Jim Lux wrote:
>
> But, to return to the Capacitance issue, and whether the C holds the
> entire charge, or if it's just a component in the circuit... we can
> ballpark this a bit, to see if it's reasonable. Two ways to consider it:
>
> First, let's consider it as a sphere in free space (usually a lower
> bound on C), for which C = about 110pF/m radius. Let's say that it's a
> 3 meter radius sphere. The C is then about 330 pF. The charge transfer
> in a typical lightning stroke is around a Coulomb, so we can figure what
> voltage this would be: Q=CV; V=Q/C; V=1/0.33E-9; V=3E9 volts... Nope,
> way, way too big to be even quasi reasonable.
>
> Let's try another strategy.. Let's say the slab is a big plate on a
> capacitor, separated by 1mm from earth. Let's further say that it's 3x3
> meters (10x10 ft, or thereabouts). C=A/d*epsilon0 = 8.84E-12 *A/d =
> 8.84E-12 * 9/1E-3 = 80E-9. Run the same voltage calculation as above:
> V=1/80E-9 = 12.5 MV... still awfully big.
>
> Now, let's look at the reactance of that C, at, say, 100 kHz (a standard
> lightning impulse is 2 microsecond rise and 50 microsecond fall, so
> let's say that it's like 100 kHz). X=1/(2*pi*100E5*80E-9) =
> 1/(2*pi*8000E-4) = 1/(6*0.8) = 1/5 -> 0.2 ohms.... That's pretty low..
> MUCH lower than the resistive impedance to the earth from a ground rod
> (5-20 ohms). With that 20 kA impulse, you're looking at a 4 kV voltage
> drop, which isn't all that huge. Given that the rise time is actually
> much faster, and the reactance lower, the voltage will be less.
>
> It would appear then, that at least as far as ballparking goes, the Ufer
> ground looks like a capacitor to ground for the lightning impulse.
> (And, the existence, or not, of an insulating vapor barrier is immaterial).
>
> Jim
As I stated originally, I read the description of the Ufer ground many
years ago and my memory might be a tad faulty! However, I think we are
converging on an explanation. I think Jim is correct in that the
capacitance of the Ufer ground provides a low impedance for the higher
frequency components of the lightning strike.
My original reason for bringing this up is that the original Ufer
lightning protection ground requires a fairly large area to be
effective. This not only provides the capacitance to shunt the high
frequencies to ground, the low resistance for the low frequencies and
DC component, but also spreads the charge over a large area of earth to
prevent current saturation.
My opinion is that a small Ufer ground, such as a tower base or the type
used in residential power distribution systems as a safety ground,
will not effectively handle the currents generated by a lightning
strike.
73, Roger
--
Remember the USS Liberty (AGTR-5)
http://ussliberty.org/
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