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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