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Lightning protector sometimes possess a [[short circuit]] to the [[Ground (electricity)|ground]] that is interrupted by a thin non-[[conductor (material)|conductor]] over which lightning jumps. Ideally, the underground part of the assembly should reside in a muddy area, or an area that tends to become so during storms. If the underground cable will resist [[corrosion]] well, it may be covered in [[salt]] to improve its electrical connection with the ground. In [[telegraphy]] and [[telephony]] a lightning protector is placed where wires enter a structure, preventing damage to electronic instruments within and ensuring the safety of individuals near them. Similarly, [[Electricity pylon|high-tension power lines]] carry a lighter conductor wire over the main power conductors. This conductor is grounded at various points along the link. Electrical substations usually have a web of the lighter conductor wires covering the whole plant.
Lightning protector sometimes possess a [[short circuit]] to the [[Ground (electricity)|ground]] that is interrupted by a thin non-[[conductor (material)|conductor]] over which lightning jumps. Ideally, the underground part of the assembly should reside in a muddy area, or an area that tends to become so during storms. If the underground cable will resist [[corrosion]] well, it may be covered in [[salt]] to improve its electrical connection with the ground. In [[telegraphy]] and [[telephony]] a lightning protector is placed where wires enter a structure, preventing damage to electronic instruments within and ensuring the safety of individuals near them. Similarly, [[Electricity pylon|high-tension power lines]] carry a lighter conductor wire over the main power conductors. This conductor is grounded at various points along the link. Electrical substations usually have a web of the lighter conductor wires covering the whole plant.


Considerable material is used in the construction of lightning arresters, so it is prudent to work out where a new arrester will have the greatest effect. Historical understanding of lightning assumed that each device protected a [[Cone (geometry)|cone]] of 45 degrees <ref>Donlon, Tim, "''[http://www.buildingconservation.com/articles/lightning/lightn.htm Lightning Protection for Historic Buildings]''". Cathedral Communications Limited, 2001.</ref>. This has been found to be unsatisfactory for protecting taller structures, as it is possible for lightning to strike the side of a building. A better technique to determine the effect of a new arrester is called the rolling sphere technique and was developed by Dr Tibor Horváth. To understand this requires knowledge of how lightning 'moves'. As the [[step leader]] of a lightning bolt jumps toward the ground, it steps toward the [[ground (electricity)|grounded]] objects nearest its path. The maximum distance that each step may travel is called the ''critical distance'' and is proportional to the electrical current. Objects are likely to be struck if they are nearer to the leader than this critical distance. It is standard practice to approximate the sphere's radius as 60 m near the ground.
Considerable material is used in the construction of lightning arresters, so it is prudent to work out where a new arrester will have the greatest effect. Historical understanding, from statements made by Ben Franklin<ref>High-voltage surge eliminator, Roy B. Carpenter, Jr., {{US patent|5532897}}. Page 1, Column 1, Line 26-27.</ref>, of lightning assumed that each device protected a [[Cone (geometry)|cone]] of 45 degrees <ref>Donlon, Tim, "''[http://www.buildingconservation.com/articles/lightning/lightn.htm Lightning Protection for Historic Buildings]''". Cathedral Communications Limited, 2001.</ref>. This has been found to be unsatisfactory for protecting taller structures, as it is possible for lightning to strike the side of a building. A better technique to determine the effect of a new arrester is called the rolling sphere technique and was developed by Dr Tibor Horváth. To understand this requires knowledge of how lightning 'moves'. As the [[step leader]] of a lightning bolt jumps toward the ground, it steps toward the [[ground (electricity)|grounded]] objects nearest its path. The maximum distance that each step may travel is called the ''critical distance'' and is proportional to the electrical current. Objects are likely to be struck if they are nearer to the leader than this critical distance. It is standard practice to approximate the sphere's radius as 60 m near the ground.


