Lightning rod
Overview
 
A lightning rod or lightning conductor (UK
British English
British English, or English , is the broad term used to distinguish the forms of the English language used in the United Kingdom from forms used elsewhere...

) is a metal rod or conductor
Electrical conductor
In physics and electrical engineering, a conductor is a material which contains movable electric charges. In metallic conductors such as copper or aluminum, the movable charged particles are electrons...

 mounted on top of a building and electrically connected to the ground
Ground (electricity)
In electrical engineering, ground or earth may be the reference point in an electrical circuit from which other voltages are measured, or a common return path for electric current, or a direct physical connection to the Earth....

 through a wire, to protect the building in the event of lightning
Lightning
Lightning is an atmospheric electrostatic discharge accompanied by thunder, which typically occurs during thunderstorms, and sometimes during volcanic eruptions or dust storms...

. 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.
Encyclopedia
A lightning rod or lightning conductor (UK
British English
British English, or English , is the broad term used to distinguish the forms of the English language used in the United Kingdom from forms used elsewhere...

) is a metal rod or conductor
Electrical conductor
In physics and electrical engineering, a conductor is a material which contains movable electric charges. In metallic conductors such as copper or aluminum, the movable charged particles are electrons...

 mounted on top of a building and electrically connected to the ground
Ground (electricity)
In electrical engineering, ground or earth may be the reference point in an electrical circuit from which other voltages are measured, or a common return path for electric current, or a direct physical connection to the Earth....

 through a wire, to protect the building in the event of lightning
Lightning
Lightning is an atmospheric electrostatic discharge accompanied by thunder, which typically occurs during thunderstorms, and sometimes during volcanic eruptions or dust storms...

. 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
Lightning protection system
A lightning protection system is a system designed to protect a structure from damage due to lightning strikes by intercepting such strikes and safely passing their extremely high voltage currents to "ground"...

. In addition to rods placed at regular intervals on the highest portions of a structure, a lightning
Lightning
Lightning is an atmospheric electrostatic discharge accompanied by thunder, which typically occurs during thunderstorms, and sometimes during volcanic eruptions or dust storms...

 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
Benjamin Franklin
Dr. Benjamin Franklin was one of the Founding Fathers of the United States. A noted polymath, Franklin was a leading author, printer, political theorist, politician, postmaster, scientist, musician, inventor, satirist, civic activist, statesman, and diplomat...

 in the Americas in 1749 and, perhaps independently, by Prokop Diviš
Václav Prokop Diviš
Václav Prokop Diviš was a Czech priest, theologian and natural scientist.Diviš was born March 26 1698 in Helvíkovice, Bohemia . It has been claimed that the lightning rod he erected in 15 June 1754 was invented independently of Benjamin Franklin, but this has been disputed by other scholars...

 in Europe in 1754.

History

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.

Asia

Some of the most ancient lightning conductors can be found in Sri Lanka
Sri Lanka
Sri Lanka, officially the Democratic Socialist Republic of Sri Lanka is a country off the southern coast of the Indian subcontinent. Known until 1972 as Ceylon , Sri Lanka is an island surrounded by the Indian Ocean, the Gulf of Mannar and the Palk Strait, and lies in the vicinity of India and the...

 in places like the Kingdom
Monarchy
A monarchy is a form of government in which the office of head of state is usually held until death or abdication and is often hereditary and includes a royal house. In some cases, the monarch is elected...

 of Anuradhapura
Anuradhapura
Anuradhapura, , is one of the ancient capitals of Sri Lanka, famous for its well-preserved ruins of ancient Lankan civilization.The city, now a UNESCO World Heritage Site, lies 205 km north of the current capital Colombo in Sri Lanka's North Central Province, on the banks of the historic...

 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.

Russia

A lightning conductor may have been intentionally used in the Leaning Tower of Nevyansk
Leaning Tower of Nevyansk
The Leaning Tower of Nevyansk is a tower in the town of Nevyansk in Sverdlovsk Oblast, Russia, built in the 18th century. Its construction was funded by Peter the Great’s associate and a famous Russian manufacturer Akinfiy Demidov .The height of the tower is 57.5 m from the ground and the...

