Lightning rod
2008/9 Schools Wikipedia Selection. Related subjects: Engineering
A lightning rod (USA) or lightning conductor (UK) 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 actual rooftop lightning rod is a metal strip or rod, usually of copper or aluminium. Lightning protection systems are installed on structures, trees, monuments, bridges and even water vessels to protect from lightning damage. Individual lightning rods are sometimes called finials, air terminals or strike termination devices. The United States Patent Office labels "Lightning protectors" in Class 174 (Electricity: conductors and insulators), Subclass 2 (Lightning protectors) and Subclass 3 (Rods).
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
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. In Europe, the lightning rod was invented by Václav Prokop Diviš between 1750-1754.
United States
In the United States, the pointed lightning rod conductor, also called a "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 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. 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 (now prized by collectors). The ornamental appeal of these glass balls has also been incorporated into weather vanes.
Balls of solid glass occasionally were 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.
Nikola Tesla's 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 DuPont Explosives manufacturing sites, which were surrounded by pine trees, used various lightning protection devices. 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.
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. The statue was designed with multiple devices that are tipped with platinum. The Washington Monument also was 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.
Structure 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. Diversion is a misnomer; modern systems intercept the charge that terminates on a structure and carry it to the ground. The diversion theory states that the lightning rod protects a structure purely because it is grounded, and thus a lightning strike that happens to attach to the protector will be diverted around the structure and "earthed" through a grounding cable or conductor. 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 manufacturers make these claims.
Lightning arresters
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 myriad 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.
Electric power system lightning protection
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.
Lightning protection of mast radiators
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 behaviour, possibly as result of structural damage, the transmitter remains switched off.
Lightning conductors and grounding precautions
Ideally, the underground part of the assembly should reside in an area of high ground conductivity. If the underground cable is 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, and/or cable radials projecting away from the building. 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.
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.
Lightning protection system design
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, assumed that each device 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 46 m near the ground.
Electricity travels mostly 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 solidly OR 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 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. 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. 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.
Should a lightning rod have a point?
This was a controversy as early as the 1700s. 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. The controversy has not been completely resolved, mostly due to the fact that 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 Moore, et al, 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.
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" by Moore et al say:
- Calculations of the relative strengths of the electric fields above similarly exposed sharp and blunt rods show that while the fields are much stronger at the tip of a sharp rod prior to any emissions, 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 over an otherwise similar, sharper rod of 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 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 have an effect.
Lightning dissipation
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. 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. 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.
Evaluations and analysis
A controversy over 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, stating that the device's exact mode of operation was something of a mystery at that point. Thus began a 250-year dispute over the dissipation theory versus diversion. 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 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). 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 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). 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 behaviour of lightning renders such observations difficult. The research situation is improving somewhat, however. There are also inventors working on this problem, 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. 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. 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 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). Members of the Scientific Committee of the International Conference on Lightning Protection has issued a joint statement stating their opposition to dissipater technology.
Various investigators believe the natural downward lightning strokes to be unpreventable. 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. 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". William Rison states that in his opinion the underlying theory of dissipation is "scientific nonsense". 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. 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.
Aircraft protectors
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 the electronic equipment which is critical to aircraft flight and equipment which is not critical to aircraft flight. 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 which 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 carry one or more conductors enclosed by a conductive shield having one end connected to grounding element to provide protection from electrostatic interference. Such systems reduce the electromagnetically induced voltage in a shielded conductor and provides protection from the induced electromagnetic interference of lightning. This network provides a high impedance and changing to a very-low impedance in response to a momentary voltage surge electromagnetically induced in the shield, thereby establishing a conductive circuit path between the shield and ground. Any surge voltage from lightning drives a shield current through the circuit to provide an electromagnetic field of the opposite direction canceling and reducing the magnitude of the overall electromagnetic field that links the shielded cable.
Watercraft protectors
A Lightning protection installation on a watercraft comprises a lightning protector mounted on the top of the mast or high on the superstructure of a vessel that does not have a mast. Electrical conductors are attached to the device and run downward to a " grounding" conductor in contact with the water. 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.