Electricity travels along the path of least resistance, so an object outside the critical distance is unlikely to be struck by the leader if there is a grounded object within the critical distance. Noting this, locations that are safe from lightning can be determined by imagining a leader's potential paths as a [[sphere]] that travels from the cloud to the ground. For lightning protection it suffices to consider all possible spheres as they touch potential strike points. To determine which strike points consider a sphere rolling over the terrain. At each point we are simulating a potential leader position and where the sphere touches the ground the lightning is most likely to strike. Points which the sphere cannot roll across and touch are safest from lightning. Lightning protector should be placed where they will prevent the sphere from touching a structure.
Electricity travels along the path of least resistance, so an object outside the critical distance is unlikely to be struck by the leader if there is a grounded object within the critical distance. Noting this, locations that are safe from lightning can be determined by imagining a leader's potential paths as a [[sphere]] that travels from the cloud to the ground. For lightning protection it suffices to consider all possible spheres as they touch potential strike points. To determine which strike points consider a sphere rolling over the terrain. At each point we are simulating a potential leader position and where the sphere touches the ground the lightning is most likely to strike. Points which the sphere cannot roll across and touch are safest from lightning. Lightning protector should be placed where they will prevent the sphere from touching a structure.

Revision as of 22:55, 18 April 2007

An example of a standard, pointed-tip air terminal

A lightning rod (or lightning protector) is a metal strip or rod, usually of copper or similar conductive material, used as part of lightning safety to protect tall or isolated structures (such as the roof of a building or the mast of a vessel) from lightning damage. Its formal name is lightning finial or air terminal. Sometimes, the system is informally referred to as a lightning conductor, lightning arrester, or lightning discharger; however, these terms actually refer to lightning protection systems in general or specific components within them. The United States Patent Office labels "Lightning protectors" in Class 174 (Electricity: conductors and insulators), Subclass 2 (Lightning protectors) and Subclass 3 (Rods).

The term 'lightning rod' is also used as a metaphorical term to describe those who attract controversy.

History

Lightning damage has been with humanity since people started building structures. Early structures made of wood and stone tended to be short and in valleys and as a result lightning hit rarely. As buildings became taller, lightning became a significant threat. Lightning can damage structures made of most materials (masonry, wood, concrete and even steel) as the huge currents involved can heat materials, and especially water to high temperatures causing fire, loss of strength and explosions from superheated steam and air.

Europe

Wooden church with lightning rods and grounding cables

The church tower of many European cities, usually the highest structure, was the building often hit by lightning. Early on, Christian churches tried to prevent the occurrence of the damaging effects of lightning by prayers. Priests prayed,

temper the destruction of hail and cyclones and the force of tempests and lightning; check hostile thunders and great winds; and cast down the spirits of storms and the powers of the air.

Peter Ahlwardts ("Reasonable and Theological Considerations about Thunder and Lightning", 1745) advised individuals seeking cover from lightning to go anywhere except in or around a church.[1]

United States

In the United States, the pointed lightning rod conductor, and more accurately the "lightning attractor", was invented by Benjamin Franklin as part of his groundbreaking explorations of electricity. Franklin speculated that, with an iron rod sharpened to a point at the end,

the electrical fire would, I think, be drawn out of a cloud silently, before it could come near enough to strike [...].

Franklin had speculated about lightning rods for several years before his reported kite experiment. This experiment, in fact, took place because he was tired of waiting for Christ Church in Philadelphia to be completed so he could place a lighting rod on top of it. There was some resistance from churches who felt that it was defying divine will to install these rods. Franklin countered that there is no religious objection to roofs on buildings to resist precipitation, so lightning, which he proved to be simply a giant electrical spark, should be no different. As an act of philanthropy, Franklin decided against patenting the invention.

In the 19th century the lightning rod became a symbol of American ingenuity and a decorative motif. Lightning rods were often embellished with ornamental glass balls[2] (now prized by collectors) that also served to provide visual sign of a lightning strike (when the rod is struck the glass ball shatters and falls off, indicating to the owner which rod got struck and that they should check it and the grounding wire for damage). The ornamental appeal of these glass balls has also been incorporated into weather vanes.

