[TowerTalk] exploding foundations

[Jim Lux]

Paul Christensen wrote:
>> What exploding concrete??  Back up your claim with real life validated
>> proof.  My experience has been the opposite.
>>
>> Doug
> 
> Want evidence?  See Figure 4 (and the text below it) in the link below:
> 
> http://www.mikeholt.com/newsletters.php?action=display&letterID=407

The text says the only grounding was through the foundation and makes no 
mention of whether it was a properly constructed CEGR (i.e. 20 feet of 
conductor, etc.)

Also, the picture does not show whether a structural failure occurred as 
a result of the lightning damage (the tower is still standing, after 
all), or whether that's just surface damage.  If it's a 1/8" deep crack 
in the surface and doesn't penetrate, who cares?  For all we know from 
the limited data in the article there was a prexisting crack in the 
concrete that was enlarged as a result of steam induced spalling.


Considering that Ufer did an awful lot of tests over the years, and 
subsequent researchers have also done a lot of tests, I think that if 
there actually was a realistic concern about spalling and exploding 
foundations it would have shown up in the reviewed literature (as 
opposed to in anecdotal reports or ground rod manufacturer sales 
literature).  I have no doubt that there are cases where there has been 
lightning damage, but I would suspect that those don't fit in the 
category of a proper concrete encased grounding electrode. For instance, 
the "3 2 foot J bolts in the top of the footing" used to bolt the tower 
mounting plate is probably NOT a suitable grounding connection.

The article also sort of contradicts itself.  On the one hand it 
advocates external grounding systems, but then, it says that the 
external 20 rod grounding system showed an impedance of 150 ohms, but 
after bonding to the structural steel, the impedance dropped to 1 ohm. 
Sounds to me like the steel was a better ground than the rods.

The article also mentions the "conduit as choke" thing, which I used to 
believe in, but subsequently have not seen a good analysis to show that 
it actually works, particularly for large surge currents, taking into 
account the magnetic saturation of the steel.

> 
> According to NFPA 780, Ufer grounding is generally satisfactory provided 
> that it is not the only earth grounding mechanism and that the ground is 
> brought out beyond the concrete encasement to make the earth connection.

Which section? (just curious.. I don't have my copy of 780 here)
> 
> Paul, W9AC
> 
>