[TowerTalk] Tower grounding

[Jim Lux]

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.