TTrs; JC asked me a question, see following my response. I can answer
regarding grounding for lightning protection, but will defer to others
who know NEC much better than I do.
JC Smith wrote:
Hi Red,
Just a quick question about those six ground rods. I'm told that the NEC
requires a minimum of two ground rods located within two feet of the tower.
This prevents having them spaced twice their depth. Are you talking about
branching out from the rods at the base of the tower or are you just
installing six rods equally spaced around and away from the tower? (Which
would mean no rods within 2' of the base.)
Thanks,
JC
Hi JC,
The six rods spaced away from the tower are to dissipate lightning.
That recommendation does not preclude other rods if NEC requires them.
However, two rods within 2' of the tower will not prevent a large
voltage rise on the tower in event of a lightning strike. We have to
provide that lightning protection in addition to AC power safety. It
becomes important at the tower because the tower and the antennas on it
are typically the tallest structures in the immediate vicinity and are
prime targets for lightning.
A volume of earth approximately equal to a cylinder with a hemispheric
bottom around a ground rod will, for want of a better word, saturate
with charge from a lightning strike and the voltage will rise higher
than what we want for protecting our electronic equipment. A charge
accumulates in the capacitance of that volume and then leaks off through
the conductivity. Earth has a dielectric constant greater than one; its
conductivity is much less than that of the metal conductors we are
accustomed to using.
Think of that volume of earth as a leaky capacitor.
The object of putting six rods equidistant around the tower is to
achieve maximum capacitance; six capacitors in parallel. The seventh
capacitor is the earth the tower base is in. Adding more rods inside
that combined volume adds little lightning protection. If more
protection is desired, it is necessary to add another ring of rods
outside that volume. When using 8' rods, place the next ring of rods
16' beyond the first ring. They may be spaced 16' apart in that ring,
which will accommodate 12 more rods, or fewer if one chooses to space
them more widely in that ring. A reason for not spacing them at greater
than 16' radius is to minimize the inductance of the radial wires
connecting to the ground rods. This spacing is a widely accepted
pattern to achieve minimum impedance, Z, from the tower to ground. That
impedance is composed of resistance, inductance, and capacitance and the
design must minimize the first two and maximize the third. An exact
pattern might vary depending upon the wire used and the characteristics
of the soil. The recommended pattern is generally good. Major
exceptions are encountered in dry sand and on very rocky soil.
There is an article in the latest QEX describing a method of measuring
the conductance and permitivity of earth at RF frequencies. They vary
with frequency. The article includes information about the physics of
conductivity in earth. The conductivity is primarily an electrolytic
conduction, not a metallic conduction. Dry or freeze the earth, and the
conductivity drops to nearly nothing. The permitivity, on the other
hand, is roughly the permitivity of sand, a principle mineral
constituent of earth. Of course, that varies with different kinds of
soil but it doesn't change much with drying or freezing.
I am not aware of a NEC requirement for dual ground rods at the tower.
I thought NEC was primarily concerned with AC power safety. I am aware
that NEC requires all ground systems to be tied together. I am also
aware that many communities and states require dual ground rods. My
understanding is that they are trying to assure that there is one safety
ground even if one fails, as ground rods or connections to them often do
because they are typically installed with clamped connections and are
never serviced or even inspected after that.
There are several contributors who know the NEC much better than I. I
post this to the list in hopes that one of them will provide additional
information.
73 de Red
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