Jim Jarvis wrote:
> I'm not a student of Ufer grounds. However, I HAVE engineered some
> large towers...including a 500' self supporter. Everyting I've
> read says...provide a low impedance path AROUND the foundation,
> not through it.
>
> Think about it...1 million amperes is going to vaporize just about
> anything you put down there.
Where's that million amps coming from? Even really big lightning
strokes don't get into that kind of range.
I've seen 8" copper strap, 1/16" thick
> simply dissapear.
Not that I question the copper disappearing, but something else is going
on here. There's not enough energy to vaporize (much less melt) that
much copper. 1/2 square inch cross section is about 3.2 square cm. A
meter long hunk means 320 cubic cm. Copper is 8.9 g/cc, or about 2.8 kg
for that copper (does that match your impression of the weight?).
Copper has a specific heat of 0.385 J/gK, so it takes about 1100 Joules
to raise the temperature 1 degree. To raise that copper from 20C to
1083C (Cu MP) is about 1000 times that, or 1.1 Megajoule.
Even in the stroke itself, where air is being heated, and the losses are
much higher, the energy dissipation is on the order of 100kJ/meter.
Without actually calculating it, I'd guess that the resistance of a 1
meter long piece of that strap is about 10 microohms, so even at 100 kA,
the dissipation is 100kW. To get the copper to the melting point, you'd
have to carry that current for 10 seconds or more, which doesn't match
what lightning does.
There might be some weird magnetic force issues causing the copper to
tear itself apart with high currents. Certainly I've destroyed lots of
AWG10 wire with discharges in the tens of kA, few kJ range in a quarter
shrinker, but it's not from thermal melting, it's from the extreme
magnetic field ripping it to pieces.
If it's inside the concrete, the expanding vapor
> will cause cracking and structural failure.
>
> My sense of the Ufer ground was that it was intended to reduce
> corrosive currents, and improve conductivity, not to handle peak
> lightning strikes. Perhaps I'm wrong. I'm sure the assembled
> expertise here can correct me.
Herb Ufer invented the concrete encased grounding electrode for use in
lightning protection for ammunition bunkers in dry areas, where
conventional ground rods and buried rings were ineffective. It IS
designed to handle high peak currents, be rugged, and provide a lower
impedance to ground, particularly for transients (large area
electrode:the concrete, makes for low Z). The other original advantage
of the Ufer ground was that it used the rebar and structural steel for
the conductors, instead of copper, which was in short supply as a
strategic material (this was done during WW II). The adoption in
residential use was prompted by the observation (one of Ufer's papers
from 1964 mentions this) that the traditional water pipe ground didn't
work as well because of the increased use of insulating couplings to
reduce galvanic corrosion, and that traditional ground rods and clamps
were not reliable over time (inexpensive exothermic welding solved the
latter problem)
>
> N2EA
>
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