Jim Jarvis wrote:
> 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.
The "cone of protection" is not an absolute thing. It only means that
the chances of getting hit when under it are diminished. No 100%
guarantee of not getting hit.
>
> 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.
Anything can have streamers come off it. Even if under the cone.
The theory is that the step leaders prior to a strike move in about 150
foot steps. (thus the name step leader) That's where the 150 foot radius
ball comes from.
This also why big towers don't always get hit at the top but often get
hit on the side.
>
> 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.
Only if the tower is poorly grounded and side flashes result.
>
> And that's even IF you completely discard my notion of the induced
> currents from the previous post.
Nothing to do with induced currents. That's a different subject and of
course is a problem when any near by object is struck.
>
> 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.
A 45 foot mast does not provide much of a cone of protection. If you
roll the ball up to it you will find that only a small part of the boat
actually fits underneath. Actually about what the stays cover.
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.
This has nothing to do with the cone of protection. The cone only limits
direct strikes under it. It does not do anything for induced currents etc.
73
Gary K4FMX
>
> n2ea
> jimjarvis@ieee.org
>
>
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