Ground Rod Length

[K6AER]

Well no body quite hit the nail on the head with this one.

Ground rod have what is known as a sphere of influence. The sphere of influence is twice the depth of the ground rod time the depth of the ground rod as a radius.

A 8 foot ground rod sphere of influence would be 16 feet deep by 16 foot in diameter. A 10 foot rod would have almost twice the sphere of influence.

Now for some rules of thumb;

•   Jet the ground rod in, burry the ground rod in, screw the ground rod in it doesn’t matter. The earth will settle around the ground rod to 95% compactness within a week.
•   The deeper the ground rod the better.
•   One deep ground is much better than a dozen shallow grounds.
•   Ground rod diameter doesn’t matter except as to the length of time it takes to corrode the ground rod.
•   If your copper wire is buried to the ground rod and the ground rod is buried better to cad weld to the rod.
•   All grounds should be bonded with a number 6 solid copper wire as per NEC code. This includes the station and the AC Panel.
•   A big Hilti hammer drill is the easiest way to plant a ground rod.
•   Cover a ground rod sticking out of the ground with white PVC. They hurt like hell when you rack your ankle on one.

[AA4HA]

Since soils are not consistent in conductivity or moisture content along their entire depths it is possible that a ground rod that is penetrating a layer of poorly conductive soil (dry clays, sand, etc...) can behave like an inductive "choke", not permitting the distant end of the ground rod to be an effective part of the ground system. Multiple, shorter rods, spaced apart and driven below the moisture level of seasonal variation is a much better solution.

To quote: (Polyphaser Lightning Protection & Grounding Solutions for Communications Sites, Page 29)

"Theoretically, one ground rod with a 1” diameter driven in homogeneous 1,000-ohm per meter (ohm/meter) soil for one meter would yield 765 ohms. Driving it two meters into the soil would give 437 ohms. Going to three meters, however, does not give as great a change (309 ohms). One would get faster ohmic reduction and easier installation by using three rods, each one meter long, giving 230 ohms compared to that of one rod three meters long. This assumes they are spaced “greater than the sum of their lengths apart”. If the bare interconnecting wire is also buried below the surface, then the ground system
may be less than 200 ohms. (Having one deep ground rod, 40 feet or more, even if it reaches the water table, will not act as a good dynamic ground because the top 5 to 10 feet will conduct most of the early current rise and could become saturated. Eddy currents will form in this top layer and cause the rod’s inductance to impede the flow of current to any further depth.)

GROUND SATURATION
The statement that rods should have a separation,“greater than the sum of their lengths apart,” originates from theory, and the fact almost all ground rods will saturate the soil to which they connect. A ground rod connects to localized, irregularly sized, three-dimensional electrical clumps. Depending on the soil make-up (layering, etc.), the volume of earth a ground rod can dump charge into can be generalized as the radius of a circle equal to the length of the rod at the circle’s center. This is known as the sphere of influence of the rod. The sum of the driven depths of two rods should be, theoretically, the closest that ground rods can be placed. Anything closer will cause the soil (clumps) connected in common to saturate even faster."

I ran into this problem at a gravel mining operation that was having a large number of variable speed drives failing. The ground system at the motor house consisted of several, 10' galvanized steel rods, one at each corner of the steel skid (the size of a truck trailer, resting on the gravel). Even with this set-up the ground resistance was so high as to allow transients from lightning strikes to follow the conveyor belts back into the building, blowing out multiple $10,000 VFD's. Trenching in buried ground radials about 3' deep and 20-25' long from each ground rod point reduced the electrical potential below the point where damage was happening to the equipment. (by the time the problem was finally fixed they had spent almost $80,000 in out-of-warranty repairs and around 2 days of downtime on a commercial quarrying/mining operation).

[W6RMK]

Well no body quite hit the nail on the head with this one.

Ground rod have what is known as a sphere of influence. The sphere of influence is twice the depth of the ground rod time the depth of the ground rod as a radius.

Except it's not a sphere. Think more a half ellipse.
The practical implication is that spacing ground rods closer than twice their length (e.g. closer than 16 ft for 8 ft buried length) doesn't have as low resistance as farther apart.

A 8 foot ground rod sphere of influence would be 16 feet deep by 16 foot in diameter. A 10 foot rod would have almost twice the sphere of influence.

Not quite true, and the "volume" of the sphere isn't particularly important.  In your example, the 10 foot rod would have 75% lower resistance than the 8 ft rod. (approximately, and only applies for SMALL differences in length.)

Now for some rules of thumb;

•   Jet the ground rod in, burry the ground rod in, screw the ground rod in it doesn’t matter. The earth will settle around the ground rod to 95% compactness within a week.
•   The deeper the ground rod the better.
•   One deep ground is much better than a dozen shallow grounds.

Just not true.  Unless you're talking 10 ft and 1 ft or something extreme (or if the dozen rods are all driven in a small area).  The same reason that driving two rods too close together isn't a whole lot better than a single rod also applies to doubling the length of a single rod. That is, the top half of the rod is (if you think about it) one half rod length away from the bottom half. 
A 10 ft rod is sort of like driving 2 5 ft rods 5 ft apart.

•   Ground rod diameter doesn’t matter except as to the length of time it takes to corrode the ground rod.
•   If your copper wire is buried to the ground rod and the ground rod is buried better to cad weld to the rod.
•   All grounds should be bonded with a number 6 solid copper wire as per NEC code. This includes the station and the AC Panel.
•   A big Hilti hammer drill is the easiest way to plant a ground rod.
•   Cover a ground rod sticking out of the ground with white PVC. They hurt like hell when you rack your ankle on one.

I'll agree with all those..