[BC] Lightning and grounding ... fun for all!

[Bruce Doerle]

Alan,

Why waste the money?  Why have false expectations?  Enclosed is a technical brief by Polyphaser, a well respected company in this field and within the RF industry.  They answer some of your questions and reference situations that clearly define the value of these devices.  Testimonials from the unknowing is NOT proof but snake oil salesmanship.  In the lengthy email I posted on Radio Tech net on January 17th "Lightning abatement", I discussed other proof that CTS devices do not work.  You need to read the material before your question proof versus testimonials.

Bruce 

POLYPHASER (www.polyphaser.com)
Lightning Protection Facts and Fallacies
Document ID: PTD1021
Last Updated: 01/19/99

Summary

A brief history of lightning protection theory and mythology.

Lightning Protection Facts and Fallacies

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.

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 8kA for one down conductor and 10kA 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: "sparks 

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 welldesigned ground system will be a better investment than adding another load 

to the top of the tower.

 

>>> radiotech at bellsouth.net 05/18/05 8:11 PM >>>
Hi Bruce,

hehe Despite all the different answers you are still pushing to a single 
thought. I'm thinking you can easily have it both ways put in your 
conventional protection and then add the Dissipaters.

But again, there is no easy way to prove that the dissipaters do work.
Although their seems to be strong evidence that they do. Even though you 
don't want to accept testimonial evidence of their performance, how 
could you prove that they did or didn't prevent a lightning strike?

 
Alan Alsobrook CSRE AMD CBNT
St. Augustine Fl. 32086 904-829-8885
aalso at Bellsouth.net