract. If the live electrical wire was grasped in the palm of our hand and if the current flow reaches about 10 milliamps it is possible that the involuntary contraction of our muscles will prevent us from letting go of the wire — a decidedly unpleasant situation.
Electrocution deaths are usually caused by fibrillation of the heart. Instead of beating with a normal rhythm, the heart muscles contract rapidly and irregularly and a person with this condition will die very quickly. Fibrillation of the heart can be caused by a current flow through the body as low as 50 milliamps — not much more current than we use to recharge the average RC airborne flight pack. You might ask why we don't get a lethal shock from the wires when we connect our NiCads to a battery charger. Simple: at the output voltage of a battery charger our bodies have sufficient electrical resistance to prevent a current flow of 50 milliamps. Of course, grasping the input wires which are at 120 volts is a different story.
Generally speaking, AC voltages below 30 volts will be insufficient to cause a lethal current flow.
Battery chargers and shop safety
Not all of the chargers used for RC equipment use a transformer to obtain the correct output. Some older models frequently used a dropping resistor connected in series with a single diode to obtain the appropriate DC voltage. If you happen to have one of these, as I do, always connect the batteries to the charger before plugging the unit into the wall socket. Conversely, always disconnect the plug from the wall receptacle first. While dropping resistors work, transformers which isolate the output from the high-voltage input are much safer.
The point of all this discussion is to give you some idea of how small a current flow is required to cause an electrical shock. The actual current that will flow through the body when it contacts an electrical source at a given voltage will vary according to how the source is contacted (finger touch versus whole-hand grasp, etc.) and how well we are grounded.
Control lines and insulation
Returning to the subject of contact with overhead power lines, almost all uninsulated power lines that are likely to be located in or around our flying sites are going to be at potentials of several thousand volts. Lest it be thought by some that the relatively long and small diameter of the model control lines will produce an effective dropping resistor between the electrical source and the flier, let us correct you.
We know of a case in which a person was flying a kite with a line that contained a metallized tracer. Such a thread had a much greater resistance than even the smallest diameter steel wire used in control-line flying. The person was electrocuted when the kite fell across a power line.
It has been suggested that some form of insulator be incorporated in the control lines and the handle should be used. It is doubtful that an insulator that would protect a person from a source of over 100,000 volts (some power lines exceed this figure) could be devised that would not also interfere with the control of a model with a conventional two-line system. An insulating device was suggested in an article that appeared in 1976 in the Journal of the Institute of Electrical Inspectors. The writer of the article also suggested, "How about Underwriters (Labs) taking this up and coming up with a Listing and set of standards for airplanes?" How about it indeed?
Do we want this to happen, or would we rather avoid an accident by flying only in locations devoid of power lines? Although we seldom fly CL models these days, we know which alternative we prefer. Although it's your life and while you may think that you have the right to do with it what you wish, give some thought to the fact that your name on a coroner's report may be one step closer to having some outside legislation affecting the hobby for the rest of us.
DO NOT FLY NEAR OVERHEAD POWER LINES.
John Preston 7012 Elvira Ct. Falls Church, VA 22042
Transcribed from original scans by AI. Minor OCR errors may remain.
