Lightning Arrester

A lightning rod (or lightning protector) is a metal strip or rod, usually of copper or similar conductive material, used as part of lightning safety to protect tall or isolated structures (such as the roof of a building or the mast of a vessel) from lightning damage. Its formal name is lightning finial or air terminal. Sometimes, the system is informally referred to as a lightning conductor, lightning arrester, or lightning discharger; however, these terms actually refer to lightning protection systems in general or specific components within them.

Construction and uses

A lightning rod is connected via a low-resistance wire or cable to the earth or water below, where the charge may be safely dissipated. Lightning rods sometimes possess a short circuit to the ground that is interrupted by a thin non-conductor over which lightning jumps. Ideally, the underground part of the assembly should reside in a muddy area, or an area that tends to become so during storms. If the underground cable will resist corrosion well, it may be covered in salt to improve its electrical connection with the ground.

In telegraphy and telephony a lightning rod is placed where wires enter a structure, preventing damage to electronic instruments within and ensuring the safety of individuals near them. Similarly, high-tension power lines carry a lighter conductor wire over the main power conductors. This conductor is grounded at various points along the link. Electrical substations usually have a web of the lighter conductor wires covering the whole plant.

Considerable material is used in the construction of lightning arresters, so it is prudent to work out where a new arrester will have the greatest effect. Historical understanding of lightning assumed that each rod protected a cone of 45 degrees. This has been found to be unsatisfactory for protecting taller structures, as it is possible for lightning to strike the side of a building.

A better technique to determine the effect of a new arrester is called the rolling sphere technique and was developed by Dr Tibor Horváth. To understand this requires knowledge of how lightning 'moves'. As the step leader of a lightning bolt jumps toward the ground, it steps toward the grounded objects nearest its path. The maximum distance that each step may travel is called the critical distance and is proportional to the electrical current. Objects are likely to be struck if they are nearer to the leader than this critical distance. It is standard practice to approximate the sphere's radius as 60 m near the ground.

Electricity travels along the path of least resistance, so an object outside the critical distance is unlikely to be struck by the leader if there is a grounded object within the critical distance. Noting this, locations that are safe from lightning can be determined by imagining a leader's potential paths as a sphere that travels from the cloud to the ground.

For lightning protection it suffices to consider all possible spheres as they touch potential strike points. To determine which strike points consider a sphere rolling over the terrain. At each point we are simulating a potential leader position and where the sphere touches the ground the lightning is most likely to strike. Points which the sphere cannot roll across and touch are safest from lightning. Lightning rods should be placed where they will prevent the sphere from touching a structure.

It is commonly believed, erroneously, that a rod ending in a sharp point at the peak is the best means to conduct the current of a lightning strike to the ground. According to field research, a rod with a rounded or spherical end is better. "Lightning Rod Improvement Studies" by Moore et al say:

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Calculations of the relative strengths of the electric fields above similarly exposed sharp and blunt rods show that although the fields, prior to any emissions, are much stronger at the tip of a sharp rod, they decrease more rapidly with distance. As a result, at a few centimeters above the tip of a 20-mm-diameter blunt rod, the strength of the field is greater than that over an otherwise similar, sharper rod at the same height. Since the field strength at the tip of a sharpened rod tends to be limited by the easy formation of ions in the surrounding air, the field strengths over blunt rods can be much stronger than those at distances greater than 1 cm over sharper ones. The results of this study suggest that moderately blunt metal rods (with tip height–to–tip radius of curvature ratios of about 680:1) are better lightning strike receptors than are sharper rods or very blunt ones.

Lightning prevention

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

This controversy dates back to the 1700's, when Franklin himself stated that his lightning rods protected buildings by dissipating electric charge. He later retracted the statement with a disclaimer stating that the exact mode of operation of the device was something of a mystery at that point.

Thus began a 250-year dispute between the dissipation theory and the diversion theory of lightning protection.

