[TowerTalk] Tower grounding

[K8RI]

On 7/18/2013 12:40 AM, Jim Lux wrote:
> 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.

As I said the tower here took at least 17 hits. There was mechanical 
damage on only one hit.  It blew all the water proofing off the 
connectors and removed all the silver plating. No other physical damage 
was visible.  All the antennas were fine.  There were large chunks of 
the weatherproofing laying on the ground. They all looked like pieces of 
expanded metal.

I also lost a 2-meter rig and computer

This was early on before all the grounding was in, but I doubt if that 
would have made any difference on the mechanical damage. Since all the 
grounding system was finished the tower took most of the hits and I've 
had no damage to equipment and nothing visible "up there".



>
> 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.

We had about a 30' pine tree across the road take a hit and literally 
explode, leaving about a 3' stump. The longest piece was about 3 or 4 
feet long.  It was stuck in the ground about 30 to 40 feet from the 
stump. It was in so deep we couldn't pull it out, but had to dig it out. 
That neighbor had no damage, but the one on the other side of him lost 
most of the appliances in the house and his well, telephone and breaker 
panel.  It never touched the tower with the top of the array at 130' 
which was closer to the tree than the neighbor that had all the damage.


73

Roger (K8RI)

>
> 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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