A lightning rod (US, AUS) or lightning conductor (UK) is a metal rod or conductor mounted on top of a building and electrically connected to the ground through a wire, to protect the building in the event of lightning. If lightning strikes the building it will preferentially strike the rod, and be conducted harmlessly to ground through the wire, instead of passing through the building, where it could start a fire or cause electrocution. A lightning rod is a single component in a lightning protection system. In addition to rods placed at regular intervals on the highest portions of a structure, a lightning protection system typically includes a rooftop network of conductors, multiple conductive paths from the roof to the ground, bonding connections to metallic objects within the structure and a grounding network. The rooftop lightning rod is a metal strip or rod, usually of copper or aluminum. Lightning protection systems are installed on structures, trees, monuments, bridges or water vessels to protect from lightning damage. Individual lightning rods are sometimes called finials, air terminals or strike termination devices. The lightning rod was invented by Benjamin Franklin in the Americas in 1749[1] and, perhaps independently, by Prokop Diviš in Europe in 1754.[2]
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As buildings became taller, lightning becomes more of a threat. Lightning can damage structures made of most materials (masonry, wood, concrete and steel) as the huge currents involved can heat materials, causing a potential for fire, and also water to high temperatures.
Some of the most ancient lightning conductors can be found in Sri Lanka in places like the Kingdom of Anuradhapura that dates back to thousands of years. The Sinhalese kings who mastered construction of stupas and advanced building structures, installed a metal tip made of Silver or Copper on the highest point of every building to conduct any lightning charge. In many parts of the world ancient Buddhist monuments have been destroyed by lightning strikes but not in Sri Lanka.
A lightning conductor may have been intentionally used in the Leaning Tower of Nevyansk. The spire of the tower is crowned with a metallic rod in the shape of a gilded sphere with spikes. This lightning rod is grounded through the rebar carcass, which pierces the entire building.
The Nevyansk Tower was built between 1725 and 1732, on the orders of industrialist Akinfiy Demidov. The Nevyansk Tower was built 25 years before Benjamin Franklin's experiment and scientific explanation.[4] However, the true intent behind the metal rooftop and rebars remains unknown.[5]
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,
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.[6] In Europe, the lightning rod may have been independently invented by Czech priest Václav Prokop Diviš between 1750 to 1754.
In the United States, the pointed lightning rod conductor, also called a "lightning attractor" or "Franklin rod," was invented by Benjamin Franklin in 1749 as part of his groundbreaking explorations of electricity. Although not the first to suggest a correlation between electricity and lightning, Franklin was the first to propose a workable system for testing his hypothesis.[7] Franklin speculated that, with an iron rod sharpened to a point at the end,
Franklin 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 lightning rod on top of it.
In the 19th century, the lightning rod became a decorative motif. Lightning rods were embellished with ornamental glass balls[8] (now prized by collectors). The ornamental appeal of these glass balls has been used in weather vanes. The main purpose of these balls, however, is to provide evidence of a lightning strike by shattering or falling off. If after a storm a ball is discovered missing or broken, the property owner should then check the building, rod, and grounding wire for damage.
Balls of solid glass occasionally were used in a method purported to prevent lightning strikes to ships and other objects. The idea 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 combined with observers' confirmation bias 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.
The first lightning conductors on ships were supposed to be hoisted when lightning was anticipated, and had a low success rate. In 1820 William Snow Harris invented a successful system for fitting lightning protection to the wooden sailing ships of the day, but despite successful trials which began in 1830, the British Royal Navy did not adopt the system until 1842, by which time the Imperial Russian Navy had already adopted the system.
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.
