fertile field for itinerant "lightning-rod-men" whoseexcesses and practices cast doubts on the utility oflightning protection. In 1879, in his book, LightningConductors-Their History, Nature, and Mode ofApplication, Richard Anderson wrote: "Americastands preeminent above all other countries in the nu-merous schemes that have been devised for the pro-tection of buildings from the effects of lightning, andprobably no other nation has been so systematicallyvictimized and swindled in the matter. The tramping'lightning-rod men' of the United States have beennotorious for extortion and ignorance; they use allkinds of fantastic and peculiar shaped terminal rodsand conductors, the main object apparently being tomake as great a show with as little material as possible.Their work is almost entirely confined to the upperportion of the conductor, to the absolute neglect ofthe most important part-the earth terminal. Themajority of the lightning conductors in America areconsequently untrustworthy...." (Anderson 1879).On the recognition in 1878 that there were no autho-rized or well-matured directions for the installationof lightning-protection systems in England and thatthe practices in vogue were so varied and anomalous,the Royal Meteorological Society convened a light-ning-rod conference in an effort to take action for thepublic. The Report of the Lightning Rod Conference(Symons 1882) provided the code of rules for thosewho installed lightning protection systems in Britain.

In 1904, following the British code, W. S. Lemmon,B. H. Loomis, and R. P. Barbour prepared Specifica-tions for Protection of Buildings Against Lightning(Lemmon et al. 1904), which was adopted for Ameri-can use by the National Fire Protection Association(NFPA) in Quincy, Massachusetts. After periodic re-visions over the years as more was learned about thephysics of lightning and as the technology has ad-vanced, these specifications have been issued as NFPA780, the current standard for the installation of light-ning protection systems (NFPA 2001).

THE PROTECTION PROVIDED BY LIGHT-NING RODS EQUIPPED WITH DOWNCONDUCTORS. In the years since the specifica-tions for protection of buildings against lightning

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were adopted and suitable protection systems wereinstalled on buildings, significant protection againstthe damage caused by lightning has been demon-strated. During a seven-year period beginning in1907, the reports of fire losses to protected farmbuildings in Iowa (composing about one-half of theinsured buildings) were 1.3% of the damage to un-protected buildings. During the years 1919-24, thevalue of the damage to the half of the insured build-ings that were rodded was about 7% of that of theunrodded structures. About 5% of the lightning-caused fires took place in barns supposedly protectedby lightning rods, but on investigation, about one-third of these so-called "rodded" barns were found tohave had defective protection systems. When opin-ions were expressed by the fire-insurance companies,they were to the effect that if buildings were properly"rodded," they would be practically safe from dam-age by lightning.

During World War II, the decision as to whetherlightning protection would be provided for certainmanufacturing facilities operated by private contrac-tors for Army Ordnance was left up to the contrac-tor. Records show that from 1943 to 1946, the unpro-tected Indiana Ordnance Works lost approximately500,000 pounds of explosives, an explosives magazine,a shipping house, and an operating building, and hadother damage exceeding $100,000 as the result of sixlightning strikes. In contrast, the immediately adja-cent protected Hoosier Ordnance Plant reported nolosses from lightning. A survey by Army Ordnanceof the lightning experience for the period 1944-48shows that protected structures were struck 330 timeswith negligible damage whereas the unprotectedstructures were struck 52 times and the damage ex-ceeded $130,000. The efficacy of lightning rods inpreventing damage from strikes has been establishedso well over the years that lightning protection is nowroutinely required on large buildings open to thepublic.

THE INITIATION OF LIGHTNING STRIKES.Lightning strikes usually begin in thundercloudsfrom which a "leader" carrying negative charges de-scends toward the earth. Schonland and Collens(1934) discovered that the negative leader descendsin a step-wise fashion from a thundercloud, provok-ing upward-going positive streamers from exposedobjects on the ground as a result of the local inten-sification of the atmospheric electric field beneath thenegative-stepped leader. One or more of the rising

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positive streamers may intensify into a positive leaderthat connects to the approaching negative leader andallows negative charge to drain to the earth as a "returnstroke" when the earth potential moves up the negativechannel. Less frequently, lightning strikes lower posi-tive charges to the earth. When these strikes occur,they are often more damaging than the negative ones;positive strikes are usually more energetic and havesustained "continuing currents" that can ignite fires.

Of special interest for lightning protection is thedetermination of which object on the earth is mostlikely to furnish the successful upward-going streamerthat provides the connection to the descending leader.From studies of the physics of lightning and of longsparks, it is now known that the development andpropagation of plasma streamers and of the moreconductive leaders occur only when the local electricfields are very strong. It is also known that the tips ofcurved electrodes concentrate and intensify any am-bient electric fields to which they are exposed. Thisprovides an explanation for why elevated objects arethe preferred recipients of lightning strikes; the strongfields around their tips caused by the charges carriedby approaching leaders provide the conditions nec-essary for the generation of upwardly propagatingleaders that rise to connect with the downward-mov-ing leader, thus completing the strike discharge pathto the earth and determining the strike point.

