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[edit]Lightning formation - a brief overview
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A thundercloud, is a naturally occurring electric field on a large scale. The highly complex electrical dynamics within the cloud, and their formation are not important to this discussion, however it is well accepted in the scientific community that the bottom of the thundercloud is generally understood to carry a relative negative charge.[1][2] This in turn induces a net positive charge on the Earth's surface in the thundercloud's shadow, in equal but opposite polarity to the intensity of the charge on the cloud surface, thereby establishing an electric field within the air.[3] Lightning, an electrical spark, occurs once the intensity of the electrical field exceeds the conductive resistance of air through a complex process described simplistically below, and is a momentary, spatial equalization of the opposite charges on both sides of the lightning flash.
In a negative cloud-to-ground lightning flash, two events occur prior to one seeing the actual “bolt” of lightning. First, a downward (stepped) leader composed of ionic plasma, generally a concentration of the negative charges, comes down from the clouds in a zigzagged manner that may branch resulting in multiple leaders near the earth. Second, as the stepped leader(s) approaches the ground, the positive charge on the surface concentrates and forms multiple plasma upward streamers, which rise from surface features offering connection points for the downward leader. Once any of the downward leaders connect to any of the upward streamers available, in a process referred to as attachment, a single ionic channel is created and an electrical pathway/circuit is formed.[4] The visible portion of lightning then occurs, which is the discharge or temporary equalization of the opposite charges between the thundercloud and earth.[5]
The exact physics behind the attachment process is not fully understood and there is still some debate in the scientific community.[2] [6] [7]
Charge neutralization and electric field dynamics
[edit]As described above, the lightning flash is a temporary equalization of charge between the cloud and ground through the ionic channel. During the milliseconds of time within which the equalization occurs, there are rapid changes to the electric field. These field dynamics affect everything at both the termination point and in general proximity to the flash channel.
Near instantaneous current fluctuations release an enormous quantity of heat, measured upwards of 50,000 degrees F. As an ignition source, to both natural objects and man-made structures, it was the main concern of lightning protection systems prior to modern times, and is referred to as primary effects. Injury, or even death, to both humans or livestock through a direct strike or the subsequent fires they caused, made it imperative for humanity to address this uncontrollable natural phenomenon and allow for the advancement of society as a whole. The development of the lightning rod system in the mid 18th century was the answer to the primary effects of lighting terminations, and it has been adopted world-wide with only slight modifications to the initial designs.[8][9][10]
However, with the electrification of the world in the 20th century, and the importance of electrical systems and micro-electronics in almost every aspect of today's technological world, dynamic electric field fluctuations or secondary effects of a lightning termination are equally destructive, although often not as openly apparent as was a building set on fire in the past.[11][12][13][14] Types of secondary effects that can cause damages not directly attributable to the direct lightning strike include; atmospheric transients, earth current transients, electro-magnetic pulses and ground rise potential.[15][16]
Strike collection versus charge transfer technology
[edit]The primary element of the traditional LPS or Franklin system, in reference to its inventor, is the lightning rod or air terminal. When a thundercloud is present, a lightning rod efficiently begins the process of point ionization when bonded to earth and exposed to the electric field. A lightning rod reaching an ionization threshold initiates an upward streamer to which an incoming downward stepped leader can attach to, thus the lightning is said to be "collected" and routed to ground through a network of conductors.[17]
Charge transfer, in the genre of lightning protection systems, is an application of point ionization used to reduce the localized electric field potential through a concentration of multiple points. This is done by transferring the thundercloud induced electric charge, available on the grounded structures under the cloud, into the air through point ionization, en mass.[18][19] The reduced electric potential retards streamer development thereby lowering the probability of leader attachment and subsequent lightning strike.[20][21]
Strike protection and generalized standards
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Benjamin Franklin's famous electricity experiments in the 18th century set the stage for a world-wide standardized approach to addressing the need to protect structures from damages caused by lightning. Over the intervening 250 plus years, both scientific investigation and trial and error approaches have been used in an attempt to best understand the myriad complexities of a lightning strike and its interaction with ground based structures. Traditional standards commonly used to define a lightning protection system (LPS) focus on that interaction and the primary effects of lightning through strike collection only. The standards do not take into consideration the secondary effects of lightning involved in nearby terminations or the strike collection itself.
The British Standards Institution (BSI), International Electrotechnical Commission (IEC), the National Fire Protection Association (NFPA) & Underwriters Laboratories (UL) have implemented best practices, BS EN 62305-1:2011[22] , IEC 62305-1[23], NFPA 780[24] & UL 96A[25], respectively, to standardize the application of traditional air terminal systems.
