CONVENTIONAL PROTECTION

Professor Martin A. Uman, Fellow, IEEE Chief Scientist, BOLT, Inc.

History and Overview

In the latter part of the eighteenth century Benjamin Franklin proposed the original concept of lightning protection systems (LPSs) for buildings. His well-known idea was to position a metallic lightning rod (called an air terminal in modern parlance) above the building to intercept lightning flashes that otherwise would strike the building. The lightning rods were attached to down conductors, and the down conductors were attached to ground rods driven into the soil adjacent to the building. Franklin thought that his lightning protection system achieved two functions: 1) to divert the lightning flash from the building and conduct its energy harmlessly to ground, and 2) to dissipate the electrical energy in the cloud and thus prevent lightning flashes. This somewhat scientific and somewhat intuitive opinion of Franklin was formed at the advent of scientific study of electrical phenomena, so scant theoretical or experimental science existed to support his opinion. However, the Franklin LPS and subsequent LPSs based on his concept substantially reduced lightning damage to the buildings and ships of his era, namely wooden or masonry (non-conducting) structures. Today, all interested in LPS and lightning phenomena are in debt to Benjamin Franklin.

Over 150 years would pass before science effectively directed its attention to lightning phenomenology and subsequently to valid engineering methods for existing and future LPSs. Recent and sound scientific analyses and experiments now reveal 1) that the Franklin type LPS is effective at preventing fires in non-conducting structures, 2) that it is ineffective for the protection of sensitive contents, such as electronics or explosives, from high electric fields and arcing within conducting structures (modern structures with a steel skeleton and/or metallic skin), and 3) that the Franklin LPS and its variants do not effectively dissipate electric charge from clouds and thus do not prevent lightning flashes.

Today perhaps even more than in Franklin's day, we are motivated to protect critical assets and infrastructure from lightning:

Stored explosives including nuclear and chemical weapons present a high consequence human and political risk.

Hydrocarbon energy processing, transmission, and storage present a high consequence human and economic risk.

Some computation centers (FAA flight control, central computers, server farms, industrial controls) present a high human and/or economic risk.

These are but a few examples of assets or infrastructure where effective protection against the effects of the infrequent or occasional occurrence of

an attached lightning flash is essential. Fortunately, we now know more about lightning phenomena and have means to evaluate LPS performance. However, much of the lightning phenomena science has been produced in the last 30 years, and much of the LPS science has been produced in the last 10 years. Thus, new and better lightning protection methods and products based on this emerging science are just now becoming available.

In view of the recent lightning science, below we present information on conventional lightning protection (time honored variants and extensions of the Franklin concept) and non-conventional protection (suspect digressions from the Franklin concept).

Conventional Lightning Protection

Conventional lightning protection systems are intended to intercept a lightning strike and conduct its energy away from the building to ground, thus protecting the building and its contents. Among the strategies used are 1) systems directly attached to the building and 2) systems unattached and above or around the building.

The Franklin-type system was the first example of a directly attached system, and it remains the dominant lightning protection method. As stated, it has worked "statistically" well for historical (non-conductive) construction for many years. As will be discussed in the next menu item, Modern Protection, it does not effectively prevent lightning induced arcs and the high electric fields that cause them. Extensive specifications (e. g., NFPA 780) have been developed for the height and configuration of air terminals (lightning rods), the size of down conductors, and the implementation and characterization of the grounding system for Franklin-type systems. These conventional systems effectively conduct the low frequency components of a lightning strike (see About Lightning, Lightning Damage) to ground, and thus they are fairly effective at preventing fires when they intercept the strike. The Franklin-type system and its variants have two major shortcomings:

1. Lightning can be rather arbitrary (perhaps perverse) in the path it takes to its attachment point. Many observations have revealed lightning bypassing the air terminals and attaching directly to a building, even when numerous, high air terminals are used. Nevertheless, lightning more often than not attaches to the air terminals rather than the building, and the system indeed then diverts the bulk of the lightning energy harmlessly to ground. The statistical effectiveness (% of air terminal attachments versus building attachments) of these type systems is not well quantified.

2. Even when lightning attaches to an air terminal, the typical down conductor is ineffective for conducting the high frequency components of the strike to ground when substantial structural metal is nearby, as is often the case in modern industrial construction. Rather, the current is conducted through the lower inductance structural metal in the building. Further, the relatively high

inductance down conductor causes high electric fields in response to the rapid rise time of current in a lightning stroke. These high electric fields can cause arcing and damage to electronic systems.