. The spire
Spire
A spire is a tapering conical or pyramidal structure on the top of a building, particularly a church tower. Etymologically, the word is derived from the Old English word spir, meaning a sprout, shoot, or stalk of grass....

 of the tower is crowned with a metallic rod in the shape of a gild
Gilding
The term gilding covers a number of decorative techniques for applying fine gold leaf or powder to solid surfaces such as wood, stone, or metal to give a thin coating of gold. A gilded object is described as "gilt"...

ed sphere
Sphere
A sphere is a perfectly round geometrical object in three-dimensional space, such as the shape of a round ball. Like a circle in two dimensions, a perfect sphere is completely symmetrical around its center, with all points on the surface lying the same distance r from the center point...

 with spikes. This lightning rod is grounded
Ground (electricity)
In electrical engineering, ground or earth may be the reference point in an electrical circuit from which other voltages are measured, or a common return path for electric current, or a direct physical connection to the Earth....

 through the rebar
Rebar
A rebar , also known as reinforcing steel, reinforcement steel, rerod, or a deformed bar, is a common steel bar, and is commonly used as a tensioning device in reinforced concrete and reinforced masonry structures holding the concrete in compression...

 carcass, which pierces the entire building.

The Nevyansk Tower was built between 1725 and 1732, on the orders of industrialist Akinfiy Demidov
Akinfiy Demidov
Akinfiy Nikitich Demidov was a Russian industrialist of the Demidov family.-Life:He was the eldest son of Nikita Demidov and increased the family fortune, raising it to one of Russia's most important industrial dynasties. He set up at least 9 steel foundries and munitions factories from 1717 to...

. The Nevyansk Tower was built 25 years before Benjamin Franklin's experiment and scientific explanation. However, the true intent behind the metal rooftop and rebars remains unknown.

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 may have been independently invented by Czech priest
Priest
A priest is a person authorized to perform the sacred rites of a religion, especially as a mediatory agent between humans and deities. They also have the authority or power to administer religious rites; in particular, rites of sacrifice to, and propitiation of, a deity or deities...

 Václav Prokop Diviš
Václav Prokop Diviš
Václav Prokop Diviš was a Czech priest, theologian and natural scientist.Diviš was born March 26 1698 in Helvíkovice, Bohemia . It has been claimed that the lightning rod he erected in 15 June 1754 was invented independently of Benjamin Franklin, but this has been disputed by other scholars...

 between 1750 to 1754.

United States

In the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

, 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
Electricity
Electricity is a general term encompassing a variety of phenomena resulting from the presence and flow of electric charge. These include many easily recognizable phenomena, such as lightning, static electricity, and the flow of electrical current in an electrical wire...

. 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. Franklin speculated that, with an iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

 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
Christ Church, Philadelphia
Christ Church is an Episcopal church located in Philadelphia, Pennsylvania. It was founded in 1695 by members of the Church of England, who built a small wooden church on the site by the next year. When the congregation outgrew this structure some twenty years later, they decided to erect a new...

 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
Glass
Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...

 balls (now prized by collectors). The ornamental appeal of these glass balls has been used in weather vane
Weather vane
A weather vane is an instrument for showing the direction of the wind. They are typically used as an architectural ornament to the highest point of a building....

s. 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
Confirmation bias
Confirmation bias is a tendency for people to favor information that confirms their preconceptions or hypotheses regardless of whether the information is true.David Perkins, a geneticist, coined the term "myside bias" referring to a preference for "my" side of an issue...

 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
William Snow Harris
Sir William Snow Harris was an English physician and electrical researcher, nicknamed Thunder-and-Lightning Harris, and noted for his invention of a successful system of lightning conductors for ships...

 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
Royal Navy
The Royal Navy is the naval warfare service branch of the British Armed Forces. Founded in the 16th century, it is the oldest service branch and is known as the Senior Service...

 did not adopt the system until 1842, by which time the Imperial Russian Navy
Imperial Russian Navy
The Imperial Russian Navy refers to the Tsarist fleets prior to the February Revolution.-First Romanovs:Under Tsar Mikhail Feodorovich, construction of the first three-masted ship, actually built within Russia, was completed in 1636. It was built in Balakhna by Danish shipbuilders from Holstein...

 had already adopted the system.

Nikola Tesla
Nikola Tesla
Nikola Tesla was a Serbian-American inventor, mechanical engineer, and electrical engineer...

'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
Logic
In philosophy, Logic is the formal systematic study of the principles of valid inference and correct reasoning. Logic is used in most intellectual activities, but is studied primarily in the disciplines of philosophy, mathematics, semantics, and computer science...

