[TowerTalk] Ground resistance measurements

[Dick Green]

Coincidentally with all this talk about lightning protection and ground
systems, I recently bought a used ground resistance tester. It's a
professional model, AEMC #136.100. It appears to be in good condition,
complete with documentation. It was such a deal...

The manual does a good job explaining how to measure the ground resistance
for a single ground rod using the Fall of Potential and 62% methods.
However, these tests seem to be designed to measure the resistance to ground
for a single isolated ground rod. You have to disconnect the ground wire
from the rod, and pound in two test electrodes to do the measurements. The
Polyphaser book shows a similar test setup.

First, the manual doesn't say how long the test electrodes should be (i.e.,
how far into the ground they should be pounded.) Does anyone know? Can I use
a couple of those short TV-type rods from Rad Shack for the electrodes, or
do I need to use some 8' rods? Second, since all my ground wires are
Cadwelded to the existing rods, I can't disconnect them. There's one rod
attached to the tower where I could disconnect the lead at the tower base to
isolate the rod, but that would be rather a chore.

I'm wondering if any meaningful measurements can be obtained by taking
readings on a ground rod that is connected to the rest of my grounding
system. Anybody know? I'm guessing that the readings would be lower than for
a single rod because the rods and radials would provide additional paths
from the test electrodes to the rod under measurement. True? Would that
number still be meaningful?

I have some spare ground rods. I suppose I could just pound one in and take
some readings on it, but I'd have to try it at the various grounding
locations on my property. That means pulling the darned thing back out or
wasting a bunch of ground rods. Any ideas?

73, Dick, WC1M



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