An example can illustrate this second point. A typical down conductor might have a resistance of 0.2 milliOhms/m and an inductance of 1000 nanoHenry/meter. For a 5.0 meter high building, R=1.0 milliOhm and L=5000 nanoHenry. Using the lightning parameters for an extreme lightning strike from the Lightning Damage section in the About Lightning menu, the maximum electric field generated is in the order of

= (1.0*10-3)*(200*103) + (5000*10-9)(400*103/10-6) volts = ~1000 kV

The electric field would be ~200 kV/m more than enough to cause arcing and disrupt electronics.

Note: In the menu item Modern Protection, FC/LPS Overview, building measurements and analyses reveal that electric fields less than 100th of those resulting from conventional protection can be achieved using modern protection based on Faraday cages. This significant improvement results from the substantially lower "transfer inductance" of a well-bonded "elemental Faraday cage" (~10 nanoHenries) in comparison to the inductance of a Franklin-type down-conductor (~5000 nanoHenries). These substantially lower electric fields effectively guard against damage or disruption caused by arcs or high voltages.

Several variations of air terminals are used. Some evidence exists that moderately blunt tip rods perform better than sharply pointed tip rods.

Unattached lightning diversion systems are often called catenary systems or catch-net systems. They typically consist of an overhead grid of conductors (overhead ground wires) suspended from masts with down conductors to a grounding system. The overhead conductors are typically spaced tens of feet apart. These systems have the advantage that when successful, they intercept the lightning strike some distance from the building to be protected. They have both the shortcomings noted above, but perhaps to a lesser degree. They are typically more expensive to implement than attached systems, they do not support surge protection, and their advantages are hard to quantify. Uman cited an example [All About Lightning] of a simple catenary system used in rural Poland consisting of a single wire suspended over small wood and thatched-roof farmhouses. This simple system reduced lightning caused house fires by 90%.

Conventional lightning protection systems have served us well for many decades. Their frequent success and occasional failure has been well observed. However, modern construction techniques and pervasive and crucial electronic systems have prompted protection requirements that conventional lightning protection systems cannot well meet. The determination of the use of conventional protection and/or modern protection (Modern Protection) rests heavily on the value and sensitivity of the resources to be protected and on the nature of building construction. Franklin-type protection (NFPA 780), when properly implemented, provides effective protection from fire and other damage to the building "structure." Modern protection guards the interior of the structure and its contents from high electric fields and resultant arcs and high voltages. For many construction types, the two protection methods are compatible and can beneficially coexist. For some construction types (e. g., sheet metal buildings), the elemental Faraday cage, see Modern Protection, FC/LPS Overview, if effectively grounded, also provides protection from structure fire and other damage, and need not be supplemented by a Franklin-type system. BOLT can help you determine what protection method or combination of methods is best for your investments, facilities and operations.

Non-Conventional Lightning Protection

Some non-conventional alternatives to the general Franklin concept have emerged over the decades and then been abandoned as ineffective. Some non-conventional protection systems have a commercial presence today; prominent examples are 1) early streamer emission systems and 2) lightning prevention systems.

Advocates of early streamer emission systems claim these systems launch an upward leader earlier than would a Franklin-type rod at the same height. Thus, lightning purportedly attaches to them preferentially. Some interesting means are used in the attempt to induce early upward leaders including radioactive sources and electrical actuation of the rod. Most lightning protection scientists reject the claims made by advocates of early streamer emission systems, and this scientific opinion is reflected in reputable scientific literature (see References, in this menu category). Nevertheless, the systems are being deployed today.

An extremely desirable alternative to intercepting and diverting lightning is to prevent lightning from striking, at least in the local area housing your investments. An intuitively compelling method to prevent lightning is to discharge a thundercloud before lightning can strike. Or less intuitively, generating a cloud of positive charges over the local region might divert downward leaders from that region, thus preventing attachment of a strike to that region. Several types of charge transfer devices have been conjured and promoted for these purposes. Most consist of numerous sharp discharge points attached to ground. These discharge points may be localized on a mast (think of a metal brush on a lightning rod) or may be distributed about your facility (think of large grounded mesh of barbwire).

These devices do indeed provide a discharge mechanism for transferring positive charge from the earth to the atmosphere. The "devil in the details" is that thunderclouds generate charge quickly and move rapidly. Present cloud discharge systems or downward leader diverting systems have yet to demonstrate convincingly that they can transfer charge rapidly enough to prevent a lightning strike to the local region. No persuasive statistical or scientific studies are found in the open and refereed scientific literature validating lightning prevention by efforts to either discharge thunderclouds or divert downward leaders. Nevertheless, individuals with passionately advocated anecdotes supporting either method can be found. However, reputable scientific work does refute the claims for some types of charge transfer devices or systems (see References, in this menu category).