The dissipation theory states that a lightning strike to a structure can be prevented by reducing the electrical potential between the structure and the thundercloud by transferring electric charge from the nearby earth to the sky. This is done by erecting some sort of tower equipped with one or more sharply-pointed rods upon the structure. While it is true that sharply-pointed objects will indeed transfer charge to the surrounding atmosphere, and it is also true that a considerable electric current through the tower can be measured when thunderclouds are overhead, there is no proof that such an arrangement is at all effective. All DuPont Explosives manufacturing sites were surrounding by pine trees. During the 1950's, DuPont was making nitroglycerin in some buildings and moving it in 'Angel Buggies' to the packing building. Employees at those sites were very sensitive to potential lightning strikes.[citation needed]

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

The diversion theory states that the lightning rod protects a building purely because it is grounded, and thus a lightning stroke that happens to attach to the rod will be diverted around the structure and down to earth through a ground cable. There is some uncertainty as to why a

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lightning strike might preferentially attach to a lightning rod; the leading assumption is that the air near the rod becomes ionized and thus conductive due to the intense electric field.

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

Moreover, a lightning strike to a metallic structure frequently leaves no evidence excepting perhaps a small pit in the metal. This means that a strike on an un-instrumented structure must be visually confirmed, and the random behavior of lightning renders such observations difficult.

Thus if we strip away the two centuries of legal actions, political activity and general outrage exhibited by both sides, we find that the current state of the dissipation/diversion controversy is a draw; that neither theory has or can be proven, and that essentially all data pertaining to the behavior of lightning on structures must be considered anecdotal.

The research situation is improving somewhat, however. While controlled experiments may be far in the future, very good data is being obtained through techniques which use a network of radio receivers that watch for the characteristic electrical 'signature' of lightning strikes using fixed directional antennas. Through accurate timing and triangulation techniques, lightning strikes can be located with great precision, and so strikes on specific objects can often be confirmed with confidence.

The most common individual dissipator rods (or dissipator elements) appear as slightly-blunted metal spikes sticking out in all directions from a metal conductor. These elements are mounted on short metal arms at the very top of a radio antenna or tower, the area by far most likely to be struck. According to various manufacture claims, there is supposedly a reduction in the potential difference (voltage) between the structure and the storm cloud, miles above, allegedly reducing, but not eliminating, the risk of lightning strikes.

There have been attempts to introduce lightning protection systems into standards. The NFPA's independent third party panel found that "the [Early Streamer Emission] lightning protection technology appears to be technically sound" and that there was an "adequate theoretical basis for the [Early Streamer Emission] air terminal concept and design from a physical viewpoint". (Bryan, 1999) The same panel also concluded that "the recommended [NFPA 780 standard] lightning protection system has never been scientifically or technically validated and the Franklin rod air terminals have not been validated in field tests under thunderstorm conditions." In response, the American Geophysical Union concluded that "[t]he Bryan Panel reviewed essentially none of the studies and literature on the effectiveness and scientific basis of traditional lightning protection systems and was erroneous in its conclusion that there was no basis for the Standard." AGU did not attempt to assess the effectiveness of any proposed modifications to traditional systems in its report.

No major standards body, such as the NFPA, UL, and the NLSI, has currently endorsed a device that can prevent or reduce lightning strikes. The NFPA Standards Council, following a request for a project to address Dissipation Array Systems and Charge Transfer Systems, denied the request to begin forming standards on such technology (though the Council did not foreclose on future standards development after reliable sources demonstrating the validity of the basic technology and science were submitted). Members of the Scientific Committee of the International Conference on Lightning Protection has issued a joint statement stating their opposition to dissipater technology.

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Investigators believe the natural downward lightning strokes to be unpreventable. Induced upward lightning strokes occurring on tall structures (effective heights of 300 m or more) can be reduced by altering the shape of the structure. According to opponents of the technology, the various designs indirectly "eliminate" lightning via the alteration and dissipaters only have a small effect (either intended or not) because there is no significant reduction the susceptibility of the tower to the generation of upward lightning strokes. Some field investigations of dissipaters show that their performance is comparable to conventional terminals and possess no great enhancement of protection. According to these field studies, these devices have not shown that they eliminate lightning strikes.

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