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And then there was the lightning rod

May 9.2011 -- This technical treatise on lightning rods (air terminals) came from Mr. Amir Rizk's writings and the company Lightning Electrotechnologies, Inc.

In 2000, a paper was published in the Journal of Applied Meteorology entitled “Lightning Rod Improvement Studies”. This paper marked a turning point in the history of lightning strike protection. Although the analysis of the data has evolved considerably since this publication, the experimental design and data collection continues to improve.

Field studies showed that lightning rods exposed to real lightning whether sharp rods, moderately blunt rods or very blunt rods, placed at the same height in the same location, would interact differently 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.

However, in the American field experiments the 'largest' most blunt rod (51 mm) were never struck either. It was observed that these larger blunter rods produced even more charges during the slowly varying electric field than the moderately blunt rod (19 mm). 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 (19 mm diameter and thus about 14% 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, 6 m masts, produced space potentials (effective applied voltages) very close to the level where the 19 mm rod would just begin to produce discharges under the slowly varying field. Had the tests been conducted with mast heights of say 10 m, 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 one meter, none of the rods would have produced discharges, and again there would have been no observable difference.

The mechanism of space charge shielding goes beyond simple rods but can also explain much of the seemingly mysterious behavior of lightning. For example, the reason tall towers are sometimes hit below the top may simply be that discharge activity from the tower top during the slowly varying electric field, shields the tower top in the same way as was observed with the sharp rods. And so when the descending leader arrives, a lower part of the tower is more prone to producing the upward connecting leader as compared to the tower top. But it will only occur if the slowly varying field reaches critical values for sufficient durations relative to the timing of the arrival of the descending leader. It is predictable but sporadic.

There is absolutely nothing that can be done to influence any aspect of the descending stepped leader. However it is possible to influence a grounded structure's propensity to produce upward leaders and from where on the structure such leaders would emanate. The taller the structure and the more charge associated with the particular descending stepped lightning leader, the more the length of the upward leader from the structure can grow. So for a low structure with a large foot print, the probability of being struck will not be greatly influenced by the building's ability to launch leaders. However, for tall slender towers, their ability to launch long leaders is the reason they are struck so frequently. Influencing the possibility of producing upward leaders from a structure will thus have a much greater effect on taller more slender structures as compared to low massive ones.

For very tall towers, in the 100 meter range and more, the possibility of another kind of lightning strike comes into play. These structures can generate their own lightning. An upward flash is a lightning strike where the upward connecting leader is created without there having been any descending leader to which to connect. It is true that such strikes can be initiated by nearby flashes but the distinction is that these upward flashes do not connect with any descending lightning leader but rather, move towards localized charge centers in the clouds. Return strokes of comparable magnitudes to downward lightning result.

It is with these structures that the most can be done to influence the probability of a direct strike. The production of charges at the tops of these very tall towers can shield them enough to raise the required space potential for upward leader initiation to the point of significantly suppressing their occurrence. It is not clear however that this effect can be significantly enhanced through the use of devices that use sharp conductive points. Any conducting protruding object or even the edges or corners of such tall structures would also produce charges via the same mechanism, streamers and in similar rates. Thus, the effect is mostly determined by the particular nature or waveform produced by the slowly varying field.

When Ben Franklin first proposed mounting needle sharp iron rods at the tops of buildings, he didn't think that they would be hit by lightning. He initially thought that such rods would discharge the clouds and so his choice of using sharp tipped rods had no connection with the mechanism which he had accidentally discovered, i.e., upward leader initiation. Had Ben used any grounded conducting object that did not include a protruding point, it would have been slightly less prone to discharge activity and performed better as an initiator of upward leaders, with fewer failures. Franklin's genius was to mount any grounded metallic object above a structure and to adapt his views to what he observed rather than proposed.

So for the last 250 years we have been using a device, the sharp tipped lightning rod, in a situation for which it is perfectly unsuited. Of course the effect of rain limits the performance of any other shape which is why this detail has eluded us for so long. In addition, the effect becomes more pronounced with increasing height, as we continue to build taller structures and thus the more these effects become apparent.

Interestingly Nicola Tesla nearly solved the problem. He somehow recognized that the sharp tipped rod's propensity for discharges was a bad thing. And so he designed a device, with the intent of resisting the production of electric discharges. However what Tesla did not recognize is that when exposed to rain and covered in water droplets, like almost all objects, his device produced discharges comparable to a sharp rod and thus function as a ordinary rod and not as Tesla had intended. With a clear understanding of how a lightning rod is supposed to work, and with full consideration of the effects of space charge shielding, Lightning Electrotechnologies Inc has developed the first ever lightning rods based in peer reviewed science.

The F-SAT (Field Sensitive Air Terminal) A passive electrode/lightning rod that resists the production of electric discharges under intense electric fields and under rain and thus never self protects via space charge shielding and always maintains the maximum attractive area for its position. When properly placed, lightning protection systems employing the F-SAT ensures that any upward leaders emanating from a structure will be from the F-SAT and not below or nearby. The F-SAT is suitable wherever lightning protection is needed and can be used to enhance an NFPA 780 compliant system.

Other installation methodology options using currently employed and recommended practices for power lines can be applied to any structure with the possibility of significant savings and without compromising security. The F-SAT is not a one size-fits-all solution. A 60m high building corner does not experience the same electric field intensification as a 60m high slender tower or a low 20m building. F-SATs come in a number of sizes designed to meet these varying conditions.

It is important to note that the F-SAT does not offer an enhanced zone of protection as compared to any conducting object; it merely prevents the type of degradation to a lightning rod’s attractive area described above. Particularly under rain, which is closely associated with lightning, all conducting objects except the F-SAT are subject to that kind of degradation.

Need a lightning consultant to protect your building or tower correctly with the latest in air terminal design for your structure? If you do then LPGI & Affiliates is your answer.


LPGI & Affiliates
962 Coronado Drive
Sedalia, CO 80135-8303
303-688-5800
Fax: 303-688-5551

 

 

 

 

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