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Lightning rod

2007 Schools Wikipedia Selection. Related subjects: Engineering

   An example of a standard, pointed-tip air terminal
   Enlarge
   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 term 'lightning rod' is also used as a metaphorical term to
   describe those who attract controversy.

Construction and uses

   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 . 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 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" 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.

History

   Wooden church with lightning rods and grounding cables
   Enlarge
   Wooden church with lightning rods and grounding cables

   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

   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.

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.

   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 (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 behaviour 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.

   As a point of fact, as an act of philanthropy, Benjamin Franklin
   decided against patenting the invention.

Lightning prevention

   Lightning rod dissipaters (known as Early Streamer Emission,
   Dissipation Array Systems, and Charge Transfer Systems) claim to make a
   structure less attractive to lightning. These generally encompass
   systems and equipment for the preventative protection of objects
   located on the surface of the earth from the effects of atmospherics.
   Scientists claim that these devices are nothing more than expensive
   lightning rods and that they, unlike traditional methods, are not based
   on "scientifically proven and indisputable technical arguments" or that
   the underlying theory is "scientific nonsense".

   This controversy dates back to the 1700's, when Franklin himself stated
   that his lightning rods 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. This is done by erecting some sort of tower
   equipped with one or more sharply-pointed rods upon the structure.
   While it is true that sharply-pointed objects will indeed transfer
   charge to the surrounding atmosphere, and it is also true that a
   considerable electric current through the tower can be measured when
   thunderclouds are overhead, there is no proof that such an arrangement
   is at all effective. All DuPont Explosives manufacturing sites were
   surrounding by pine trees. During the 1950's, 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.

   It should be noted, however, that there is also no proof that the
   dissipation theory is incorrect, and it is worth considering that these
   devices have been around for a long time. For example, the statue of
   Freedom atop the United States Capitol building in Washington is
   equipped with multiple 'lightning points' which are tipped with
   platinum, and that these were replaced as originally constructed when
   the statue was restored in the 1990's. The original aluminum cap of the
   Washington Monument was also equipped with multiple lightning points,
   and the rays that radiate from the crown of the Statue of Liberty in
   New York Harbour constitute a lightning-dissipation device as well.

   The diversion theory states that the lightning rod protects a building
   purely because it is grounded, and thus a lightning stroke that happens
   to attach to the rod will be diverted around the structure and down to
   earth through a ground cable. There is some uncertainty as to why a
   lightning strike might preferentially attach to a lightning rod; the
   leading assumption is that the air near the rod becomes ionized and
   thus conductive due to the intense electric field.

   A close reading of the scant scientific literature on the subject will
   reveal much of the problem, which is that thus far it has proven
   impossible to conduct a controlled experiment with natural lightning. A
   test structure that is equipped with lightning instrumentation may
   languish for years without a strike, and then be subjected to a strike
   that destroys the instrumentation.

   Moreover, a lightning strike to a metallic structure frequently leaves
   no evidence excepting perhaps a small pit in the metal. This means that
   a strike on an un-instrumented structure must be visually confirmed,
   and the random behaviour 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 behaviour of lightning on structures must be
   considered anecdotal.

   The research situation is improving somewhat, however. While controlled
   experiments may be far in the future, very good data is being obtained
   through techniques which use a network of radio receivers that watch
   for the characteristic electrical 'signature' of lightning strikes
   using fixed directional antennas. 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.

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

   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) 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.

   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). Members of the Scientific
   Committee of the International Conference on Lightning Protection has
   issued a joint statement stating their opposition to dissipater
   technology.

   Investigators believe the natural downward lightning strokes to be
   unpreventable. 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. 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 eliminate lightning strikes.

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
     * U.S. Patent 20877 - Haskins, "Protecting vessels from lightning"
     * U.S. Patent 29398 - Patterson, "Lightning rod"
     * U.S. Patent 75492 - Varley, "Telegraph pole"
     * U.S. Patent 84210 - Munson, "Lightning rod"
     * U.S. Patent 11217 - Forbes , "Lightning rod". Jul., 1854.
     * U.S. Patent 367435 - O'Brien, "Lightning arrestor for the
       protection of oil tanks"
     * U.S. Patent 911260 - Pennock, "Apparatus for collecting atmospheric
       electricity"
     * U.S. Patent 938137 - Goetz, "Lightning Rod"
     * U.S. Patent 1266175 - Tesla, "Lightning-Protector"
     * U.S. Patent 1617788 - Baldwin, "Device for the preventing
       electrical ignition of stored inflammable fluids"
     * U.S. Patent 1678539 - Ticehurst, "Oil reserve safety appliance"
     * U.S. Patent 1743526 - Cage, "Lightning protector"
     * U.S. Patent 1916335 - Schaeffer, "System for the preventing
       electrical ignition of reservoir stored flammables"
     * U.S. Patent 4180698 - Carpenter, Jr., "System and equipment for
       atmospherics conditioning"
     * U.S. Patent 6474595 - Herman, "Electrical energy
       depletion/collection system". November 5, 2002. (Aeronautics,
       Lightning arresters and static eliminators; Safety and protection
       of systems and devices, High voltage dissipation (e.g., lightning
       arrester) )
     * U.S. Patent 6708638 - Thomson, "Method and apparatus for lightning
       protection", University of Florida Research Foundation, Inc.
       (Gainesville, FL)
     * U.S. Patent 4910636 - Sadler, "Static electricity dissipator".
     * U.S. Patent 4017767 - Ball, "Laser lightning rod system"
     * U.S. Patent 4605814 - Gillem "Lightning deterrent"
     * U.S. Patent 6657120 - Smith, "Lightning shelters"
     * U.S. Patent 6875915 - Chung, "Lightning arrester"

   Reissued
     * U.S. Patent RE6835 - Spang, "Lightning-rod"
     * U.S. Patent RE9934 - Spang, "Lightning rod"
     * U.S. Patent RE25417 - Amason, "Lightning arrestors for radomes"

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