While it is correct to say that the concrete base of a tower does
contribute to the overall ground for the tower, it is important to
remember that the real purpose of the concrete base is to support
the tower! Anything that compromises the ability of the base to do
its primary function represents the potential for the entire tower
to fail.
The average lightning stroke has a peak current on the order of
40,000 amps. A direct hit on a tower that is only grounded through
the concrete base results all of the current flowing through the
base. Concrete, even after it has cured, contains some water. The
ligntning strike passing through the concrete can cause this water
to turn to steam. As the water expands it can cause cracks in the
concrete. Worst case the concrete will "explode" much like a tree
will during a direct hit. However, even if the concrete doesn't
explode, the damage done by the lightning strike could significantly
weaken the base and in turn its ability to support the tower.
For this reason separate ground rods (or other grounding techniques)
should be used to shunt the lightning energy around the concrete
base.
Mike Zeug K9EC
Michael.Zeug@ameritech.com
|