PROTECTION AGAINST DIRECT LIGHT-NING STRIKES. The fundamental principle in theprotection of life and property against lightning is toprovide a means by which a lightning discharge canenter or leave the earth without resulting damage orloss. In practice, this is accomplished by providingpreferred receptors for nearby strikes and then con-veying these discharges to the earth and distributingthe charges brought down by lightning into theground so that they do not cause local hazards.

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As pointed out in the 1882 Report of the LightningRod Conference, it is important that the tips of thedesignated strike receptors "be high enough to be themost salient features of the building no matter fromwhat direction the storm cloud may come." WhileFranklin invented lightning rods with sharp tips be-cause he thought that their emissions could neutral-ize a cloud and prevent lightning, we now know thatthis does not occur. Instead, the primary function ofa Franklin rod is to be a strike receptor. For this pur-pose, a sharp tip is not ideal. The emissions fromsharp-tipped lightning rods under the influence ofstrong electric fields act to weaken the fields locally,thus, decreasing the likelihood that the tip will serveas a strike receptor. In order that a sharp-tipped rodbe "struck," it is necessary that the local electric fieldbe intensified more rapidly than it is weakened by theformation and motion of ions around its tip and thatthe rod respond successfully before competingstreamers form above some other objects in its vicin-ity. The early successes with Franklin rods came fromsharp-tipped rods exposed on tall structures (asFranklin taught), well above any competing objects.The optimum configuration for the tip of a lightningrod has been studied recently and was addressed inthe recent edition of NFPA 780.

Another aspect of lightning physics that has beenstudied recently is that of the efficacy of ground rodsin transferring lightning discharges to the earth. It iswell known that large potential gradients develop atground level when lightning strikes an isolated tree;the resulting voltages appearing on the ground sur-face between the legs of animals standing nearby mayexceed hundreds of kilovolts, causing fatal currents toflow through their legs. Arcing occurs radially outwardfrom ground rods when the current from lightningdischarges conducted to the rod exceeds 20 kA. Theuse of radial ground conductors buried below groundlevel is now recommended in the 2000 edition of

NFPA 780 as a means of distributing lightningcharges to their ultimate destination, the surface ofthe Earth.

THE NEED FOR A STANDARD TO GUIDETHE INSTALLATION OF LIGHTNING PRO-TECTION SYSTEMS. It is now well establishedthat properly installed and maintained lightning-rod-based protection systems significantly decrease light-ning damage. Since the installation of an adequatelightning protection system is not simple, a standardis needed to ensure that proper procedures are fol-lowed by the installer. To be effective in providingprotection for a structure that is struck, the follow-ing requirements must be met.

1) A sufficient number of rods must extend above theupper portions of the structure to be protected, andtheir tips must be so exposed that one of them be-comes the locally preferred strike receptor uponthe close approach of an initiating leader, descend-ing from a thundercloud.

2) The connection between the strike receptor and theearth, the "down conductor," must be able to carrythe lightning current without significant heating:

3) The impedance to the flow of current in the downconductor must be sufficiently low that "sideflashes" to objects in the vicinity do not occur as aresult of high voltages developed by the passage ofthe current.

4) The connection from the down conductor to theearth must allow the lightning current to flow intothe ground without the development of large elec-trical potential differences on the Earth's surfaceand without creating hazards to personnel orstructures nearby.

5) All large metal components of the structure shouldbe connected electrically to its down-conductorsystem to minimize capacitance effects and to al-low the transfer to the earth of the "displacementcurrents" that flow when the external electric fieldsare changed abruptly by the lightning discharge.

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6) Surge protection should be provided for the elec-trical service and for all electronic equipmentwithin the structure.

Given these complexities, standards specifying therequirements that must be met in order to ensure theadequacy of each lightning protection installation areessential and are required by `most industrial coun-tries. As given above, the standard that is widely usedin the United States is the NFPA standard for the in-stallation of Lightning Protection Systems (NFPA780), which originated in 1904 and is revised everythree years by an NFPA Technical Committee to in-corporate new findings and experiences. This Ameri-can standard represents the consensus of specialistsin the physics of lightning, of manufacturers of light-ning protection equipment, and of lightning protec-tion installers.

-C. B. MOORE (NEW MEXICO INSTITUTE OF MININGAND TECHNOLOGY)

Anderson, R., 1879: Lightning Conductors-Their His-tory, Nature and Mode of Application. E. & F. N.Spon, 470 pp.

Franklin, B., 1941: How to secure Houses &c. from light-ning. Benjamin Franklin's Experiments: A New Editionof Franklin's Experiments & Observations on Electric-

ity, I. B. Cohen, Ed., Harvard University Press, 388-

392. First published in Poor Richard's Almanack (1753).

Lemmon, W. S., B. H. Loomis, and R. P. Barbour, 1904:Specifications for Protection of Buildings Against

Lightning. NFPA.NFPA, 2001: The Standard for the Installation of Light-

ning Protections Systems. NFPA 780, National FireProtection Association. (Available online atwww.nfpa.org/Codes/index.asp).

Schonland, B. F. J., and H. Collens, 1934: Progressivelightning. Proc. Roy. Soc. London, 114, 654-674.

Symons, G. J., Ed., 1882:Report on the Lighting Rod Con-

ference. E. & F. N. Spon.

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