At 08:47 AM 7/3/2006, Roger D Johnson wrote:
>Jim Lux wrote:
> > At 07:44 PM 7/2/2006, JC Smith wrote:
> ><various stuff about Ufer grounds snipped>
> >
> >
> > 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.
Or, just a big lump of semiconductive material: concrete, which looks like
a lossy RC to a fast impulse.
>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.
Depends... The resistance from a 20 ft long wire to the concrete isn't all
that high. You could calculate how much heat is going to be dissipated
from a notional 20 kA stroke. Say the resistance, overall, is on the order
of 5-10 ohms. 20kA into 10 ohms is about 4000 MW, but lasting only about
2-20 microseconds, so 8-80 kJ or so, which isn't a huge amount of heat to
inject into 20 ft of wire and concrete. The capacitance of the block of
concrete to the surrounding soil will be fairly large and the resistance low.
The NEC is definitely more focussed on dissipating transient energy (from
near strikes, etc.) and fault currents, not on dissipating a direct stroke,
so the 20ft encased in concrete thing might be insufficient for lightning
protection.
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