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Lightning rod

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An example of a conventional standard, pointed rod with blunt 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. Different systems of terminals are employed according to protection requirements. Its formal name is lightning finial or air terminal. Sometimes, the system is informally referred to as:

  • a lightning conductor,
  • a lightning arrester, or
  • a lightning discharger.

However, these terms actually refer to lightning protection systems in general or specific components within them. Lightning protection systems alter lightning streamer behavior. The field of lightning protection is almost totally void of systems or concepts designed to deal with the general problem area as a whole.

Introduction

Protection systems as a whole are almost totally void of concepts designed to deal with the typical lightning discharge, electrostatic disturbance, and electromagnetic atmospherics. Approaches toward dealing with the problem of and concerns over lightning have encompassed dispensing of chaff or silver iodide crystals into the atmosphere as a prevention measure to the use of lightning arresters as a remedial measure to deal with just one specific manifestation of natural atmospheric discharges. The use of lightning arrester equipment was established to deal with lightning only, one electrical manifestation of atmospheric electricity as it impacts facilities within its sphere of influence. The plausiblity of a preventative protection which can be made effective through the provision of a plurality of adequately devised neutralizing elements together with an appropriate charge collector and an interconnecting system have been researched, though.

Arresters

In the event of a lightning strike, a lightning arrester is used. It is a device that shunts or diverts the massive voltage and electrical current of a lightning strike to an earthed ground. Electrical equipment can be protected from lightning by an arrester, a device that contains one or more gas-filled spark gaps between the equipment's cables and earth. An arrester is designed to handle much higher jolts of electricity than a surge protector, which cannot handle a direct strike at all.

It was originally thought that the lightning rods served to release and neutralize, selectively, a charge at a relatively slow rate. In reality, lightning arresters (though leaking off some current) reach a saturation under prestroke conditions resulting in a breakdown voltage at the point of installation and a subsequent upward moving streamer joins with the lightning leader already moving toward earth. When lightning strike a building, the current will travel through the conductor rather than through the fabric of the building, causing less damage. A typ[ical strike results in a deluge of current (around a magnitude of 20 coulombs) flowing into the rod and through the grounding system, all in a very short period of time.

Lightning arresters typically involves one or more spark gaps, across which a normal voltage cannot arc. When lightning strikes one of the nodes, the high voltage will cause a gap to break down and become a conductor, providing a path for the lightning to reach the ground without passing through the equipment. When lightning exceeds the arrester's breakdown voltage, the currents arcs to the ground and prevents arcing around inside sensitive electronic equipment connected further downline. The spark gap may be filled with a noble gas, or with air. Other types may work by blocking normal alternating current (AC), but allowing the direct current (DC) from a lightning discharge.

Lightning arresters are typically installed on electric power transmission lines, and on radio tower feedlines between the radio antenna and transmitter. Smaller ones can also be installed on the mains electricity service coming into a building (even a home), just before the circuit breaker panel. Telephone wires also have fusible links sometimes where they enter a building, connected by carbon which will vaporize with very high current.

Construction and uses

Nikola Tesla's
"Lightning-Protector"
U.S. patent 1,266,175; A type of dissipater-arrester, prevent and safely dissipate lightning strikes

A lightning rod is connected via a low-resistance wire or cable to the earth or water below, where the charge may be safely dissipated. Lightning rods 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 rod 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 of lightning assumed that each rod protected a cone of 45 degrees [1]. 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 60m 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 rods should be placed where they will prevent the sphere from touching a structure.

It is commonly believed, erroneously, that a rod 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" [2] 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.

Dissipaters

Lightning rod dissipaters (known as Early Streamer Emission, Dissipation Array Systems, and Charge Transfer Systems) theoretically make a structure less attractive by which charges can flow to the Earth's atmosphere around it. These gernerally encompass systems and equipment for the preventative protection of objects located on the surface of the earth from the effects of atmospherics. Protection from the effects of atmospherics include, but not be limited to, such phenomena as lightning, electrostatic fields, electromagnetic fields, field transients, static charges, and any other related phenomena.

Dissipater systems comprise of an ionizer structure, ground current collector, and a plurality of service wires. One or more ionizer structure usually rise above the earth's surface and the to be protected objects, selectively, extending upwardly to a specific elevation calculable for the at least one specific intended area of protection provided by the ionizer structure under a uniform field shape, avoiding field reinforcing at any point. The ground current collector structure has one or more downwardly extending grounded charge collecting rods. These ground current collector structures are usually disposed horizontally about the base of the ionizer structure and contiguous to the earth's surface, so as to surround the to be protected objects on the earth's surface and thereby establishing an electrically floating island having the area of said calculable intended protection. The service wires connect the base of the ionizing structure with operationally significant points of the ground collector structure and those of objects on the earth's surface and located within the perimeter of the ground current collector structure, selectively.