DuPont Explosives manufacturing sites were surrounded by pine trees. The needles on many pine trees can act as tens of thousands of points for higher voltages to flow current into the air and lower the difference between the cloud to ground, and reduce the number of strikes per square mile of area. During the 1950s, DuPont made nitroglycerin in some buildings and moved it in "Angel Buggies" to the packing building. Employees at those sites were very sensitive to potential lightning strikes.[9]
In the 1990s, the 'lightning points' were replaced as originally constructed when the statue of Freedom atop the United States Capitol building in Washington, D.C. was restored.[10] The statue was designed with multiple devices that are tipped with platinum. The Washington Monument also was equipped with multiple lightning points,[11] and the Statue of Liberty in New York Harbor gets hit with lightning which is grounded out.
Lightning rods convey the current from a strike to the ground or water via a low-resistance conductor. A lightning strike is thus said to be diverted from the protected structure. However, diversion is a misnomer. Rather, the lightning rod widely used attracts and intercepts a strike that terminates near a protected structure.[13] There is some uncertainty as to why a lightning strike is attracted to a lightning rod or similar protector, the leading assumption being that the air near the rod becomes ionized during an electrical storm, and thus highly conductive relative to the surrounding air.[13]
In telegraphy and telephony, a lightning arrester is placed where wires enter a structure, preventing damage to electronic instruments within and ensuring the safety of individuals near them. Lightning arresters, also called surge protectors, are devices that are connected between each electrical conductor in a power and communications systems and the Earth. These provide a short circuit to the ground that is interrupted by a non-conductor, over which lightning jumps. Its purpose is to limit the rise in voltage when a communications or power line is struck by lightning.
The non-conducting material may consist of a semi-conducting material such as silicon carbide or zinc oxide, or a spark gap. Primitive varieties of such spark gaps are simply open to the air, but more modern varieties are filled with dry gas and have a small amount of radioactive material to encourage the gas to ionize when the voltage across the gap reaches a specified level. Other designs of lightning arresters use a glow-discharge tube (essentially like a neon glow lamp) connected between the protected conductor and ground, or voltage-activated solid-state switches called varistors or MOVs. Lightning arresters built for substation use are impressive devices, consisting of a porcelain tube several feet long and several inches in diameter, filled with disks of zinc oxide. A safety port on the side of the device vents the occasional internal explosion without shattering the porcelain cylinder.'
High-tension power lines carry a lighter conductor (sometimes called a 'pilot' or 'shield') wire over the main power conductors. This conductor is grounded at various points along the link, or insulated from the tower structures by small insulators that are easily jumped by lightning voltages. The latter allows the pilot wire to be used for communications purposes, or to carry current for aircraft clearance lights. Electrical substations may have a web of grounded wires covering the whole plant.
Mast radiators are insulated from the ground by a gap at the base. When lightning hits the mast, it jumps this gap. A small inductivity in the feed line between the mast and the tuning unit (usually one winding) limits the voltage increase, protecting the transmitter from dangerously high voltages. The transmitter must be equipped with a device to monitor the antenna's electrical properties. This is very important, as a charge could remain after a lightning strike, damaging the gap or the insulators. The monitoring device switches off the transmitter when the antenna shows incorrect behavior, e.g. as a result of undesired electrical charge. When the transmitter is switched off, these charges dissipate. The monitoring device makes several attempts to switch back on. If after several attempts the antenna continues to show improper behavior, possibly as result of structural damage, the transmitter remains switched off.