 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.

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.
Washington, D.C.
Washington, D.C., formally the District of Columbia and commonly referred to as Washington, "the District", or simply D.C., is the capital of the United States. On July 16, 1790, the United States Congress approved the creation of a permanent national capital as permitted by the U.S. Constitution....

 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 Statue of Liberty
Statue of Liberty
The Statue of Liberty is a colossal neoclassical sculpture on Liberty Island in New York Harbor, designed by Frédéric Bartholdi and dedicated on October 28, 1886...

 in New York Harbor gets hit with lightning which is grounded out.

Structure protectors

Lightning diversion

Lightning rods convey the current from a strike to the ground or water via a low-resistance
Electrical resistance
The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...

 conductor
Electrical conductor
In physics and electrical engineering, a conductor is a material which contains movable electric charges. In metallic conductors such as copper or aluminum, the movable charged particles are electrons...

. 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. 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
Electrical storm
Electrical storm may refer to:* A thunderstorm* A medical condition of chaotic electrical activity of the heart, usually manifested by ventricular tachycardia* "Electrical Storm" , song by U2...

, and thus highly conductive relative to the surrounding air.

Lightning arrester

In telegraphy
Telegraphy
Telegraphy is the long-distance transmission of messages via some form of signalling technology. Telegraphy requires messages to be converted to a code which is known to both sender and receiver...

 and telephony
Telephony
In telecommunications, telephony encompasses the general use of equipment to provide communication over distances, specifically by connecting telephones to each other....

, 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 protector
Surge protector
A surge protector is an appliance designed to protect electrical devices from voltage spikes. A surge protector attempts to limit the voltage supplied to an electric device by either blocking or by shorting to ground any unwanted voltages above a safe threshold...

s, are devices that are connected between each electrical conductor in a power and communications systems and the Earth. These provide a short circuit
Short circuit
A short circuit in an electrical circuit that allows a current to travel along an unintended path, often where essentially no electrical impedance is encountered....

 to the ground
Ground (electricity)
In electrical engineering, ground or earth may be the reference point in an electrical circuit from which other voltages are measured, or a common return path for electric current, or a direct physical connection to the Earth....

 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 varistor
Varistor
A varistor is an electronic component with a "diode-like" nonlinear current–voltage characteristic. The name is a portmanteau of variable resistor...

s or MOV
Varistor
A varistor is an electronic component with a "diode-like" nonlinear current–voltage characteristic. The name is a portmanteau of variable resistor...

s. 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
Electricity pylon
A transmission tower is a tall structure, usually a steel lattice tower, used to support an overhead power line. They are used in high-voltage AC and DC systems, and come in a wide variety of shapes and sizes...

 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 radiator
Mast radiator
A mast radiator is a radio mast or tower in which the whole structure itself functions as an antenna. This design is commonly used for transmitting antennas operating at low frequencies, in the VLF, LF and MF ranges, in particular those used for AM broadcasting. The metal mast is electrically...

s 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.

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 able to resist corrosion
Corrosion
Corrosion is the disintegration of an engineered material into its constituent atoms due to chemical reactions with its surroundings. In the most common use of the word, this means electrochemical oxidation of metals in reaction with an oxidant such as oxygen...

 well, it can be covered in salt
Salt
In chemistry, salts are ionic compounds that result from the neutralization reaction of an acid and a base. They are composed of cations and anions so that the product is electrically neutral...

 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
Counterpoise (ground system)
A counterpoise is a type of electrical ground that is not connected to earth. It is used in radio antenna systems when a normal earth ground cannot be used because of high soil resistance It consists of a network of wires or cables running parallel to the ground, suspended from a few centimetres...

 (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
Ufer Ground
The Ufer Ground is an electrical earth grounding method developed during World War II. It uses a concrete-encased electrode to improve grounding in dry areas. The technique is used in construction of concrete foundations.-History :...

). 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
Cone (geometry)
A cone is an n-dimensional geometric shape that tapers smoothly from a base to a point called the apex or vertex. Formally, it is the solid figure formed by the locus of all straight line segments that join the apex to the base...

 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
Ground (electricity)
In electrical engineering, ground or earth may be the reference point in an electrical circuit from which other voltages are measured, or a common return path for electric current, or a direct physical connection to the Earth....

