Lightning Explained, Part 2
The millisecond-by-millisecond account of a lightning strike provided in Part I was based on modeling of a competition between the building corner and a 0.3m rod installed as per code to protect that corner. It was found that the corner had a higher probability of being struck. This occurs with prospective return stroke currents as low as 12kA and the situation gets worse as the size of the strike increases.
In such cases, the installation of rods as per the standards on such structures has little to no influence on the protection provided. The entire protection function is performed by the grounding and down-conductor system that collects a portion of the lightning current after direct attachment has occurred to the structure itself and not to the rods. This still limits damage but the stray portions of the lightning current degrades infrastructure and enhances electromagnetic disturbances. It would be far better if the entire lightning current was distributed and conducted to ground via the lightning protection system, but this requires that the lightning rods are struck directly.
There exists a whole class of products called charge transfer systems or dissipators. They all use large numbers of sharp conductive points and the explanation as to the mode of operation offered by their proponents keeps changing. First it was claimed that they could prevent lightning from occurring by discharging the clouds, but that was invalidated. Then it was claimed that the charge of the descending leader could be neutralized, but that was invalidated. They still claim that these devices can dissipate, bleed off or otherwise remove the induced image charge produced by the cloud charges and change the potential between the clouds and the ground.
This is of course impossible since induced image charges are just that, induced images. So long as the inducing charges that are the source of the image remain, the resulting image remains. Also altering the voltage between the clouds and the ground would necessarily mean changing the potential of the clouds since, unless hit by lightning, ground remains at ground potential. Changing the potential of the clouds is not an option.
However, rather than bleeding off or transferring any image charge, something else is happening here that is of some scientific interest.
It has been observed that conventional rods are occasionally bypassed and lightning attaches in their immediate vicinity. As well, in experimental field studies with real lightning, it was observed that in head-to-head competition, blunt rods are favored to be struck when compared to sharp rods. The results were most recently explained by a mechanism known as "space charge shielding". (More on space charge shielding in Part 3.)
-- continued on page 3
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