Ideally, the underground part of the assembly should reside in an area of high ground conductivity. If the underground cable is able to resist corrosion well, it can be covered in salt to improve its electrical connection with the ground. While the electrical resistance of the lightning conductor between the air terminal and the Earth is concerning, the inductive reactance of the conductor could be more important. For this reason, the down conductor route is kept short, and any curves have a large radius. If these measures are not taken, lightning current may arc over an obstruction, resistive or reactive, that it encounters in the conductor. At the very least, the arc current will damage the lightning conductor and can easily find another conductive path, such as building wiring or plumbing, and cause fires or other disasters. Grounding systems without low resistivity to the ground can still be effective in protecting a structure from lightning damage. When ground soil has poor conductivity, is very shallow, or non-existent, a grounding system can be augmented by adding ground rods, counterpoise (ground ring) conductor, cable radials projecting away from the building, or a concrete building's reinforcing bars can be used for a ground conductor (Ufer Ground). These additions, while still not reducing the resistance of the system in some instances, will allow the dissipation of the lightning into the earth without damage to the structure.[14]
Additional precautions must be taken to prevent side-flashes between conductive objects on or in the structure and the lightning protection system. The surge of lightning current through a lightning protection conductor will create a voltage difference between it and any conductive objects that are near it. This voltage difference can be large enough to cause a dangerous side-flash (spark) between the two that can cause significant damage, especially on structures housing flammable or explosive materials. The most effective way to prevent this potential damage is to ensure the electrical continuity between the lightning protection system and any objects susceptible to a side-flash. Effective bonding will allow the voltage potential of the two objects to rise and fall in tandem, thereby eliminating any risk of a side-flash.[15]
Considerable material is used to make up lightning protection systems, so it is prudent to consider carefully where a rod structure will have the greatest effect. Historical understanding of lightning, from statements made by Ben Franklin,[16] assumed that each device protected a cone of 45 degrees.[17] 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 46 m near the ground.[18]
An object outside the critical distance is unlikely to be struck by the leader if there is a solidly grounded object within the critical distance. Locations that are considered 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 strike points, consider a sphere rolling over the terrain. At each point, we are simulating a potential leader position. Lightning is most likely to strike where the sphere touches the ground. Points that the sphere cannot roll across and touch are safest from lightning. Lightning protectors should be placed where they will prevent the sphere from touching a structure. A weak point in most lightning diversion systems is in transporting the captured discharge from the lightning rod to the ground, though.[19] Lightning rods are typically installed around the perimeter of flat roofs, or along the peaks of sloped roofs at intervals of 6.1 m or 7.6 m, depending on the height of the rod.[20] When a flat roof has dimensions greater than 15 m by 15 m, additional air terminals will be installed in the middle of the roof at intervals of 15 m or less in a rectangular grid pattern.[21]
This was a controversy as early as the 18th century. In the midst of political confrontation between Britain and its American colonies, British scientists maintained that a lightning rod should have a ball on its end. American scientists maintained that there should be a point. As of 2003[update], the controversy had not been completely resolved.[22] It is difficult to resolve the controversy because proper controlled experiments are nearly impossible in such work; in spite of the work of Moore, et al. [described below] most lightning rods seen on buildings have sharp points. Work performed by Charles B. Moore, et al.[23], in 2000 has helped this issue, finding that moderately rounded or blunt-tipped lightning rods act as marginally better strike receptors. [described below] As a result, round-tipped rods are installed the majority of the time on new systems in the United States. To quote:
In addition, the height of the lightning protector relative to the structure to be protected and the Earth itself will have an effect.[24][25]
Lightning dissipators have been widely discredited and criticized by lightning researchers over the last 30 years. These terminals (known as Dissipation Array Systems, and Charge Transfer Systems) claim to make a structure less attractive to lightning and other charges that 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 are alleged to deal with the phenomena such as electrostatic fields, electromagnetic fields, field transients, static charges, and any other related atmospheric electricity phenomena.