Balls of solid glass were occasionally used in a method purported to prevent lightning strikes to ships. It is worth noting here not because it worked, which it didn't, but because it reveals a lot about pre-scientific thought. The basic principle was that glass objects, being non-conductors, are seldom struck by lightning. Therefore, goes the theory, there must be something about glass that repels lightning. Hence the best method for preventing a lightning strike to a wooden ship was to bury a small solid glass ball in the tip of the highest mast. The random behavior of lightning ensured that the method gained a good bit of credence even after the development of the marine lightning rod soon after Franklin's initial work.

Nikola Tesla's U.S. patent 1,266,175 was an improvement in lightning protectors. The patent was granted due to a fault in Franklin's original theory of operation; the pointed lightning rod actually ionizes the air around itself, rendering the air conductive, which in turn raises the probability of a strike. Many years after receiving his patent, in 1919 Dr. Tesla wrote an article for The Electrical Experimenter entitled "Famous Scientific Illusions", in which he explains the logic of Franklin's pointed lightning rod and discloses his improved method and apparatus.

Some of DuPont Explosives manufacturing sites, which were surrounding by pine trees, used various lightning protection devices. During the 1950s, DuPont was making nitroglycerin in some buildings and moving it in 'Angel Buggies' to the packing building. Employees at those sites were very sensitive to potential lightning strikes.[citation needed]

In the 1990s, the 'lightning points' were replaced as originally constructed when the statue of Freedom atop the United States Capitol building in Washington was restored.[citation needed] The statue was designed with multiple devices which are tipped with platinum. The original aluminum cap of the Washington Monument was also equipped with multiple lightning points,[citation needed] and the rays that radiate from the crown of the Statue of Liberty in New York Harbor constitute a lightning-dissipation device. [citation needed]

Lightning protectors

Lightning diversion

Conventional lightning rods are connected via a low-resistance wire or cable to the earth or water below, where the charge may be safely dissipated. The diversion theory states that the lightning rod protects a structure purely because it is grounded, and thus a lightning stroke that happens to attach to the protector will be diverted around the structure and "earthed" through a grounding cable or conductor.[3] There is some uncertainty as to why a lightning strike might preferentially attach to a lightning protector; the leading assumption is that the air near the protector becomes ionized and thus conductive due to the intense electric field. Various manufacture make these these claims.

Lightning protector sometimes possess a short circuit to the ground that is interrupted by a thin non-conductor over which lightning jumps. Ideally, the underground part of the assembly should reside in a muddy area, or an area that tends to become so during storms. If the underground cable will resist corrosion well, it may be covered in salt to improve its electrical connection with the ground. In telegraphy and telephony a lightning protector is placed where wires enter a structure, preventing damage to electronic instruments within and ensuring the safety of individuals near them. Similarly, high-tension power lines carry a lighter conductor wire over the main power conductors. This conductor is grounded at various points along the link. Electrical substations usually have a web of the lighter conductor wires covering the whole plant.

Considerable material is used in the construction of lightning arresters, so it is prudent to work out where a new arrester will have the greatest effect. Historical understanding, from statements made by Ben Franklin[4], of lightning assumed that each device protected a cone of 45 degrees [5]. This has been found to be unsatisfactory for protecting taller structures, as it is possible for lightning to strike the side of a building. A better technique to determine the effect of a new arrester is called the rolling sphere technique and was developed by Dr Tibor Horváth. To understand this requires knowledge of how lightning 'moves'. As the step leader of a lightning bolt jumps toward the ground, it steps toward the grounded objects nearest its path. The maximum distance that each step may travel is called the critical distance and is proportional to the electrical current. Objects are likely to be struck if they are nearer to the leader than this critical distance. It is standard practice to approximate the sphere's radius as 60 m near the ground.

Electricity travels along the path of least resistance, so an object outside the critical distance is unlikely to be struck by the leader if there is a grounded object within the critical distance. Noting this, locations that are safe from lightning can be determined by imagining a leader's potential paths as a sphere that travels from the cloud to the ground. For lightning protection it suffices to consider all possible spheres as they touch potential strike points. To determine which strike points consider a sphere rolling over the terrain. At each point we are simulating a potential leader position and where the sphere touches the ground the lightning is most likely to strike. Points which the sphere cannot roll across and touch are safest from lightning. Lightning protector should be placed where they will prevent the sphere from touching a structure.

