Electrical Systems / Industrial Safety / Energy Management

Keep lightning away from your structures

Russ Kratowicz, P.E., CMRP, executive editor, reviews ways to safely channel meteorological sparks away from structures.

By Russ Kratowicz, P.E., CMRP, executive editor

During his investigations into the weapons of Zeus, Ben Franklin made some interesting observations about lightning. The only aspect of lightning that changed since the 1700s is our knowledge about the phenomenon. According to the U.S. Department of Commerce, National Oceanic and Atmospheric Administration's National Weather Service, at any given moment, nearly 2,000 thunderstorms are in progress somewhere in the world, and lightning strikes the Earth 100 times each second. Around the world, there are about 45,000 thunderstorms daily and 16 million annually. There are at least 100,000 thunderstorms annually across the United States.

Lightning is common during thunderstorms. The upper cloud layers develop a positive electrical charge. The corresponding negative charge, plus a small area of secondary positive charge, resides in the lower portions of the cloud. While the Earth is normally negatively charged with respect to the atmosphere, the huge negative charge at the cloud bottom can reverse the normal situation and induce a positive charge on anything under the cloud. When conditions are just right, the electrical potential between cloud and structures on the earth — sometimes as high as 100 million volts — produces a corona consisting of ionized streamers that extend upward to meet the cloud. Once the conductive path is sufficiently complete, there ensues a spark carrying 35,000 to 40,000 amperes. Researchers claim that lightning has a rise time corresponding to more than half a million amperes per microsecond.

Franklin's investigations into the nature of electricity so long ago resulted in a low-tech approach to protection — the lightning rod with the needle-like tip and a good earth ground system. In the intervening 300 years, farmers and factory owners equipped thousands of buildings with these skyward-pointing metal rods. Some now argue, however, that during a strike, standard passive Franklin lightning rods actually increase the risk of damage to electronic components because they bring the high-voltage lightning pulse in closer proximity than it would normally be. Another argument is that a passive lightning rod may not give adequate protection because the upward corona that promotes a strike may be initiated from more favorably situated parts of the structure itself.

The market and innovation being what they are, there is an alternative concept called early streamer emission. This version of lightning rod features an active component powered by radioactivity, electricity or, as some claim, by the energy of the storm itself. The idea is that if a ground-based device can be forced to produce an upward corona earlier in the cycle, it can overcome the distance advantage of the more favorably situated, but passive, parts of the building. By extension, the argument is that the corona from one centrally located early streamer emission lightning protection device could replace the corona from several standard Franklin lightning rods.

Quite naturally, the National Fire Protection Association is concerned with lightning and protection against loss caused by fire. This organization publishes NFPA-780, Standard for the Installation of Lightning Protection Systems. The publication, however, mentions only the standard, sharp-pointed Franklin lightning rod. It disregards devices that are based on the idea of early streamer emission. Backers of early streamer emission sought to have another standard issued that would cover their technology.

The National Fire Protection Association commissioned a report that evaluated early streamer emission lightning protection technology. The third-party independent evaluation panel addressed two major issues:

  • Whether early streamer emission lightning protection technology is scientifically and technically sound.
  • Whether the early streamer emission lightning protection technology is supported by an adequate scientific theoretical basis and laboratory testing.

In 1999, the panel submitted the Report of the Third-Party Independent Evaluation Panel on the Early Streamer Emission Lightning Protection Technology to the National Fire Protection Association Standards Council. In it, the panel reviewed 377 documents, including personal communications, reports, papers, and other relevant documents.

Soundness of the technology

The panel considered reports from laboratory tests that were designed to simulate lightning. Several of the researchers concluded that the difference in scale between a lab and the great outdoors prevents laboratory simulations from adequately representing the real thing. With that caveat, the test results showed inconclusive results. Some indicated that the early streamer emission technology showed a very slight advantage over the Franklin rod.

One round of tests showed that the early streamer emission to respond faster than the passive rod, but the measured difference was a matter of nanoseconds. Another laboratory recorded 420 discharges, of which 48% struck a Franklin rod, 39% an early streamer emission device and 13% struck neither.

The panel also considered the results of field testing using naturally occurring lightning. Real-world testing is inherently problematic because storm clouds are not standardized, atmospheric conditions vary widely and nobody can produce lightning storms at will. As one laboratory report put it, "Any claims of 100% efficiency in the performance of a lightning attractor should be viewed with skepticism."

One set of research data found that blunt Franklin rods appeared to be more effective than adjacent sharp-pointed rods. Another field test used an early streamer emission device on a 20-foot mast placed four meters below the summit of a 3,300-meter-high peak. Video cameras monitored the area when storms were present. Although the manufacturer of the device claims it offers protection within 100 meters, during a five-week study, there were two recorded strikes about 80 meters from the device. But, near the end of the test, researchers found that a weld on the device had been broken. Examination under a microscope showed evidence of melting. Perhaps lightning struck while the cameras were idle.

Another research team summarized the field tests spanning six summers for a radioactive early streamer emission device. They found six strikes within 100 meters of the device, but found no evidence of a direct strike to the unit itself. In fact, the researcher made digitized measurements of the currents flowing from the device. The analysis from this report indicated that when lightning struck a blunt rod 20 meters away, no corona streamers were recorded coming from the early streamer emission device.

Theoretical scientific basis

According to the panel's report, "there appears to be no correlation between the performance of the ESE or Franklin air terminals in the high voltage laboratory situations and the field test results under natural lightning storm conditions." Another researcher concluded that although there is a greater probability of the early streamer emission device initiating a streamer, the precise amount by which this enhancement improves attraction efficiency remains questionable.

Still another research report reviewed by the panel showed three early streamers from a standard Franklin rod starting about a quarter of a second before a lightning strike 155 meters away. None of the three connected with the lightning, but they qualify as "early streamers." The same document points out that the "factor that determines the continued propagation of an upward-going streamer is the strength of the electric field ahead of the leader, not ionization back at the air terminal."

When considering the performance of an entire system, not just the Franklin rod or the early streamer emission device, neither seems to be the final answer. One document covered in the panel's reported the failure of a Franklin rod mounted on a building just four feet away from a camera. It was the camera that took the strike, not the rod. Other researchers reported cases of failure of early streamer emission devices mounted on buildings. These failure reports lack critical information such as the age of the system, maintenance records, condition of the system when the strike occurred and other data that a plant maintenance department would document as a matter of routine.

The panel's recommendations

Addressing the question of whether early streamer emission lightning protection technology is scientifically and technically sound, the panel had three conclusions. First, early streamer emission technology appears to be technically sound since it is generally equivalent to the standard Franklin rod. Second, neither the Franklin rod nor the early streamer emission technology appear to be technically sound when evaluated against real lightning. Third, the early streamer emission technology does not appear to be sound in relation to the claimed areas of protection or essentials of the grounding system.

Addressing the question of whether the early streamer emission lightning protection technology is supported by an adequate scientific theoretical basis and laboratory testing, the panel issued three determinations. First, there does seem to be an adequate theoretical basis for the concept and design of the technology from a physical viewpoint. Second, there does not appear to be an adequate basis for the claims of enhanced areas of protection. Third, although laboratory tests of the early streamer emission technology appear to be adequate, they are not equivalent to the evaluation of a complete system with real lightning.

If you are interested in a copy of the latest version of NFPA 780: "Standard for the Installation of Lightning Protection Systems," you can get full details at http://www.nfpa.org/catalog/search.asp?action=search&query=780.

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