Individual dissipator rods may appear as slightly-blunted metal spikes sticking out in all directions from a metal conductor.[26] These elements are mounted on short metal arms at the top of a radio antenna or tower, the area most likely to be struck. The dissipation theory states an alteration in the potential difference (voltage) between the structure and the storm cloud miles above theoretically reduces but does not eliminate risk of lightning strikes.[27] Various manufacturers make these claims. Induced upward lightning strikes occurring on tall structures (effective heights of 300 m or more) can be reduced by altering the shape of the structure.[28]
A controversy over the assortment of operation theories dates back to the 18th century, when Franklin himself stated that his lightning protectors protected buildings by dissipating electric charge. He later retracted the statement, stating that the device's exact mode of operation was something of a mystery at that point. Diversion is a misnomer; no modern systems are claimed to divert anything, but rather to intercept the charge that terminates on a structure and carry it to the ground. The energy in a lightning strike is measured in joules. The reason that lightning does damage is that this energy is released in a matter of microseconds (typically 30 to 50 microseconds). If the same energy could be released slowly over a period of many seconds or minutes, the current flow would be in milliamperes or a few amperes at most. This is the intent of charge dissipation.[29]
The dissipation theory states that a lightning strike to a structure can be prevented by altering the electrical potential between the structure and the thundercloud. This is done by transferring electric charge (such as from the nearby Earth to the sky or vice versa).[27][30] Transferring electric charge from the Earth to the sky 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[31][32] and that a considerable electric current through the tower can be measured when thunderclouds are overhead.
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).[33] When there is no evidence, analyzing the strikes is difficult. This means that a strike on an uninstrumented structure must be visually confirmed, and the random behavior of lightning renders such observations difficult.[33][34][35][36] There are also inventors working on this problem,[37][38] 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.[39][40][41][42] Through accurate timing and triangulation techniques, lightning strikes can be located with great precision, so strikes on specific objects often can be confirmed with confidence.
The introduction of lightning protection systems into standards allowed various manufactures to develop protector systems to a multitude of specifications and there are various lightning rod standards.[43][44][45][46][47][48][49][50][51][52] 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[53]) 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.[54]
No major standards body, such as the NFPA or UL, 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).[55] Members of the Scientific Committee of the International Conference on Lightning Protection has issued a joint statement stating their opposition to dissipater technology.[56]
Various investigators believe the natural downward lightning strokes to be unpreventable.[28] 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.[57] Scientists from the National Lightning Safety Institute claim that these dissipation devices are nothing more than expensive lightning protectors and that they, unlike traditional methods, are not based on "scientifically proven and indisputable technical arguments".[58] William Rison states that in his opinion the underlying theory of dissipation is "scientific nonsense".[59] 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 building'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.[28] 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.[60]
Lightning protection for aircraft is provided by mounting devices on the aircraft structure. The protectors are provided with extensions through the structure of the aircraft's outer surface and within a static discharger. Protection systems for use in aircraft must protect critical and non-critical electronic equipment. Aircraft lightning protection provides an electrical path having a plurality of conductive segments, continuous or discontinuous, that upon exposure to a high voltage field form an ionization channel due to the system's breakdown voltage. Various lightning protection systems must reject the surge currents associated with the lightning strikes. Lightning protection means for aircraft include components which are dielectrics and metallic layers applied to the ordinarily lightning-accessible surfaces of composite structures. Various ground connection means to the layers comprises a section of wire mesh fusing the various layers to an attachment connecting the structure to an adjacent ground structure. Composite-to-metal or composite-to-composite structural joints are protected by making the interface areas conductive for transfer of lightning current.
Some aircraft lightning protection systems use a shielded cable system. These systems consist of one or more conductors enclosed by a conductive shield. The cable has both conductors of one end connected to a grounding element. This is intended to provide protection from electromagnetic interference. Such systems reduce the electromagnetically induced voltage in a shielded conductor. This is intended to provide protection against induced electromagnetic interference from lightning. This network provides a normally-high impedance which breaks down to a very low impedance in response to a momentary voltage surge electromagnetically induced in the shield. This establishes a conductive path between the shield and ground. Any surge voltage from lightning creates a current through the cable. This results in an electromagnetic field of the opposite direction, which cancels or reduces the magnitude of the electromagnetic field within the shielded cable.
A lightning protection installation on a watercraft comprises a lightning protector mounted on the top of the mast or superstructure and a grounding conductor in contact with the water. Electrical conductors attach to the protector and run downward to the conductor. For a vessel with a conducting (iron or steel) hull, the grounding conductor is the hull. For a vessel with a non-conducting hull, the grounding conductor may be retractable, part of the hull, or attached to a centerboard.