[TowerTalk] Ground Rod Under Tower Base

[Eric Gustafson]

Hi Ron,

It seems to me that there is a LOT of confusion on this subject.


>From: "Ron Castro" <ronc@sonic.net>
>Date: Fri, 19 Mar 1999 20:50:10 -0800
>
>I would worry that the current that flows through the conductor
>leading to the bottom of the foundation could cause sufficient
>heat, even though only for a fraction of a second, that the base
>could fracture.

You should not worry about stroke current heating an
appropriately sized conductor (bigger than #6 solid copper).
There just isn't time enough for significant heat to be generated
in the conductor.

High temperatures are present in a well designed (and
implemented) lightning protection system at only two points.
Sometimes only one.

The first point and the one that is always present is the place
where the lightning plasma touches the struck system.  This
_should_ be on the end of a diversion electrode.  In the typical
amateur antenna system, it is usually either the top of the mast
or somewhere on the antenna itself.  Even in these places, the
zone of damage caused by heating is usually quite small.
Depending on the stroke current, the damaged area can range from
pinhole sized up to half inch diameter melt zones (depending also
on the type of material and its thickness).

If the local ground is "good" and the system is well designed,
the stroke attachment point is the only hot spot in the system.

If the local ground is not good, and cannot dissipate the charge
rapidly enough to prevent the structure from attaining very high
potential relative to its surroundings, then the other hot spot
is located at the point(s) where the stroke energy leaves the
conductors of the protected system and continues into the earth
to be dissipated.  Where the charge jumps off the conductive
system, it forms another plasma channel.  The point where this
plasma touches the system is subject to high temperatures.

That is the primary reason why ground rods are effective for the
purpose of protecting the system from damage even in marginal
soil.  The plasma leaving the system is (if the soil/rod can't
soak up all the charge without forming a plasma connection to
earth) located 9 or 10 feet below grade at the end of the ground
rod where the tiny amount of melting on the rod end is not
significant to the system's owner.



>Concrete is semi-permeable, and generally contains moisture,
>even in dry climates.  The instant vaporization of the moisture
>(water expands to 800 times its volume when it turns to steam)
>might blast the foundation enough to do permanent damage.

This statement about concrete is true.  And damage to a concrete
base is _possible_.  But it is an extremely rare occurrence.  It
normally happens when two major errors have been committed in the
design of the system.  First, the tower base is connected only to
the concrete support and there is no other sufficiently low
impedance conductive path to permit the charge to leave the
system except by going through the concrete to get to earth.
And, second, the base is not of sufficient bulk to soak up enough
of the charge to prevent potentials from getting high enough to
form a plasma.

Bottom line is that you don't want the plasma forming in the
concrete itself.

But it is perfectly safe to run a lightning conductor through the
concrete pour _SO LONG AS_ it runs far enough out of the concrete
to prevent the plasma from touching concrete when (if) it forms.

However, there are other considerations which might cause a
decision to not place _THE_ ground rod in the bottom of the base
excavation.  There are also rational reasons to do so.  But if it
is done, it should probably NOT be a driven steel (copper plated,
galvanized, or not) rod.  And subgrade connections to it should
be exothermic weld bonds since inspection after installation will
be impossible.


Snip...

73, Eric  N7CL

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