[TowerTalk] Lightning... and pointed rods!

[David Jordan]

For those of you who want to see how Uncle Sam performs lightning
protection and grounding you can download a copy of MIL-HDBK-419A
It was published in 1987 but as far as I can tell is very current.

For example...here is a piece regarding the concern of grounding with
minimal impendance:

1.5.1.1 Multipoint Ground System, The multipoint ground system requires
the existence of an equipotential
ground plane for the system. Such an equipotential plane exists in a
building with a metal floor or ceiling grid
electrically bonded together, or in a building with a concrete floor
with a ground grid embedded in it, connected
to the facility ground. Equipment cabinets are then connected to the
equipotential plane. Chassis are
connected to the equipment cabinets and all components, signal return
leads, etc., are connected to the chassis.
The equipotential plane is then terminated to the earth electrode
subsystem to insure personnel safety and a
low impedance path for lower frequency signals.
1-118
At higher frequencies, the large conducting surface, embedded in the
floor or the metallic raised floor under
the equipments to be grounded, presents a much lower characteristic
impedance than a signal wire, even if both
were improperly terminated. This is true because the characteristic
impedance (Z0 ) is a function of L/C. As
capacity to earth increases, Z0 decreases. Normally, the capacity of a
metallic sheet to earth is higher than
that of wire. If the size of the sheet is increased and allowed to
encompass more area, the capacitance
increases. Also, the unit length inductance decreases with width, which
further decreases Z0 . If the dimensions
of a metallic sheet increase extensively (as in the case of a conducting
subfloor), the characteristic impedance
approaches a very low value. In this case, even if improperly
terminated, the impedance would be quite low
throughout a large portion of the spectrum. This, in turn, would
establish an equipotential reference plane for
all equipments bonded to it. With this reference plane bonded to earth,
the following advantages are obtained:
a. Any "noisy" cable or conductor connected to the receptor through or
along such a ground plane will
have its fiels contained between the conductor and the ground plane. The
noise field can be "shorted out" by
filters and bond straps because the distance between these "transmission
line" conductors is very small.
Shorting out the noise field has the desirable effect of keeping noise
current from flowing over the receptor
case and along any antenna input cables.

By the way...H.G. Ufer's "Investigation and Testing of Footing-Type
Grounding Electrodes  for Electrical Installations," if referenced in
the document.


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