The most common individual dissipater rods (or dissipator elements) appear as slightly-blunted metal spikes sticking out in all directions from a metal conductor.[3] 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 effectiveness of stand-alone dissipater rods has not been confirmed. According to various manufacture claims, there is supposedly a reduction in the voltage between the structure and the storm cloud, miles above, reducing, but not eliminating, the risk of lightning strikes.

Commercial "elimination" claims of lightning are rightly greeted with a skeptical reception. The NFPA's independent third party panel found that "the [Early Streamer Emission] lightning protection technology appears to be technically sound". (Bryan, 1999) The panel stated that there was an "adequate theoretical basis for the [Early Streamer Emission] air terminal concept and design from a physical viewpoint".(Bryan, 1999) Though, there is no firm evidence that these devices prevent lightning strikes.[4][5] No device has rigorously been proven to prevent or reduce the number of lightning strikes and there is no device that is endorsed to prevent lightning strikes by a major standards body, including the NFPA, UL or the NLSI. 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). [6] Members of the Scientific Committee of the International Conference on Lightning Protection has issued a joint statement stating thier opposition to dissipater technology. [7]

In investigations, the natural downward lightning strokes are not believed to be preventable. 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. According to opponents of the technology, the various designs indirectly "eliminate" lightning via the alteration and dissipaters only have a small effect (either intended or not) because there is no significant reduction the susceptibility of the tower to the generation of upward lightning strokes. [8] 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 do "eliminate" lightning strikes. [9] According to some, many of these devices are likely to be nothing more than expensive lightning rods.[10] Reguardless, many of these devices are designed to act as an arrester if a lightning strike does hit.

History

Lightning damage has been with humanity since we 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

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) gave information to individuals seeking cover from lightning to go anywhere except in or around a church.[11]

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. Franklin's kite experiment is debated topic. Some believe that, in fact, that it 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.[12] Others have doubted the authenticity of such records. This group, which includes Tom Tucker, has stated that this experiment was a myth propagated by Franklin. [13] Among the reason are that Franklin did not say that he did conduct the experiment when he described it in the newspaper, the Pennsylvania Gazette. Reguardless, there was some resistance from churches to lightning rods because they felt that it was defying divine will to install the devices. 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.

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[14] (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. As a point in fact, as an act of philanthropy, Benjamin Franklin decided against patenting the invention.

Nikola Tesla's U.S. patent 1,266,175 was an early 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.

References

General
Citations
  • J. L. Bryan, R. G. Biermann and G. A. Erickson, "Report of the Third-Party Independent Evaluation Panel on the Early Streamer Emission Lightning Protection Technology". National Fire Protection Association, Quincy, Mass., 1999.
  • ^ Donlon, Tim, "Lightning Protection for Historic Buildings". Cathedral Communications Limited, 2001.
  • ^ 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. (ed. lightning rod design)
  • ^ "Antique Lightning Rod Ball Hall of Fame". Antique Bottle Collectors Haven. (glass lightning balls collection)
  • ^ Seckel, Al, and John Edwards, "Franklin's Unholy Lightning Rod". 1984.
  • ^ Mousa, Abdul M. (1999). "Scientists Oppose Early Streamer Air Terminals". National Lightning Safety Institute. Retrieved December 11, 2005. {{cite web}}: Cite has empty unknown parameter: |accessyear= (help); External link in |publisher= (help)
  • ^ Rison, William (2001). "There Is No Magic To Lightning Protection: Charge Transfer Systems Do Not Prevent Lightning Strikes" (pdf). National Lightning Safety Institute. Retrieved December 11, 2005. {{cite web}}: Cite has empty unknown parameter: |accessyear= (help); External link in |publisher= (help)
  • ^ U.S. patent D478,294 - Haygood, "Lightning dissipation assembly "
  • ^ Abdul M. Mousa, "The applicability of Lightning Elinination Devices to Substations and Power Lines". British Columbia Hydro, Burnaby, British Columbia, Canada V3N 4X8.
  • ^ Abdul M. Mousa, "Scientists Oppose Early Streamer Air Terminals", 1999.
  • ^ 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
  • ^ Casey C. Grant, "To: Interested Parties" (ed. Attachment of the final decision of the Standards Council.)
  • ^ "The Lightning Rod; Point of Invention". sln.org, 2001.
  • ^ Tom Tucker, "Bolt of Fate: Benjamin Franklin and His Electric Kite Hoax". PublicAffairs, June 2003. ISBN 1891620703

Patents

The United States Patent Office labels "Lightning protectors" in Class 174 (Electricity: conductors and insulators), Subclass 2 (Lightning protectors) and Subclass 3 (Rods).

U.S. Patent Documents

Original

Reissued

External articles and other resources

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