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@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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