Scott Neader wrote:
>
> induced. Ufers should always be used to augment your grounding system and
> not to be the entire system. Radials or radials with ground rods should be
> used together with the Ufer...
>
> given area. This is known as current density J. The more surface area you
> have to spread out the current, the less the current density. YOUR TOWER'S
> ANCHOR BOLTS ARE IN THE CONCRETE ANYWAY. If your ground system is poor,
> the current density surrounding the bolts will be high and can blow apart
> your concrete. At least if you tie in your rebar, the area is increased
> and the current density is decreased."
>
> Any arguments??
an article in the proceedings from a recent uhv transmission line
conference
in montreal would seem to lend more support to these discussions.
granted
that the bases are bigger, but so are the towers and the lightning
capture
area. if i remember right it was a japanese research group that tested
the impedance of various ground systems for uhv power lines. just using
the concrete base provided a surprisingly good ground, and even more
interesting was that it was virtually linear, not presenting the
non-linear
characteristics shown by rods as the current density gets high enough to
break down the ground around them. if i remember right they could only
go
up to about 40ka, which covers the smaller lightning strokes anyway.
they
also tested some combinations in parallel with the base which did
improve
the performance. one description of a radial system sounded like a
buried
porcupine (well, 4 rods with spikes anyway).
on another thread, there was also an article in those proceedings about
the
use of 'porcupine' air terminals. the theory of operation of the spiked
ball is much different than the common franklin type rods. the old
style
franklin type rods are meant to provide a point to attract near by
strokes
to a safe downlead. as such they are reasonably effective if the area
to
be protected is well covered with them, especially around the
perimeter.
the porcupines are meant to provide a large area of ionization around
them
essentially reducing the electric field gradient near the ground. by
reducing the field gradient near the ground or structure there is less
of
an incentive for the upward streamer from the ground to form to meet the
downward progressing leader from the cloud. the researchers from this
paper seemed to downplay their conclusions a bit (or maybe it was just
the
translation) saying that more work was needed to come up with guidelines
for effective use. i would wonder about reaction time (can they build
up
ionization fast enough to be effective), how do they work in rain (does
the
rain reduce the ionization), or wind (does the ionized area blow away),
their
lab experiments and simulations didn't address those factors.
--
David Robbins K1TTT (ex KY1H)
k1ttt@berkshire.net or robbins@berkshire.net
http://www.berkshire.net/~robbins/k1ttt.html
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