Since Benjamin Franklin demonstrated the effectiveness of lightning rods in preventing or greatly reducing the damage from direct strikes, there have been many attempts to market other types of lightning protection systems (LPSs). These systems, which were touted by aggressive salespeople eager to share testimonials from satisfied customers, had no scientific or empirical evidence supporting their effectiveness. In 1856, Herman Melville vividly depicted such scenarios in his short story, The Lightning-Rod Man. Nearly 150 years later, claims are still being made. In November 2001, Power Quality magazine published an article by Donald Zipse titled “Prevent Lightning Strikes with Charge Transfer Systems.” In the article, the author makes two claims: First, there is no scientific proof that conventional LPSs are effective; and second, properly designed charge transfer systems (CTSs) can prevent lightning strikes to protected regions. In this article, I'll offer evidence to refute both of these claims.
The Science Behind LPSs
To support the claim that conventional LPSs lack scientific merit, Zipse cites the National Fire Protection Association's (NFPA) decision to withhold publication of a revision to Standard 780, the “Standard for the Installation of Lightning Protection Systems.” According to the article, a committee appointed by the NFPA to review Standard 780 concluded that “lightning rods lack scientific and technical merits.” This is incorrect. In fact, the committee that reached this conclusion (the Bryan Panel) had been appointed by the NFPA to review early streamer emission air terminals, another type of nonconventional LPS . Based on the Bryan Panel's conclusion, the NFPA issued a ruling that stated it would withdraw NFPA 780 unless it received sufficient evidence of a scientific basis for conventional LPSs .
In response to this ruling, the NFPA received several submissions that provided strong scientific evidence supporting the effectiveness of conventional LPSs [3, 4]. The NFPA has decided to continue its lightning protection project and has released the revised 2000 edition of Standard 780 . As for the Bryan Panel's report, the NFPA wrote: “It is now apparent …that a large body of literature confirming the basic principles of conventional lightning protection technology was not considered by the Bryan Panel.”
At the Langmuir Laboratory for Atmospheric Research, we've been conducting research in a high-lightning environment for the past 35 years. This mountain-top thunderstorm research laboratory at the New Mexico Institute of Mining and Technology features a specially built instrumentation shelter called Kiva II. The shelter is designed to record data during direct strikes.
On many occasions, I've been inside Kiva II (along with other researchers and observers) when it has been struck by lightning. During these times, the lightning terminated on a current-measuring shunt on the top of Kiva II, about 2 ft above our heads (see the photo, on page 30). Because Kiva II was properly designed, personnel and instrumentation inside were 100% protected during these direct lightning strikes.
Challenging CTS Theory
Now let's examine the second claim made by Zipse. The article asserts that a CTS injects several coulombs of positive charge into the air above a protected structure, which neutralizes the charge in an approaching lightning leader. The equation for determining the number of points, N, needed to provide this charge was given as follows:
In this equation, Q is the amount of charge (in coulombs) needed to neutralize the leader, Ip is the amount of corona current emitted by a single point under a thunderstorm, and t is the amount of time needed to accumulate charge Q. These numbers imply that, in order to provide 2.5 C of charge to neutralize a leader, a system needs 1500 points, and each point would need to emit 170µA of current for 10 sec. This is physically impossible.
The force which drives the corona current from a point under a thunderstorm is the electrical force from the (commonly) negative charge in the base of a thundercloud. The electrical field on the ground from this negative charge is typically 2000V/m to 5000V/m. When the electrical field under a thunderstorm reaches about 1000V/m, sharp objects start emitting corona current.
The charge carriers in corona current are not free electrons that can move rapidly away from the CTS; they are ionized molecules that drift slowly in the atmosphere while colliding with other molecules. These ions move with a speed of approximately 10 m/sec under the forces induced by the fields near the ground. Thus, the ions can move about 100 m in 10 sec, which means the space charge created by the corona current is confined to a region of 100 m above the CTS. Corona current cannot continue after the field is reduced below the 1000V/m needed to initiate it.
