Lightning protection system
A lightning protection system is a system that protects a structure from damage due to
lightningstrikes, either through safely conducting the strike to the ground, or preventing the structure from being struck. Most lightning protection systems are composed of a network of lightning rods, metallic cable conductors, and ground electrodes designed to provide a low impedancepath for the lightning to travel through towards the ground.
The majority of lightning protection systems in use today are of the traditional Franklin design. [ [http://scitation.aip.org/journals/doc/PHTOAD-ft/vol_59/iss_1/42_1.shtml Benjamin Franklin and Lightning Rods — Physics Today January 2006] , Accessed 2008-06-1 9:00pm GMT.] The fundamental principle used in Franklin-type lightning protections systems is to provide a sufficiently low impedance path for the lightning to travel through to reach ground without damaging the building. This is accomplished by surrounding the building in a kind of
Faraday cage. A system of lightning protection conductors and lightning rods are installed on the roof of the building to intercept any lightning before it strikes the building.
Non-traditional systems aim to provide the same or similar protection with fewer components. Although some are sold and installed, no such system has demonstrated this ability. This category can be further divided into improved lightning rods that claim an increased zone of protection (and are otherwise similar to a Franklin-type system), and systems that claim to eliminate lightning strikes altogether. The first subcategory includes early streamer emission (ESE) systems, radioactive rod systems, and laser induced systems. An example of a system that claims to eliminate lightning strikes is the charge transfer system (CTS).
The traditional Franklin type lightning protection system has 3 main parts.
*The roof circuit
*Interconnection to grounding electrodes
The roof circuit is composed of a system of copper or aluminum [LPI-175 Standard of Practice for the Design - Installation - Inspection of Lightning Protection Systems - Paragraph 15] lightning rods and cables on or in the roof of a structure. These are arranged to provide a zone of protection around the building. Lightning rods are typically installed around the perimeter of flat roofs, or along the peaks of sloped roofs at intervals of 20ft (6.1m) or 25ft (7.6m), [NFPA-780 Standard for the Installation of Lightning Protection Systems 2008 Edition - Section 126.96.36.199-188.8.131.52] depending on the height of the rod. When a flat roof has dimensions greater than 50ft by 50ft (15m x 15m), additional lightning rods will be installed in the middle of the roof at intervals of 50ft (15m) or less in a rectangular grid pattern. [NFPA-780 Standard for the Installation of Lightning Protection Systems 2008 Edition - Section 184.108.40.206]
The currently accepted theory of lightning propagation is referred to as the rolling sphere method. [NFPA-780 Standard for the Installation of Lightning Protection Systems 2008 Edition - Section 4.7.3] The basic premise of the rolling sphere method is that near ground level, lightning has a strike distance of 150ft (46m). What this means is that given an imaginary sphere with a radius of 150ft (46m), one can find the parts of a structure that are susceptible to a lightning strike by rolling this sphere over the building at all possible positions. Any part of the building that can be touched by this sphere is susceptible to a lightning strike. Any part of the building that cannot be touched by the sphere is considered to be under a zone of protection. Using this method, a properly designed lightning protection system will have lightning rods installs such that when the sphere is rolled over the building, no part of the building can be touched by the sphere except the tips of the lightning rods. There is one common exception to this, though, as the sides of very tall structures do not typically require lightning rods.NFPA-780 Standard for the Installation of Lightning Protection Systems 2008 Edition - Annex B.3.2.2]
If one imagines the center of the sphere moving at a constant rate as it is rolled over the building, the parts of the building that are in contact with the sphere the longest are those that are most likely to be struck by lighting. A typical example of a part of a building that would be a higher risk would be the edge of the roof as the sphere rests on this longer than any flat portion of the roof. This is a simple explanation of why more air lightning rods are required on peaks or the edges of flat roofs than are required on the middle of flat roofs.
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. [NFPA-780 Standard for the Installation of Lightning Protection Systems 2008 Edition - Annex C]
Interconnection to grounding electrodes
The roof circuit must be electrically continuous with some method of grounding. This is usually accomplished in one of two ways. The first way is to use copper or aluminum cables to connect the roof circuit to the grounding electrodes. Virtually all structures require two or more connections from the roof to the ground. [LPI-175 Standard of Practice for the Design - Installation - Inspection of Lightning Protection Systems - Paragraph 75] If a structure's perimeter is greater than 76 m, the structure will require a connection for every 100ft (30.5m) of perimeter or fraction thereof. [UL-96A Installation Requirements for Lightning Protection Systems - Section 9.3.2] For example, a structure with a perimeter of 315ft (96m) will require 4 connections from the roof circuit to the grounding electrodes.
The second method is to utilize the structural steel present in a structure to carry the lightning current between the roof and the ground. When this method is used, more connections to the grounding electrodes are usually required. The number of connections at roof level usually does not change, [UL-96A Installation Requirements for Lightning Protection Systems 2007 Edition - Section 15.3] but the number of connections from the building steel to the grounding electrodes should average no less than 1 connection for every 60ft (18.3m) of building perimeter. [UL-96A Installation Requirements for Lightning Protection Systems 2007 Edition - Section 15.5]
Proper grounding of a lightning protection system is critical for the protection of a structure. Failure to provide sufficient grounding could result in the damage or loss of property and lives. The most common methods for grounding a lightning protection system include ground rods, ground plates, Ufer grounds or a ground ring (counterpoise). [UL-96A Installation Requirements for Lightning Protection Systems 2007 Edition - Section 10] In poor grounding conditions such as rocky soil or shallow topsoil, additional grounding measures should be taken to ensure adequate grounding for the system.
