[TowerTalk] lightning umbrella

[Jim Jarvis]

Gary Schafer wrote:

To determine the "cone of protection" that the tower will provide
make a ball with a 150 foot radius. Roll that ball on the ground up to
the tower until it touches the top of the tower. Everything under the
curve of the ball from the top of the tower to where the ball touches
the ground will be the cone of protection.

If the top of the house does not fall under that curved line then it
will not be protected.

-0-

While Gary repeats the canonical wisdom about the "cone of protection",
in my humble opinion, there is no such thing.

You MAY cause a direct strike to be diverted to the tower, rather than
hitting your chimney, or roof, or wiring in the attic...but you're just
as likely to have streamers come off to any of those points.

My interpretation of the zone of protection would be, if your house
is anywhere WITHIN that radius, you are MORE likely to be hit than
if the tower were located further from the house, outside the
'protection' radius.

And that's even IF you completely discard my notion of the induced
currents from the previous post.

I'm not thumping on Gary, here...just reiterating the notion that
even after we take all the precautions we can, we're still trying to
avoid a very high-energy event.  And if you don't visualize a 12" bend
radius in a 3" ground strap as an inductor...you're not thinking like a
lightning strike.

-0-

A (hopefully) brief story about the "cone of protection".  Consider
a 30' sailboat, with a 45' grounded mast.  Multiple thundercells came up in
August,
with cloud to ground lightning 7-8 miles away.  Cells all around us...
cloud to cloud discharges.  Best we could make was 6kts, and 2 miles to safe
harbor.  Everybody in center of boat...stay away from lifelines and metal...
no strikes to the boat, but strikes to gnd 7 miles away resulted in the
knotmeter,
and windspeed indicator being pegged,  and the depthsounder doing a full
reset.
Turned off the instruments so they wouldn't wierd out the guests.  The
bloody
instrument lights blinked on with every strike.

Induced current.  IN the cone of protection.

n2ea
jimjarvis at ieee.org