Lightning rod

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Image:Pointed Lightning Rod.jpg

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,
  • a lightning arrester, or
  • a lightning discharger.

However, these terms actually refer to lightning protection systems in general or specific components within them.

Contents

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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 Template:Ref. 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 60m 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" Template:Ref by Moore et al say:

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

Lightning damage has been with humanity since we 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.

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Europe

The church tower of many European cities, usually the highest structure, was the building often hit by lightning. Early on, Christian churches tried to prevent the occurrence of the damaging effects of lightning by prayers. Priests prayed,

temper the destruction of hail and cyclones and the force of tempests and lightning; check hostile thunders and great winds; and cast down the spirits of storms and the powers of the air.

Peter Ahlwardts ("Reasonable and Theological Considerations about Thunder and Lightning", 1745) gave information to individuals seeking cover from lightning to go anywhere except in or around a church.Template:Ref

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United States

In the United States, the pointed lightning rod conductor, and more accurately the "lightning attractor", was invented by Benjamin Franklin as part of his groundbreaking explorations of electricity. Franklin speculated that, with an iron rod sharpened to a point at the end,

the electrical fire would, I think, be drawn out of a cloud silently, before it could come near enough to strike [...].

Franklin had speculated about lightning rods for several years before his reported kite experiment.

This experiment, in fact, took place because he was tired of waiting for Christ Church in Philadelphia to be completed so he could place a lighting rod on top of it. There was some resistance from churches who felt that it was defying divine will to install these rods. Franklin countered that there is no religious objection to roofs on buildings to resist precipitation, so lightning, which he proved to be simply a giant electrical spark, should be no different.

In the 19th century the lightning rod became a symbol of American ingenuity and a decorative motif. Lightning rods were often embellished with ornamental glass ballsTemplate:Ref (now prized by collectors) that also served to provide visual sign of a lightning strike (when the rod is struck the glass ball shatters and falls off, indicating to the owner which rod got struck and that they should check it and the grounding wire for damage). The ornamental appeal of these glass balls has also been incorporated into weather vanes.

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

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Lightning prevention

Image:Lightning rod.png Lightning rod dissipaters (known as Early Streamer Emission, Dissipation Array Systems, and Charge Transfer Systems) claim to make a structure less attractive by which charges can flow to 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 effectiveness of lightning rod dissipators has not been confirmed.

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

Commercial "elimination" claims of lightning are rightly greeted with a skeptical reception. The NFPA's independent third party panel found that "the [Early Streamer Emission] lightning protection technology appears to be technically sound". (Bryan, 1999) The panel stated that there was an "adequate theoretical basis for the [Early Streamer Emission] air terminal concept and design from a physical viewpoint".(Bryan, 1999) Though, there is no firm evidence that these devices prevent lightning strikes.Template:RefTemplate:Ref No device has rigorously been proven to prevent or reduce the number of lightning strikes and there is no device that is endorsed to prevent lightning strikes by a major standards body, including the NFPA, UL or the NLSI. 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). Template:Ref Members of the Scientific Committee of the International Conference on Lightning Protection has issued a joint statement stating their opposition to dissipater technology. Template:Ref

In investigations, the natural downward lightning strokes are not believed to be preventable. 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. Template:Ref 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 do "eliminate" lightning strikes. Template:Ref However, these devices are likely to be nothing more than expensive lightning rods.Template:Ref

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References

General
Citations 
| author=Mousa, Abdul M.
| year=1999
| url=http://www.lightningsafety.com/nlsi_lhm/charge_transfer_opp.html
| title=Scientists Oppose Early Streamer Air Terminals
| format=
| work=
| publisher=National Lightning Safety Institute
| accessdate=December 11, 2005
| accessyear=}}

| author=Rison, William
| year=2001
| url=http://www.lightningsafety.com/nlsi_lhm/magic.pdf
| title=There Is No Magic To Lightning Protection: Charge Transfer Systems Do Not Prevent Lightning Strikes
| format=pdf
| work=
| publisher=National Lightning Safety Institute
| accessdate=December 11, 2005
| accessyear=}}
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Patents

The United States Patent Office labels "Lightning protectors" in Class 174 (Electricity: conductors and insulators), Subclass 2 (Lightning protectors) and Subclass 3 (Rods).

U.S. Patent Documents

Original

Reissued

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External articles and other resources

de:Blitzableiter es:Pararrayos fr:Paratonnerre it:Parafulmine nl:Bliksemafleider ja:避雷針 pt:Pára-raios sl:Strelovod zh:避雷针

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