[TowerTalk] Tower grounding

[Red]

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