This brief consideration of conventional and non-conventional lightning protection reflects the opinion of the scientific community. Please read the menu item, Conventional Protection, Warning and Disclaimer.

Gallery of Photographs

Conventional protection can fail to prevent fire and explosion.

Lightning can bypass conventional protection and directly strike the "protected" structure.

Lightning strikes cause ground flashes even for well-grounded systems.

Fulgurites demonstrate the extreme temperatures of lightning and the 10s of feet lightning can travel through earth to couple to a conductor (power cables or pipes).

Disclaimer and Warning

The brief descriptions of conventional lightning protection were prepared by the BOLT scientific staff to assist you in understanding your lightning protection options. These descriptions are consistent with the opinion of the scientific community. As stated, conventional lightning protection can have effective protective value, particularly for preventing fire in non-conductive buildings. This value is typically difficult to quantify. Also, conventional protection may be prudent for buildings with low value contents.

Prospective customers for lightning protection are urged by BOLT to seek independent and unbiased scientific advice before committing to substantial investments for lightning protection or depending confidently on the performance of conventional lightning protection. Before purchasing any lightning protection system, prospective customers and their scientific advisors are advised to evaluate the underlying lightning protection technologies using scientific papers in refereed and reputable journals or using acknowledged scientific texts. For some non-conventional lightning protection technologies, no reputably published scientific support will be found, and for some, scientific counter evidence will be found. Some germane scientific papers and texts are listed in the next menu item.

References

1. Uman, M., Lightning, McGraw Hill, 1969. 2. Uman. M., All About Lightning, Dover Publications, 1986. 3. Golde, R. H., Lightning, Volume 1 and 2, Academic Press, New York, 1977. 4. Bazelyan and Raizer, Lightning Physics and Lightning Protection, Institute

of Physics Publishing, 2000 5. Bell, T., Thunderstorms, Viking Press, 1960. 6. Viemeister, P., The Lightning Book, MIT Press, 1961. 7. Marckerous, D., et. al., Review of claimed enhanced lightning protection of

buildings by early streamer emission air terminals, IEE Proc. - Scientific Measurement Technology, Vol. 144 No. 1, January 1997.

8. Mousa, A., et. al., The applicability of lightning elimination devices to substations and power lines, IEEE Transactions on Power Delivery, Vol. 13, No. 4, October 1998.

9. Grzybowski, S., et. al., Effectiveness of dissipators used for lightning protection of 13 kV distribution lines - model tests, Ninth Annual Symposium on High Voltage Engineering, August 28 - Sept 1, 1995.

10. Grzybowski, S., et. al., Effectiveness of spline ball dissipators used for lightning protection of 115 kV distribution lines - model tests, 22nd International Conference on Lightning (ICLP), R2-03, September, 1994.

11. NFPA 780, Standard for the Installation of Lightning Protection Systems, 1997 Edition.

12. Moore, C., et al, Lightning Rod Improvements, Journal of Applied Meteorology, 39, pp. 593-609, 2000.

13. Moore, C., Improved Configurations of Lightning Rods and Air Terminals, Journal of Franklin Institute, 315, pp. 61-85,1982.

14. Tobias, J., et al, The Basis of Conventional Lightning Protection Technology, Report of the Federal Interagency Lightning Protection Users Group, June 2000.

15. Grzybowski, S., et. al., Effectiveness of dissipators used for lightning protection of 13 kV distribution lines - model tests, Ninth Annual Symposium on High Voltage Engineering, August 28 - Sept 1, 1995.

16. Van Brunt, R.J., et. al., Early Streamer Emission Air Terminals Lightning Protection Systems, National Institute of Standards and Technology (Gaithersburg, Maryland), Report for National Fire Protection Research Foundation, Batterymarch Park, Quincy, Mass., Jan. 31, 1995.

17. Grzybowski, S., et. al., Effectiveness of spline ball dissipators used for lightning protection of 115 kV distribution lines - model tests, 22nd International Conference on Lightning (ICLP), R2-03, September, 1994

18. Zipse, D. Lightning protection systems: Advantages and disadvantages, IEEE Trans Ind. Appl. 30: 1351-61, 1994.

19. Rourke, C., A review of lightning-related operating events at nuclear power plants, IEEE Transactions on Energy Conversion, 9: 636-641, 1994.

20. Mackerras, D. et. al., Standard and non-standard lightning protection methods, Journal of Electrical and Electronics Engineering, Austria, 7:133-140, 1987.

21. Moore, C., Improved Configurations of Lightning Rods and Air Terminals, Journal of Franklin Institute, 315, pp. 61-85,1982.

22. Zeleny, J. Do lightning rods prevent lightning? Science 79: 269-71, 1934.