On Wed,1/21/2015 1:01 PM, Patrick Greenlee wrote:
I'm confused... (still, yet, again...)
There are recommendations to place ground rods (8 footers) 8 ft apart
in a circle (octagon.) If my 14 ft apart Ufer grounds are too close
together so as to act as a single point, what about the 8 ft rods 8 ft
apart?
Forget about "single point" with respect to these various earth
electrodes. Multiple electrodes (rods, Ufer, etc.) simply provides
multiple paths in parallel to discharge a strike. Each of those paths is
R + jwL, and the impedances to earth of the combination of those paths
adds like any other combination of resistance and inductance, except to
the extent that mutual coupling between the electrodes increases the
combined impedance.
Lightning is NOT a DC event, it is an RF event, with the energy
concentrated in very broad spectrum, roughly centered (on a log
frequency scale) around 1 MHz (in other words, spread roughly between 50
kHz and 20 MHz). Thus, the inductance dominates the impedance.
Resistance matters primarily to the extent that we would like the
conductor to carry as much as possible of the strike current before it
vaporizes. :)
73, Jim K9YC
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