The field from a charge is given by this formula:
Producing a field of 4000V/m (enough to reduce the 5000V/m field typical during thunderstorms to the corona onset strength of 1000V/m) at a distance of 100 m (the maximum distance the charge could travel in 10 sec) would require a charge of 4.4 × 10-3 C, almost a thousand times smaller than the 2.5 C claimed to be needed to neutralize a leader.
The problem with the first equation is this: It assumes the current from an array of points is the current from a single isolated point multiplied by the number of points in the array. This is simply not the case. As an analogy, consider the water delivered from a system of fire hydrants. During a fire, one or a few hydrants can produce a prodigious flow of water under high pressure. However, if every fire hydrant in the city is opened, the flow of water from a single hydrant is considerably less than the flow of a few active hydrants. This is because there are physical constraints (water pressure and size of pipes) that limit the total amount of water that a system of hydrants can deliver.
Similarly, there are physical constraints that limit the amount of current that can be emitted by an array of points. A CTS cannot release any more charge after it has released enough charge to reduce the ambient field to about 1000V/m. The second formula demonstrates that an array of points can release a maximum of about 4.4 × 10-3 C of charge in a 10-sec interval, regardless of the number of points in the array.
We have done studies on current emissions from multipoint arrays at Langmuir Laboratory. In our experiments, we have found that an array of 80 points emits a corona current about twice the value of that from a single isolated point (see the photo, above).
The real way to test a CTS is with field studies conducted by independent organizations. In the early 1970s, NASA was looking for a way to protect the Space Shuttle and other manned vehicles while they were on the launch pad awaiting launch. NASA did an extensive study of CTSs. The studies found that the frequency of lightning to the towers with CTSs was not significantly different than those without CTSs . CTSs did not prevent or significantly reduce the probability of lightning strikes to a tower. NASA decided to protect the Space Shuttle with a wire above it to act as a preferential strike point for lightning. There are many documented incidences of lightning striking the overhead wire, protecting the Shuttle as designed. Since the NASA studies, several other independent studies have reached the same conclusion.
Although CTSs do not prevent lightning strikes, they do work well as conventional LPSs. They provide a system of overhead wires that function as preferential strike points. They also provide a good grounding system and conductors to connect the overhead wires to the grounding system. This probably explains the testimonials from satisfied customers. However, conventional LPSs are equally effective in protecting structures, at a fraction of the cost.
I am opposed to CTSs because the underlying theory lacks scientific credibility and because every independent study demonstrates that they do not prevent lightning strikes. CTSs are modern incarnations of the “magic” wares peddled by Melville's Lightning-Rod Man — ornate devices with no independent evidence that they live up to their fantastic claims.
William Rison is a professor of electrical engineering at the New Mexico Institute of Mining and Technology in Socorro, N.M. You can reach him at [email protected].
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.
NFPA Decision D#00-30, NFPA Standards Council Decision about Retraction of NFPA 780, Quincy, Mass., 2000. Available on NFPA's Web site at www.nfpa.org.
Federal Interagency Lightning Protection User Group, “The Basis of Conventional Lightning Protection Technology: A Review of the Scientific Development of Conventional Lightning Protection Technologies and Standards,” 2001. Available on the Web at http://www.lightningsafety.com/nlsi_lhm/conventionalLPT.pdf.
Report of the Committee on Atmospheric and Space Electricity of the American Geophysical Union on the Scientific Basis for Traditional Lightning Protection Systems, 2001. Available on the Web at http://case.agu.org/NFPAreport.pdf.
NFPA Decision D#01-26, NFPA Standards Council Decision to Retain NFPA 780, Quincy, Mass., Oct., 2001. Available on NFPA's Web site at www.nfpa.org.
Conference sponsored by Office of Naval Research, NASA, FAA, and U.S. Air Force, “Review of Lightning Protection Technology for Tall Structures,” Houston, Texas, Nov. 6, 1975.