Nikola Tesla's
"Lightning-Protector"
U.S. patent 1,266,175; An early type of dissipater-arrester, which the patent states to prevent and safely dissipate lightning strikes

It is commonly believed, erroneously, that a protector ending in a sharp point at the peak is the best means to conduct the current of a lightning strike to the ground. According to field research, a rod with a rounded or spherical end is better. "Lightning Rod Improvement Studies" [6] by Moore et al say:

Calculations of the relative strengths of the electric fields above similarly exposed sharp and blunt rods show that although the fields, prior to any emissions, are much stronger at the tip of a sharp rod, they decrease more rapidly with distance. As a result, at a few centimeters above the tip of a 20-mm-diameter blunt rod, the strength of the field is greater than that over an otherwise similar, sharper rod at the same height. Since the field strength at the tip of a sharpened rod tends to be limited by the easy formation of ions in the surrounding air, the field strengths over blunt rods can be much stronger than those at distances greater than 1 cm over sharper ones.
The results of this study suggest that moderately blunt metal rods (with tip height–to–tip radius of curvature ratios of about 680:1) are better lightning strike receptors than are sharper rods or very blunt ones.

In addition, the height of the lightning protector relative to the structure to be protected and the Earth itself will also have an effect.[7][8]

Lightning dissipation

Lightning dissipaters (known as Early Streamer Emission, Dissipation Array Systems, and Charge Transfer Systems) claim to make a structure less attractive to lightning and other charges which can flow through the Earth's atmosphere around it. These generally encompass systems and equipment for the preventative protection of objects located on the surface of the earth from the effects of atmospherics. The devices deal with the phenomena such as electrostatic fields, electromagnetic fields, field transients, static charges, and any other related atmospheric electricity phenomena.

The most common individual dissipator rods (or dissipator elements) appear as slightly-blunted metal spikes sticking out in all directions from a metal conductor.[9] These elements are mounted on short metal arms at the very top of a radio antenna or tower, the area by far most likely to be struck. The dissipation theory states a reduction in the potential difference (voltage) between the structure and the storm cloud, miles above, theoretically reducing, but not eliminating, the risk of lightning strikes.[10] Various manufacture make these these claims. Induced upward lightning strokes occurring on tall structures (effective heights of 300 m or more) can be reduced by altering the shape of the structure.[11]

Evaluations and analysis

A controversy regarding the assortment of operation theories dates back to the 1700s, when Franklin himself stated that his lightning protectors protected buildings by dissipating electric charge. He later retracted the statement with a disclaimer stating that the exact mode of operation of the device was something of a mystery at that point. Thus began a 250-year dispute between the dissipation theory and the diversion theory of lightning protection. The dissipation theory states that a lightning strike to a structure can be prevented by reducing the electrical potential between the structure and the thundercloud by transferring electric charge from the nearby earth to the sky.[12] This is done by erecting some sort of tower equipped with one or more sharply-pointed protectors upon the structure. It is noted that sharply-pointed objects will indeed transfer charge to the surrounding atmosphere[13][14] and that a considerable electric current through the tower can be measured when thunderclouds are overhead. It is worth considering that these dissipators and arrestors have been around for a long time (see the history section above) and that there is also no direct proof that the dissipation theory is incorrect. A reading of the scientific literature on the subject will reveal much of the problem. It has been reguarded as impossible to conduct a controlled experiment with voltages approaching natural lightning.[15] Also, test structures that are equipped with lightning instrumentation may languish for years without a strike, and then be subjected to a strike that destroys the instrumentation.

Lightning strikes to a metallic structure can vary from leaving no evidence excepting perhaps a small pit in the metal to the complete destruction of the structure. (Rakov, Page 364[16]). When there is no evidence, attempts at making analysis of such strikes is difficult. This means that a strike on an un-instrumented structure must be visually confirmed, and the random behavior of lightning renders such observations difficult. Thus if we strip away the two centuries of legal actions, political activity and general outrage exhibited by both sides, we find that the current state of the dissipation/diversion controversy is a draw; that neither theory has or can be proven, and that essentially all data pertaining to the behavior of lightning on structures must be considered anecdotal. The research situation is improving somewhat, however.[17][16][18][19] There are also inventors working on this problem,[20][21] such as through a lightning rocket. While controlled experiments may be off in the future, very good data is being obtained through techniques which use radio receivers that watch for the characteristic electrical 'signature' of lightning strikes using fixed directional antennas.[22][23][24][25] Through accurate timing and triangulation techniques, lightning strikes can be located with great precision, and so strikes on specific objects can often be confirmed with confidence.

