On 7/17/13 6:55 PM, David Gilbert wrote:
Your hysterics aside, almost all of that is patently false, and in most
locations is directly contrary to code. You really need to study what a
Ufer ground is and why they work.
By the way, this same old wives tale comes up every so often here and on
other forums, and each time it does I spend about a half hour doing
Google searches to find any documented instances of a block of concrete
exploding because of steam expansion from a lightning strike. I have
found TONS of discussion and endless repetitions of the same old
exhortation you just made, but not a single first hand account of
exploding concrete due to steam. Concrete is quite brittle and
fractures easily, and I could far better imagine that the intense
thermal shock generated by a lightning hit would instead be the
culprit. I have indeed witnessed first hand a lightning strike blasting
a chunk out of solid granite rock alongside the road I was driving on
(scared the hell out of me), and I guarantee that rock was dry as a bone.
I would agree here. I've seen lightning damaged concrete, but there
were no conductors anywhere near it. It was a strike in the middle of a
concrete pad. The damage was pretty minor. You'd do more damage
beating on it with a hammer.
I've seen spots on metal where the stroke apparently attached (e.g. the
tippy top of a pointed lightning rod).
I think that to do serious mechanical damage, there has to be a set of
circumstances for the lightning current that are somewhat unusual.
You need a sort of moderate resistance.. if the resistance is low
enough, then it just conducts the tens of kA and gets warm. Something
like damp sand, though, is resistive enough that you get a lot of
dissipation, and it fuses the sand. It's a fairly narrow range of
resistivity though.. I was trying to make fulgurites in my backyard with
a big capacitor bank, and it's hard work.
Something like a tree is going to get damaged: you have a resistive
layer but still conductive damp layer under the bark. The current
flows, the thermal capacity is small, so it gets hot quick, boiling the
water, etc.
Wooden structures are probably similar...
Things where there's a spark gap that arcs over is also a problem: now
you've got a concentrated heat source.
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