[TowerTalk] Grounding base slab and pier tower bases

[Nick Pair]

I agree entirely, the concrete either has corroded rebar or had pockets from poor construction techniques when it was poured where water gathered. NEC does require a gound both for electrical safety and lightning protection with the additional requirement that they both be tied together so that they are at same potential.
  The exploding base may also have been cracked before strike for many reasons and just noticed after due to water/steam expansion.
  Some of the rebar cages in sensitive areas have be spec'd with coated rebar to prevent corrosion. That sure messes up their grounding ability, but doesn't change the CEGE(Ufer) installation.
   
  Nick WB7PEK

   
  
Jim Lux <jimlux at earthlink.net> wrote:
  At 07:44 PM 7/2/2006, JC Smith wrote:
>Hi Nick,
>
>I've been reading about these Ufer grounds recently. Never heard of them
>before, in fact it seems like the consensus has always been that you should
>NOT ground your tower to the rebar. I can recall stories about concrete
>tower foundations being cracked (almost exploding in some stories) form the
>tower being attached to the rebar when a direct lightning strike hit.
>Obviously, however, if it is in the NEC it must be a valid technique.

Well, "concrete encased grounding electrodes" have been around for a while, 
and seem to be required by many codes these days (probably because of the 
increased use of plastic pipes for water lines, so the old "cold water 
pipe" standby doesn't work as well).

However, a CEGE (Ufer) ground, while being what the NEC requires, may not 
be a good lightning ground (notwithstanding that's what Herb Ufer designed 
them for). NEC wants a ground for other reasons than lightning protection 
(electrical safety, mostly).

And, "exploding in *some stories*" is exactly what that is.. anecdotal 
evidence. One tends to hear about the disasters (because they're unusual), 
and not hear about a Ufer grounded facility, tower, etc., that takes a 
direct hit with no damage. The literature makes the claim that cases of 
failed concrete and lightning tend to be with corroded rebar that had a gap 
between rebar and concrete, the water boils and the steam explosion causes 
the damage and/or spalling. There's also the fact that there's a heck of a 
lot of energy in a lightning strike, and the results of a direct hit are, 
shall we say, not totally predictable and consistent.


Jim Lux, W6RMK 




 		
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