These ancient buildings on the Great Wall of China are protected by the modern grounded lightning mast between them.

There have been attempts to introduce lightning protection systems into standards. The NFPA's independent third party panel found that "the [Early Streamer Emission] lightning protection technology appears to be technically sound" and that there was an "adequate theoretical basis for the [Early Streamer Emission] air terminal concept and design from a physical viewpoint". (Bryan, 1999[26]) The same panel also concluded that "the recommended [NFPA 780 standard] lightning protection system has never been scientifically or technically validated and the Franklin rod air terminals have not been validated in field tests under thunderstorm conditions." In response, the American Geophysical Union concluded that "[t]he Bryan Panel reviewed essentially none of the studies and literature on the effectiveness and scientific basis of traditional lightning protection systems and was erroneous in its conclusion that there was no basis for the Standard." AGU did not attempt to assess the effectiveness of any proposed modifications to traditional systems in its report. [27]

No major standards body, such as the NFPA, UL, and the NLSI, has currently endorsed a device that can prevent or reduce lightning strikes. The NFPA Standards Council, following a request for a project to address Dissipation Array Systems and Charge Transfer Systems, denied the request to begin forming standards on such technology (though the Council did not foreclose on future standards development after reliable sources demonstrating the validity of the basic technology and science were submitted). [28] Members of the Scientific Committee of the International Conference on Lightning Protection has issued a joint statement stating their opposition to dissipater technology. [29]

Various investigators believe the natural downward lightning strokes to be unpreventable.[11] Since most lightning protectors' ground potentials are elevated, the path distance from the source to the elevated ground point will be shorter, creating a stronger field (measured in volts per unit distance) and that structure will be more prone to ionization and breakdown.[30] Scientists from the National Lightning Safety Institute claim that these dissipation devices are nothing more than expensive lightning protector and that they, unlike traditional methods, are not based on "scientifically proven and indisputable technical arguments". [31] William Rison of the National Lightning Safety Institute states that in his opinion the underlying theory of dissipation is "scientific nonsense".[32] According to these sources, there is no proof that the dissipation arrangement is at all effective. According to opponents of the dissipation technology, the various designs of dissipaters indirectly "eliminate" lightning via the alteration of a buildging's shape and only have a small effect (either intended or not) because there is no significant reduction to the susceptibility of a structure to the generation of upward lightning strokes. [11] Some field investigations of dissipaters show that their performance is comparable to conventional terminals and possess no great enhancement of protection. According to these field studies, these devices have not shown that they totally eliminated lightning strikes. [33]

