On Wed, 28 May 2008 21:36:13 -0700, David Gilbert wrote:
>so why
>would additional ground wires running from the tower through the
>concrete to ground ... an additional shunt path ... be a bad
idea?
As others have observed, the concrete forms a Ufer ground, whether
we like it or not, and it will carry current to the soil. That can
cause heating, which can crack or otherwise degrade the concrete
and cause structural failure, so flanking paths of the lowest
practical impedance are important, and more is better.
A Ufer ground (the concrete) has a rather low impedance to earth
by virtue of its large cross sectional area AND because it is
short. That makes it rather difficult to shunt! That doesn't mean
you can't shunt it, but you must work hard at it (that is,
multiple short, wide straps with minimal bends, etc. to multiple
widely spaced ground electrodes). Why wide spacing? Mutual
inductance between the rods increases their impedance to earth!
A copper path through the concrete to the soil will heat too, so
is less desirable and less effective than multiple paths external
to the concrete. Let's say that the rebar went to a 20 ft copper
rod below the concrete. The surface area of the concrete is much
much greater than the surface area of the rod and the path through
the rod has more inductance, so the concrete is still carrying
more current than the rod.
73,
Jim K9YC
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