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Lightning Protection and Grounding Facts and Fallacies

from PolyPhaser’s Technical Information Answer Center

Document ID: PTDIO2I 
Last Updated: 01/19/99

The idea of preventing a lightning strike goes back to 1754 when the master himself, Ben Franklin, was still experimenting. Prokop Divisch installed 216 earthed points on a 7.4 meter wooden frame, and a few years later it was suggested by Lichtenberg that a catenary of barbed wire over a house could prevent a strike. The idea of using multiple points to discharging a cloud and neutralize its charge has been thought about and tried for years. "The idea is just as bogus now as it was then." -- clarification added by LPGI. It may be a bit technical but read on to understand why.

R. H. Golde suggested an umbrella-shaped barbed wire device could be used on very tall towers to prevent the normally occurring streamers. Golde's concept is to meticulously form a uniform field shaped element taking into account the electrostatic effects of surrounding points. If all points are positions with the correct outward looking angle, it could spread the E field out, much like a corona preventor on a high-voltage power supply. Since it is made of discharge points, unlike the rounded corona preventor, the electrostatically inducted voltage from the tower/ground system will be spread to limit the size of the upward streamer. The effect on the downward approaching stepped leader is nil. As the stepped leader approaches the array of points, the E fields will increase above the ability of the size of the array to prevent the transition from ion-maker to streamer producer (glow to arc transition).

This is similar to (but not the same as) reaching the limit of the corona preventor on a high-voltage power supply. The air breaks down and a major streamer/arc leaps outward. The larger the array means the larger the support structure. (Ice and wind tower loading also increase.) More charge can now be stored on this structure before the array can bleed it off into the wind. This can result in larger streamers from the array as E fields increase with the approach of the stepped leader.

However, recently a few people have claimed success. They claimed first to discharge the cloud. When that was proven impossible, they claimed to prevent a strike from occurring. Various branches of the U.S. government have tested several multiple point arrays over the years without any success. One report was completed by the Office of Naval Research, NASA, and US Air Force in 1975.

A 1,200 foot tower at Eglin AFB, was fitted with a multipoint system and sustained eleven hits in three months.

Five were photographically recorded while seven other strikes were monitored, using NASA's magnetic links, as having had strikes in the 30 to 48kA range. The report also contains a video lightning strike sequence from a monitor showing NASA's Kennedy Space Center 500 foot meteorological tower, equipped with a multipoint array, being hit by lightning.

As the height of an object is increased, the number of strikes increases. This was proven in the middle of this century with testing at the Empire State Building. Most of the strikes to this structure were caused by upward streamers triggering the strike. A more recent test of multipoint arrays was done in the late 1980's by the FM (terminated on 1/11/90). The FM report also concludes that the tower arrays under test were struck and damaged.

The report further includes photos of the video tape of the strike, and the NASA magnetic links current measurement for one strike was 8 kA for one down conductor and 10 kA for the other. Other damage to the facility was listed together with eyewitness accounts.

Few array suppliers will agree whether it prevents a strike 100% or just minimizes the chances of a strike. Another argument is whether to ground the array and how important the ground is to the array operation.

Some of the arrays on the market consist of small rounded brushes which when hit, splatter molten metal as far away as 10 meters. This can be a fire hazard. The FAA report quotes the eyewitnesses to the August 27,1989 strike to the Tampa ATC Tower: Asparks like the slag you get when arc welding."

Mother Nature produces a large variation of strikes. The larger strikes will have larger E fields, and the stepped leaders will be longer. This means the larger strikes will overwhelm the array and upward streamers will reach out and grab the stepped leader anyway. Perhaps the array will slightly delay the streamer, making another target (if there is one) more attractive?

The real solution for lightning protection is to have control of the strike energy. To do this, a well designed ground system will be a better investment than adding another load to the top of the tower.

 

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