See also

External articles and references

Citations

  1. ^ Seckel, Al, and John Edwards, "Franklin's Unholy Lightning Rod". 1984.
  2. ^ "Antique Lightning Rod Ball Hall of Fame". Antique Bottle Collectors Haven. (glass lightning balls collection)
  3. ^ U.S. patent 4,429,341, Lightning protection for external surface composite material of an aircraft, Charles H. King.
  4. ^ High-voltage surge eliminator, Roy B. Carpenter, Jr., U.S. patent 5,532,897. Page 1, Column 1, Line 26-27.
  5. ^ Donlon, Tim, "Lightning Protection for Historic Buildings". Cathedral Communications Limited, 2001.
  6. ^ C. B. Moore, William Rison, James Mathis, and Graydon Aulich, "Lightning Rod Improvement Studies". Journal of Applied Meteorology: Vol. 39, No. 5, pp. 593–609. Langmuir Laboratory for Atmospheric Research, New Mexico Institute of Mining and Technology, Socorro, New Mexico. April 10, 1999.
  7. ^ U.S. patent 1,266,175, Tesla, "Lightning-Protector".
  8. ^ U.S. patent 3,371,144, Griscom, "Transmission-line lightning-proofing structures". Page 25, Column 5. (cf. [...] the charge on a leader as a function of height above ground[...])
  9. ^ U.S. patent D478,294 - Haygood, "Lightning dissipation assembly "
  10. ^ {{US patent|123456|link text}}, Richard Ralph Zini, et. al., Non-contaminating lightning protection system. Claim one and claim ten.
  11. ^ a b c Mousa, Abdul M. "The applicability of Lightning Elimination Devices to Substations and Power Lines". British Columbia Hydro, Burnaby, British Columbia, Canada V3N 4X8.
  12. ^ John Richard Gumley, U.S. patent 6,320,119, Lightning air terminals and method of design and application
  13. ^ Emitter of ions for a lightning rod with a parabolic reflector, Manuel Domingo Varela, U.S. patent 6,069,314.
  14. ^ Lightning-protector for electrical conductors, Johathan H. Vail, U.S. patent 357,050.
  15. ^ U.S. patent 5,541,385, Page 10, Column 9 Line 38 - 41. (cf., "It is possible to raise in the chamber a voltage of same grade as in the lightning and the insulation has to be of a quality that breakdowns beyond control are not possible.)
  16. ^ a b Rakov, et. al., Lightning: physics and effects
  17. ^ Martin A. Uman, Lightning Discharge. Courier Dover Publications, 2001. 377 pages. ISBN 0486414639
  18. ^ Donald R. MacGorman, The Electrical Nature of Storms. Oxford University Press (US), 1998. 432 pages. ISBN 0195073371
  19. ^ Hans Volland, Handbook of Atmospheric Electrodynamics, Volume I. CRC Press, 1995. 408 pages. ISBN 0849386470
  20. ^ Method and apparatus for the artificial triggering of lightning, Douglas A. Palmer, U.S. patent 6,012,330
  21. ^ Lightning rocket, Robert E. Betts, U.S. patent 6,597,559
  22. ^ Lightning locating system, Ralph J. Markson et. al., U.S. patent 6,246,367.
  23. ^ Lightning locating system, Airborne Research Associates, Inc., U.S. patent 5,771,020.
  24. ^ System and method of locating lightning strikes, The United States of America as represented by the Administrator of the National Aeronautics and Space Administration, U.S. patent 6,420,862
  25. ^ Single station system and method of locating lightning strikes, The United States of America as represented by the United States National Aeronautics and Space Administration, U.S. patent 6,552,521.
  26. ^ Bryan, R. G., et. al., "Report of the Third-Party Independent Evaluation Panel on the Early Streamer Emission Lightning Protection Technology".
  27. ^ Report of The Committee on Atmospheric And Space Electricity of The American Geophysical Union on The Scientific Basis for Traditional Lightning Protection Systems
  28. ^ Casey C. Grant, "To: Interested Parties"
  29. ^ Mousa, Abdul M. "Scientists Oppose Early Streamer Air Terminals", 1999.
  30. ^ U.S. patent 1,869,661, Bumbraugh, "Lightning protection system and method".
  31. ^ Mousa, Abdul M. (1999). "Scientists Oppose Early Streamer Air Terminals". National Lightning Safety Institute. Retrieved September 18, 2006. {{cite web}}: Cite has empty unknown parameter: |accessyear= (help); External link in |publisher= (help)
  32. ^ Rison, William (2001). "There Is No Magic To Lightning Protection: Charge Transfer Systems Do Not Prevent Lightning Strikes" (pdf). National Lightning Safety Institute. Retrieved September 18, 2006. {{cite web}}: Cite has empty unknown parameter: |accessyear= (help); External link in |publisher= (help)
  33. ^ Rison, W., Moore, C.B., and Aulich, G.D., "Lightning air terminals - is shape important?", Electromagnetic Compatibility, 2004. EMC 2004. 2004 InternationalSymposium on Volume 1, 9-13 Aug. 2004 Page(s):300 - 305 vol.1

General information

Patents

U.S. Patent Documents
  • Original
  • Reissued

Websites

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