Jim Lux 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
If my memory has not deteriorated too much, my understanding of a
Ufer ground is that it approximates a capacitance in parallel with
a resistance. The theory was that the capacitance would absorb the
energy of the lightning strike which would then bleed off through
the resistance of the concrete.
The Ufer ground seems to have morphed into a substitute for a ground
rod in power distribution systems using relatively short conductors
in the concrete. While this might have a low enough impedance for a
safety ground, I don't think it would provide enough area (capacitance)
for effective lightning protection.
Remember the USS Liberty (AGTR-5)
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