 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.

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
Sphere
A sphere is a perfectly round geometrical object in three-dimensional space, such as the shape of a round ball. Like a circle in two dimensions, a perfect sphere is completely symmetrical around its center, with all points on the surface lying the same distance r from the center point...

 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 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. , the controversy had not been completely resolved.
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
Charles B. Moore
Charles B. Moore, Jr. was an American physicist, engineer and meteorologist, known for his work with gas balloons...

, 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. To quote:

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
Electric field
In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...

 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
Structure
Structure is a fundamental, tangible or intangible notion referring to the recognition, observation, nature, and permanence of patterns and relationships of entities. This notion may itself be an object, such as a built structure, or an attribute, such as the structure of society...

 less attractive to lightning and other charges that flow through the Earth's atmosphere
Earth's atmosphere
The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...

 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
Atmospheric electricity
Atmospheric electricity is the regular diurnal variations of the Earth's atmospheric electromagnetic network . The Earth's surface, the ionosphere, and the atmosphere is known as the global atmospheric electrical circuit...

. The devices are alleged to deal with the phenomena such as electrostatic fields, electromagnetic field
Electromagnetic field
An electromagnetic field is a physical field produced by moving electrically charged objects. It affects the behavior of charged objects in the vicinity of the field. The electromagnetic field extends indefinitely throughout space and describes the electromagnetic interaction...

s, field transients
Transient (oscillation)
A transient event is a short-lived burst of energy in a system caused by a sudden change of state.The source of the transient energy may be an internal event or a nearby event...

, static charges, and any other related atmospheric electricity
Atmospheric electricity
Atmospheric electricity is the regular diurnal variations of the Earth's atmospheric electromagnetic network . The Earth's surface, the ionosphere, and the atmosphere is known as the global atmospheric electrical circuit...

 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
Radio masts and towers
Radio masts and towers are, typically, tall structures designed to support antennas for telecommunications and broadcasting, including television. They are among the tallest man-made structures...

, the area most likely to be struck. The dissipation theory states an alteration in the potential difference (voltage
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...

) between the structure and the storm cloud
Cloud
A cloud is a visible mass of liquid droplets or frozen crystals made of water and/or various chemicals suspended in the atmosphere above the surface of a planetary body. They are also known as aerosols. Clouds in Earth's atmosphere are studied in the cloud physics branch of meteorology...

 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
Controversy
Controversy is a state of prolonged public dispute or debate, usually concerning a matter of opinion. The word was coined from the Latin controversia, as a composite of controversus – "turned in an opposite direction," from contra – "against" – and vertere – to turn, or versus , hence, "to turn...

 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 joule
Joule
The joule ; symbol J) is a derived unit of energy or work in the International System of Units. It is equal to the energy expended in applying a force of one newton through a distance of one metre , or in passing an electric current of one ampere through a resistance of one ohm for one second...

s. 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.

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 behavior of lightning renders such observations difficult. There are also inventors working on this problem, such as through a lightning rocket
Lightning rocket
The term lightning rocket can generically describe a rocket device used to trigger lightning as well as a specific system patented by Robert E. Betts....

. 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
National Fire Protection Association
The National Fire Protection Association is a United States trade association that creates and maintains private, copywrited, standards and codes for usage and adoption by local governments...

'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
Underwriters Laboratories
Underwriters Laboratories Inc. is an independent product safety certification organization. Established in 1894, the company has its headquarters in Northbrook, Illinois. UL develops standards and test procedures for products, materials, components, assemblies, tools and equipment, chiefly dealing...

, 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
Structure
Structure is a fundamental, tangible or intangible notion referring to the recognition, observation, nature, and permanence of patterns and relationships of entities. This notion may itself be an object, such as a built structure, or an attribute, such as the structure of society...

 of the aircraft's outer surface and within a static discharger
Static discharger
Static dischargers are commonly known as static wicks or static discharge wicks. They are used on aircraft to allow the continuous satisfactory operation of onboard navigation and radio communication systems. During adverse charging conditions , they limit the potential static buildup on the...

