The BIG point you are missing is that grounding is far from an exact
science. The ground resistivity can vary quite a bit over a small area, and
also with depth. Driving one rod that happens to hit some moist clay in one
spot can give you a great resistance reading, 10' away if that rod is in a
pocket of dry sand or gravel it can be horrible. When it rains it affects
only the very top of the soil, so some rods may get much better, and others
may not change much at all. Also, measuring systems are not exact, they
depend on just where you drive the rods, how close you measure the spacing,
and even how you drive the rods... if you wiggle them too much while driving
they won't make good contact with the ground and you get bad readings. And
the formulas you will find for various ground configurations are also
approximations based on a few experiments or simple calculations. And to
top it off, when you start pumping current into a ground rod the resistance
changes as the earth ionizes. A lightning stroke will ionize the air gaps
in the soil around a rod or radial making the apparent resistance drop for a
short time, it will also create streamers underground that improve the
current dissipation.
In summary, you are probably over analyzing the problem unless you are
planning to put in an electrical substation or a military ammo bunker or
command post that requires extreme protection against lightning or EMP.
David Robbins K1TTT
e-mail: mailto:k1ttt@arrl.net
web: http://www.k1ttt.net
AR-Cluster node: 145.69MHz or telnet://dxc.k1ttt.net
> -----Original Message-----
> From: towertalk-bounces@contesting.com [mailto:towertalk-
> bounces@contesting.com] On Behalf Of Beat Meier
> Sent: Saturday, September 09, 2006 21:22
> To: towertalk@contesting.com
> Subject: Re: [TowerTalk] Soil resistivity test done: Some questions
>
> Ok
>
> I have just found the following formula:
>
> R=0.366*res/h * log ( 3* h/d)
>
> where
> res=Resitividad [ohm-meters]
> h=depth of ground road[m]
> d= diameter of ground road [m]
>
> Is this correct?
> So in my case with a ground road of 5/8'' of 2.5m and the resistivity of
> 500 ohm-meters
> I should get 50 ohm ...
> And with 2 ground roads I should get 25 ohm?
> This seems the point I have missed ...
>
> Is this right?
>
> Beat
>
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>
>
>
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