- ^ Lightning, McGraw-Hill Book Company, New York (1969), 264 pages. Russian translation (1972), revised edition, Dover, New York (1984).
- ^ a b Vladimir A. Rakov; Martin A. Uman (8 January 2007). "3". Lightning: Physics and Effects. Cambridge University Press. ISBN 978-0-521-03541-5. Retrieved 16 August 2012. Cite error: The named reference "RakovUman2007" was defined multiple times with different content (see the help page).
- ^ NOAA, National Severe Storms Laboratory. "Questions and Answers about Lightning". NOAA. Retrieved 8 August 2012.
- ^ Rupke, Ed (March 1, 2002). "Lightning Direct Effects Handbook" (PDF). AGATE - Advanced General Aviation Transport Experiments: Chapter 2, pgs 1-3. Retrieved 19 October 2012.
- ^ Zavisa, John. "How Lightning Works". How Stuff Works. Retrieved 8 August 2012.
- ^ "The Effect of a Corona Discharge on a Lightning Attachment". Plasma Physics Reports. 31 (1): 75–91. 2005. doi:10.1134/1.1856709.
{{cite journal}}: Unknown parameter|coauthors=ignored (|author=suggested) (help) - ^ "Attachment process in rocket-triggered lightning strokes" (PDF). Journal of Geophysical Research. 104 (D2): 2143–2150. January 27, 1999. doi:10.1029/1998JD200070. Retrieved 17 August 2012.
{{cite journal}}: Unknown parameter|coauthors=ignored (|author=suggested) (help) - ^ Bellis, Mary. "The Inventions and Scientific Achievements of Benjamin Franklin". About.com. Retrieved 28 September 2012.
- ^ "Lightning Fires" (PDF). Topical Fire Research Series. FEMA - US Fire Administration. Retrieved 28 September 2012.
- ^ Fowler, Norman, L. "A BRIEF HISTORY OF LIGHTNING PROTECTION" (PDF). HQ AFCESA/ENE Tyndall AFB, F1. 32403-6001. Retrieved 28 September 2012.
{{cite web}}: CS1 maint: multiple names: authors list (link) - ^ EPRI PEAC Corporation. "Some Enlightning Case Histories on Lightning Damage" (PDF). Retrieved 20 August 2012.
{{cite web}}: Unknown parameter|coauthors=ignored (|author=suggested) (help) - ^ "Lightning and Static Effects on Industrial Electronics" (PDF). Retrieved 20 August 2012.
- ^ Norton, Steven (June 28, 2012). "Thanks to Gadget Lust, Lightning Damage Claims Surge". Bloomberg Businessweek. Retrieved 20 August 2012.
- ^ "When lightning strikes, insurance can cover shocking damage costs". NetQuote.com. Retrieved 20 August 2012.
- ^ "The Secondary Effects of Lightning Activity" (PDF). pp. 3–8. Retrieved 21 August 2012.
{{cite web}}: Unknown parameter|coauthors=ignored (|author=suggested) (help) - ^ Carpenter, Jr., Roy B. (May 1997). "Lightning Protection for Gas & LNG Facilities" (PDF). pp. 9–14. Retrieved 19 October 2012.
{{cite web}}: Unknown parameter|coauthors=ignored (|author=suggested) (help)CS1 maint: date and year (link) - ^ Uman, Martin A. (1986). "How Does A Lightning Rod Work?" (PDF). New York: Dover Publications. Retrieved 19 October 2012.
- ^ U.S. patent 6,307,149, Richard Ralph Zini, et al., Non-contaminating lightning protection system. Claim one and claim ten.
- ^ John Richard Gumley, U.S. patent 6,320,119, Lightning air terminals and method of design and application
- ^ Emitter of ions for a lightning rod with a parabolic reflector, Manuel Domingo Varela, U.S. patent 6,069,314.
- ^ Lightning-protector for electrical conductors, Johathan H. Vail, U.S. patent 357,050.
- ^ Protection against lightning. General principles. Bristish Standards Institution. June 2011. Retrieved 28 September 2012.
- ^ IEC (2010–12). IEC 62305 International Standard (2.0 ed.). IEC.
{{cite book}}: Check date values in:|year=(help) - ^ National Fire Protection Association (2011). Standard for the Installation of Lightning Protection Systems, NFPA 780, 2011 Ed. NFPA. ISBN 978-161665080-3.
- ^ Underwriters Laboratories Inc. (2007). UL Standard for Safety for Installation Requirements for Lightning Protection Systems, UL 96A, Twelfth Edition. UL.