Tendencies, not absolutes. It would be foolish to assume a charge
dissipating system would work 100% of the time. Grounding runs for
lightning rods are of considerable size and are careful to bend gently
to avoid lumps of inductance. Otherwise they would be destroyed the
first time they failed to defuse a strike. Ask yourself what the sharp
thingies are on those commercial towers for. Do you suppose they might
reduce the number and severity of strikes or are they just pretty
decorations for birds and tower climbers to appreciate?
Patrick NJ5G
On 1/29/2015 7:24 AM, David Robbins wrote:
Sorry, I have been busy and trying to ignore the recent threads on lightning
and grounding, but this one needs to be debunked... again.
"Lots of folks think lightning rods (and other structures such a towers
etc.) are intended to attract lightning to protect other objects from
being hit. Not so, gentle reader, as the primary purpose of lightning
rods is to drain away static charges such as accumulate and thereby
prevent a lightning strike. If the grounded system is hit by lightning
then the protection system has failed its primary goal. A secondary
consideration is surviving the strike so as to continue with its primary
purpose of draining charge and preventing strikes in the immediate area."
No, there is no way a lightning rod system, even if using those fancy
porcupines on a stick, will ever dissipate enough
charge to prevent a structure getting hit. if that were the purpose there
would be no need for heavy conductors from
lightning rods as the current during the concentration of charge on the ground
under a convective cell is pretty small.
just think, how much current does it take to make your hair stand on end? or
for the tiny corona and sparks from pine
needles or sharp points on a structure??? its TINY... and even an entire patch
of a forest going into corona trying to
dissipate charge doesn't prevent it from being hit. lightning rods are
designed to be the preferred point that gets hit,
they do that by having a sharp point that goes into corona easily and initiates
the upward streamer where the downward
moving leader makes its connection to ground thus triggering what you see as
lightning.
Also note that the old 'rolling ball' or '30 degree angle' for shielding under
a structure or wire is not reliable. there are
many lightning triggered camera systems that have caught lightning striking
power lines directly under a shield wire
and buildings getting hit on their side instead of on top. there is a strong
random component in the movement of the
downward leaders that make the final attachment point VERY hard to control or
predict.
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