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Lightning Explained, Part 3
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. Therefore the effect is mostly determined by the particular nature or waveform produced by the slowly varying field.
How we got here
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 that he had accidentally discovered: upward leader initiation. 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.
Modern knowledge of electric discharges however confirms that the sharp-tipped rod is the worst conceivable shape for a lightning rod. This is due to an intrinsic quality of the rod's geometry, more than any other simple shape, it is prone to production of electric discharge under the slowly varying electric fields that precede lightning strikes.
So for the last 250 years we have been using a device, the sharp tipped lightning rod, in a situation for which its shape 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. Also since the effect becomes more pronounced with increasing structure height, as we continue to build taller structures, the more these effects will 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 an ordinary rod and not as Tesla had intended.
With a modern understanding of how grounded objects interact with lightning, and with full consideration for the effects of space charge shielding, Lightning Electrotechnologies Inc has developed a series of lightning rods based in peer reviewed science.
The F-SAT (Field Sensitive Air Terminal)
A patented electrode/lightning rod with a unique shape 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. (IEEE STD 1243, EPRI Reference Book)
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. 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. All conducting objects except the F-SAT are subject to that kind of degradation.
The streamer-inhibiting electrode
This is the complete opposite strategy as compared to the F-SAT. Here the objective is to maximize the effects of space charge shielding. The Streamer Inhibitor is a patented composite electrode consisting of a conducting support structure for a coil, which is made of micro thin conducting fibers. Under the same ambient field conditions this device will produce more charge than any other known configuration.
Furthermore it does so without producing streamers, it produces electric discharges exclusively in the pulse-less glow mode over a very broad voltage range. Glow-mode corona is a silent and typically invisible discharge phenomenon. It produces a dc current rather than the sub-microsecond pulses associated with streamers. Below a critical rate of some tens of kilovolts per microsecond, the Streamer Inhibitor also maintains glow mode corona under switching impulses or very fast voltage rise rates. Therefore in addition to producing large volumes of charge, even after the arrival of the descending stepped leader, the Streamer Inhibitor, unlike a point discharge device, continues to produce space charges that suppresses streamers, which in turn suppresses leaders in a defined although limited area around the Inhibitor.
The Streamer Inhibitor represents the most that can be done to reduce a structure’s lightning-attractive area, suppress leader formation and reduce the probability of a direct strike to a structure. This can have a tremendous effect on tall slender towers or isolated masts but very limited effects on low, massive structures. However the location or origin of any upward connecting leader can be significantly influenced by the simultaneous use of Streamer Inhibitors and F-SATs and reduce risk at sensitive points on low structures.
Streamer Inhibitors are modular and come in the form of toroidal air terminals of various sizes as well as a long conductor.
1. NFPA International, Standards Council Decision (Long Form): D301-26
2. F.A.M. Rizk, "Modeling of Lightning Exposure of Buildings and Massive Structures" IEEE Trans. On Power Delivery, Vol. 24, No.4, pp. 1987-1998 October 2009
3. M.A. Uman and V.A. Rakov, "A Critical review of Nonconventional Approaches to Lightning Protection" American Meteorology Society, December, 2002
4. C.B. Moore, W. Rison, J. Mathis and G. Aulich, "Lightning rod improvement studies", Journal of Applied Meteorology, Vol.39, pp.593-609, 2000
5. F.A.M. Rizk, "Modeling of Lightning Exposure of Sharp and Blunt Rods" IEEE Trans. On Power Delivery, Vol. 25, No.4, pp 3122 -3132, October, 2010
6. D. Mackerras, M. Darveniza, A.C. Liew, " Review of claimed enhanced lightning protection of buildings by early streamer emission air terminals", IEE Proc.-Sci Meas. Technol., Vol. 144, No. 1. January 1997
7. I.D. Chalmers FIEE, J.C. Evans and W.H. Siew MIEE, IEE Proc.-Sci Technol, Vol. 146, No. 2, March, 1999
8. N. Knudsen, F. Iliceto, "Flashover tests on large air gaps with dc voltages and with switching surges superimposed on dc voltage", IEEE Trans., Vol. PAS-89, No.5/6, pp. 781-788, May/June 1970
9. F.A.M. Rizk, "Rocket-Triggered Lightning: Modeling of Trigger-Wire Corona Effects" International Conference on Lightning Protection, ICLP, Cagliari, Italy, September 2010
10. F.A.M. Rizk, "Modeling of lightning Incidence to tall structures.", IEEE Trans., Vol. PWRD-9, pp. 162-171, Jan. 1994
11. US PATENT 1,266,175
12. F.A.M. Rizk, "Analysis of Space charge Generating Devices for Lightning Protection: Performance in Slow Varying Fields", IEEE Trans. On Power Delivery, Vol. 25, No. 3, pp. 1996-2006, July 2010
13. F.A.M. Rizk, "Exposure of overhead conductors to direct lightning strikes: Modeling of positive streamer Inhibition", IEEE Trans. on Power Delivery, Vol. 26, No. 2, pp. 1156 - 1165, April 2011
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