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Lightning Explained, Part 3
The scientific views expressed in this article are those of Dr. Farouk A.M. Rizk as explained by Amir Rizk. Dr. Farouk Rizk is the former VP (Research Laboratories), Hydro Quebec, and former Scientific Director at IREQ. Amir Rizk (B.Sc., phys, MBA) is VP of Lightning Electrotechnologies Inc and a student of Dr. Rizk. Any technical matters dealing with lightning protection adhered to previously published, peer reviewed material.
In 2000, a group of well-respected American scientists published a paper in the Journal of Applied Meteorology entitled “Lightning rod improvement studies”. In my view, this paper marks a turning point in the history of lightning protection science. Although the analysis of the data has evolved considerably since publication, the experimental design and data collection continues to educate and inspire.
What the American scientists showed, through long-term field studies of lightning rods exposed to real lightning, was that even objects as similar as sharp rods, moderately blunt rods and very blunt rods, placed at the same height in the same location, would interact differently with lightning and produce different results. The sharp rods were never struck whereas moderately blunt rods were struck most frequently.
Some criticism of the work was that the data was not statistically significant and this is true when the rods are grouped in terms of their geometric shape alone. However, if the rods are grouped in terms of their propensity to produce electric discharges under the slow varying electric field that precedes a lightning strike, the numbers make a very strong case.
Space charge shielding
The mechanism of space charge shielding was first observed in the 1970s during studies of design parameters for HVdc power lines. A main feature of all HV power line designs is their ability to resist the critical voltage surges that occur during normal switching operations as well as fault conditions. It was observed that if the dc line is actively producing discharges under the dc system voltage, something that is generally undesirable as it amounts to energy losses, the line is actually more resistant to the effects of these critical voltage surges when they appear.
It is interesting to note that the voltage waveform produced by these voltage surges or switching impulses, are very similar to the waveforms produced by the descending lightning leader, particularly in the last 30 percent before the peak, which is the most important part for leader initiation.
The analogy to lightning starts to become evident.
In a direct competition, under the same conditions, the object that produces the most discharge activity during the slowly varying dc-like electric field that precedes a lightning strike is the one that is the most resistant to the switching impulse-like peaks that result from a descending lightning leader. So it's not merely a question of blunt or sharp but rather a question of more preceding discharge activity or less.
In the American field experiments, like the sharp rods, the largest most blunt rod (51mm) were never struck. It can be seen from the collected data that these larger blunter rods produced even more charges during the slowly varying electric field than the moderately blunt rod (19mm). This could only be explained by the presence of water droplets on the surface of the larger rods, which acted like conducting protrusions with small radii of curvature that caused them to produce charge at rates comparable to sharp rods. The same will be true of just about any object that is exposed to rain and has a surface area large enough to collect raindrops. The presence of raindrops on the surface of a conducting object, under high voltage, causes the water droplets to elongate and typically has an electric effect comparable to covering the object with needles.
The moderately blunt rods, which were struck the most, were small enough (19mm diameter and thus about 14 percent of the surface area of the 51mm rod) such that it was unlikely for a water droplet to settle on their surface. Furthermore the height used in the field tests, 6m masts, produced space potentials (effective applied voltages) very close to the level where the 19mm rod would just begin to produce discharges under the slowly varying field. Had the tests been conducted with mast heights of say 10m, they would not have detected any difference between the rods tested since they all would have been raging with similar discharge activity. Alternatively, had they chosen a test height of 1m, none of the rods would have produced discharges and again there would have been no observable difference.
-- continued on page 2
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