A number of non-traditional systems have been proposed which aim to reduce the number of air terminals required by a traditional Franklin-type system, thereby hopefully reducing the overall cost of the protection system. These can be divided into two categories: systems that provide an increased zone of protection around each air terminal, and systems that eliminate lightning strikes altogether. These benefits are currently only claimed, and have not been substantiated by testing.
Early Streamer Emission
Early streamer emission type air terminals aim to provide a wider coverage area than traditional Franklin rods. The basic theory utilized is the idea that if an upwards step leader were able to be produced before any would naturally occur on a structure, this early step leader would be able to travel higher and be much more likely to intersect the downward step leader. By initiating a controlled step leader before any naturally occur, ESE systems should be able to direct the main lightning stroke to a very small number of lightning rods on a structure. This would allow ESE systems to use far fewer components than traditional systems.
The effectiveness of ESE systems has never been scientifically proven. All studies to date have shown that the zone of protection provided by an ESE lightning rod is exactly the same as a traditional lightning rod. [http://elistas.egrupos.net/cgi-bin/eGruposDMime.cgi?K9D9K9Q8L8xumopxC-qjd-uluCPSYTSCvthCnoqdy-qlhhyCVUQWQjfb7 Inefficacy of radioactive terminals and early streamer emission terminals] , Accessed 2008-06-13 5:42pm GMT.]
ESE Systems are generally not recognised by either the soon to be obsolete BS 6651:1999 or the newly published BS EN 62305:2006.
Radioactive Rod Systems
Radioactiverod systems' main differentiating feature from traditional Franklin type systems is the use of radioactive materials in the lightning rods. The theory behind this is that the radioactive properties of the rods can ionizethe air around the rod sufficiently to increase the likelihood of that rod being struck, rather than the building itself. This, in effect, would increase the zone of protection provided by the lightning rod. Unfortunately, the efficacy of radioactive lightning has been shown to be less than that of regular lightning rods as the radioactive materials are only able to ionize air around the lightning rod for a short distance that doesn't positively affect the chance of being struck.
Laser Induced Systems
Laser induction of lightning strikes is currently being researched by scientists. [ [http://www.eurekalert.org/pub_releases/2008-04/osoa-lte041008.php Laser triggers electrical activity in thunderstorm for the first time] , Accessed 2008-06-13 4:22pm GMT.] The main principle of these systems is that a sufficiently powerful and properly tuned laser can ionize air from the clouds to the ground level. This ionization reduces the
breakdown voltageof the air and provides a lower resistance path for the lightning to travel on. This acts like a lightning rod tall enough to reach the clouds. So far, scientists have only been able to trigger lightning activity in the clouds and have not been able to induce a cloud-to-ground strike. [Cite web|url=http://www.opticsexpress.org/abstract.cfm?id=157189|title=Electric events synchronized with laser filaments in thunderclouds|accessyear=2008|accessmonthday=June 13|publisher= Optics Express|year=2008|author=J. Kasparian, R. Ackermann, Y. -B. André, G. Méchain, G. Méjean, B. Prade, P. Rohwetter, E. Salmon, K. Stelmaszczyk, J. Yu, A. Mysyrowicz, R. Sauerbrey, L. Woeste, and J. -P. Wolf, et al|work=Vol. 16, Issue 8, pp. 5757-5763]
Charge Transfer Systems
Charge transfer systems claim to eliminate the charge buildup on a structure by transferring it to the surrounding area, thereby eliminating the potential for a lightning strike. This is the same idea that prompted
Benjamin Franklinto invent the lightning rod. The effectiveness of these systems has been called into question recently as studies have failed to show any evidence for any reduction of lightning strikes when compared to traditional Franklin systems. [ [http://www.lightningsafety.com/nlsi_lhm/charge_transfer.html Charge Transfer System is Wishful Thinking, Not Science] , Accessed 2008-06-13 1:25pm GMT.]
Some structures are inherently more or less at risk of being struck by lightning. The risk for a structure is a function of the size (area) of a structure, the height, and the number of lightning strikes per year per mi² for the region. [NFPA-780 Standard for the Installation of Lightning Protection Systems 2008 Edition - Annex L.1.3] For example, a small building will be less likely to be struck than a large one, and a building in an area with a high density of lightning strikes will be more likely to be struck than one in an area with a low density of lightning strikes. The National Fire Protection Agency provides a risk assessment worksheet in their lightning protection standard. [NFPA-780 Standard for the Installation of Lightning Protection Systems 2008 Edition - Annex L]
* J. L. Bryan, R. G. Biermann and G. A. Erickson, "Report of the Third-Party Independent Evaluation Panel on the Early Streamer Emission Lightning Protection Technology". National Fire Protection Association, Quincy, Mass., 1999.
* M. A. Uman and V. A. Rakov " [http://www.lightningsafety.com/nlsi_lhm/Uman_Rakov.pdf Critical Review of Nonconventional Approaches to Lightning Protection] ", Bulletin of the American Meteorological Society, December 2002.
* Van Brunt, R.J., Nelson, T.L., Stricklett, K.L. "Early streamer emission lightning protection systems: An overview", IEEE Electrical Insulation Magazine, Vol. 16, Iss. 1, 5-24, 2000.
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