. 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
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...

 field
Field (physics)
In physics, a field is a physical quantity associated with each point of spacetime. A field can be classified as a scalar field, a vector field, a spinor field, or a tensor field according to whether the value of the field at each point is a scalar, a vector, a spinor or, more generally, a tensor,...

 form an ionization
Ionization
Ionization is the process of converting an atom or molecule into an ion by adding or removing charged particles such as electrons or other ions. This is often confused with dissociation. A substance may dissociate without necessarily producing ions. As an example, the molecules of table sugar...

 channel due to the system's breakdown voltage
Breakdown voltage
The breakdown voltage of an insulator is the minimum voltage that causes a portion of an insulator to become electrically conductive.The breakdown voltage of a diode is the minimum reverse voltage to make the diode conduct in reverse...

. 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
Composite material
Composite materials, often shortened to composites or called composition materials, are engineered or naturally occurring materials made from two or more constituent materials with significantly different physical or chemical properties which remain separate and distinct at the macroscopic or...

 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
Shielded cable
A shielded or screened cable is an electrical cable of one or more insulated conductors enclosed by a common conductive layer. The shield may be composed of braided strands of copper , a non-braided spiral winding of copper tape, or a layer of conducting polymer. Usually, this shield is covered...

 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
Electromagnetic interference
Electromagnetic interference is disturbance that affects an electrical circuit due to either electromagnetic induction or electromagnetic radiation emitted from an external source. The disturbance may interrupt, obstruct, or otherwise degrade or limit the effective performance of the circuit...

. Such systems reduce the electromagnetically
Electromagnetism
Electromagnetism is one of the four fundamental interactions in nature. The other three are the strong interaction, the weak interaction and gravitation...

 induced
Electromagnetic induction
Electromagnetic induction is the production of an electric current across a conductor moving through a magnetic field. It underlies the operation of generators, transformers, induction motors, electric motors, synchronous motors, and solenoids....

 voltage in a shielded conductor. This is intended to provide protection against induced electromagnetic interference from lightning. This network provides a normally-high impedance
Electrical impedance
Electrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...

 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.

Watercraft protectors

A lightning protection installation on a watercraft
Watercraft
A watercraft is a vessel or craft designed to move across or through water. The name is derived from the term "craft" which was used to describe all types of water going vessels...

 comprises a lightning protector mounted on the top of the mast
Mast (sailing)
The mast of a sailing vessel is a tall, vertical, or near vertical, spar, or arrangement of spars, which supports the sails. Large ships have several masts, with the size and configuration depending on the style of ship...

 or superstructure and a grounding
Ground (electricity)
In electrical engineering, ground or earth may be the reference point in an electrical circuit from which other voltages are measured, or a common return path for electric current, or a direct physical connection to the Earth....

 conductor
in contact with the water. Electrical conductor
Electrical conductor
In physics and electrical engineering, a conductor is a material which contains movable electric charges. In metallic conductors such as copper or aluminum, the movable charged particles are electrons...

s 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
Hull (watercraft)
A hull is the watertight body of a ship or boat. Above the hull is the superstructure and/or deckhouse, where present. The line where the hull meets the water surface is called the waterline.The structure of the hull varies depending on the vessel type...

, or attached to a centerboard.

See also

  • List of lightning rod patents
  • Grounding kit
    Grounding kit
    A grounding kit / earthing kit can be described as a kind of lightning protector which avoids lightning punctures on cables. It is used for grounding /earthing coaxial cables of copper or aluminium on antenna installations for telecommunication .-Construction:Grounding kits / Earthing kits are...

  • Václav Prokop Diviš
    Václav Prokop Diviš
    Václav Prokop Diviš was a Czech priest, theologian and natural scientist.Diviš was born March 26 1698 in Helvíkovice, Bohemia . It has been claimed that the lightning rod he erected in 15 June 1754 was invented independently of Benjamin Franklin, but this has been disputed by other scholars...

     (1698–1765), constructor of the first grounded lightning rod, in Přímětice u Znojma during 1750-1754.
  • James Otis, Jr.
    James Otis, Jr.
    James Otis, Jr. was a lawyer in colonial Massachusetts, a member of the Massachusetts provincial assembly, and an early advocate of the political views that led to the American Revolution. The phrase "Taxation without Representation is Tyranny" is usually attributed to him...

    , contemporary of Ben Franklin, killed at doorway by lightning in Andover, Massachusetts
    Andover, Massachusetts
    Andover is a town in Essex County, Massachusetts, United States. It was incorporated in 1646 and as of the 2010 census, the population was 33,201...

     on May 23, 1783.

External links

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