[TowerTalk] Earthing a tower

[K8RI]

On 12/30/2012 2:14 PM, ve4xt at mymts.net wrote:
> Another repetition of the exploding base myth.
>
> If concrete is so explosive, and such a poor conductor, why is it often used to assist with rod-earth connections?
>
> As well, the very existence of the Ufer ground pretty much debunks the exploding base mythology.
>

The NWS even has  video showing what could be construed as the exploding 
base and it is no myth, BUT we need to be able to separate cause from 
hearsay.

I have seen pictures of concrete blown apart from lightning strikes and 
some of those were from the NWS so I believe them.  However in those 
cases we need to ask WHY did the concrete either blow apart, or crack so 
badly it had to be replaced.

Concrete by itself is not explosive. The resistivity in ohms per cubic 
centimeter is relatively high and after cure the moisture content is 
relatively low.  This means that in a "properly designed and constructed 
tower base" (please note the emphasis on that phrase!) that current 
density will be quite low and should cause no structural damage even 
with a so-called super strike, BUT in a poorly designed and/or 
constructed base it is easy to realize conditions where the current 
density can be very high in some localized spots/locations and in those 
instances the damage can be spectacular, particularly if you are close 
when the strike occurs!

The UFER ground depends on a low current density to work well and to 
maintain its structural integrity. It does this by having a large 
volume, a large contact area with the surrounding soil, and a large, 
intimate contact with the tower through the rerod in the base.

Typically the point where the metal enters the concrete becomes cracked 
or the metal becomes rusted.  In either case moisture can enter the 
concrete and flow along the metal. This leaves a small gap that is a 
wet, high resistance area that the high current must jump across.  This 
produces some very hot, high pressure steam almost instantly and that 
produces a tremendous pressure with a force comparable to Dynamite.

So with a properly designed ground system, be it UFER or ring It's 
likely that more damage will be caused by voltage induced into the house 
electrical wiring than will come in on the coax from the tower and this 
is assuming a good single point ground where the cables enter the house.

So, yes a super strike can blow a poorly designed and/or implemented 
ground, or base apart, but it's unlikely it will do so with an adequate one.


73

Roger (K8RI)



> 73,
> Kelly
>
> Sent from my iPhone
>
> On Dec 30, 2012, at 12:33, "Michael Mraz" <mike.n6mz at gmail.com> wrote:
>
>> My dear old SK friend Al Caplan, who used to manage the Hy-Gain amateur division when it was part of Telex, was emphatic about proper grounding when I bought my HG-52 crankup. He said that lightning could split the concrete around the re-bar if the tower took a direct strike. It was a long time ago, but my recollection was that this nasty event actually occurred to some unfortunate ham who had to abandon the old foundation and install a new one in a different location.
>>
>> 73  Mike  N6MZ
>>
>> At 17:17 30-12-12, Patrick Greenlee wrote:
>>> I have a friend who is a ham and is EE with 30+ years of antenna design experience.  He tells me emphatically, "concrete is not a good conductor." Given this, I'd be trying to not depend on concrete as part of a grounding system.  Rebar installed correctly in steel reinforced concrete is probably at least 2 inches under the surface of the concrete on any side, top, or bottom.  Weld plates installed when the concrete was wet (to give you something to anchor to) or bolts or other components set into the concrete are typically NOT in contact with the rebar cage inside the concrete (which is not a good conductor anyway.)  So again, it is better to not rely on the concrete to conduct.
>>>
>>> 73 Patrick AF5CK
>>
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