Resistance measurements of the ground is fine for powerline frequencies but not
very good
for lightning. You need to measure the impedance of the ground system at
frequencies
up into the megahertz range. The inductance of long leads makes many systems
less than
optimum for lightning.
73, Roger
On 8/12/2015 1:18 PM, Grant Saviers wrote:
I'm using a clamp on ground resistance tester Duoyi DY1000A to measure some
new ground rods and Ufer grounds.
New ground rods driven into bone dry soil 8' x 5/8 rods, #6 to tower base
south 85 ohms
north 0.05 ohms since connection is a loop and rod end is buried
west 88 ohms
2x#6 Cu in 45' trench 8" deep 65 ohms (some watered soil)
Interpretation: as I understand how the instrument works, if I don't have a
very low resistance ground in the network, I am measuring the sum of the
resistances, or nearly so, of an isolated branch. So, it seems likely that
the actual ground rods are around 40 to 50 ohms each.
Old tower ufer measurements
east tower (18 yards concrete 6 x 10' x 8') 1.5 ohms, known wet spot,
water table was about 5' down in fine clay this time of year, partially
touching an abandoned septic tank
west tower (5x5x8) 16 ohms, didn't hit the water table with this foundation
Interpretation: The difference is a bit surprising. For both towers, the
reference ground resistance is only coax braids and AC earth return paths to
the shop
Ufer rebar out of foundation near mains entrance (250' perimeter x 12" wide
down 16") 0.041ohms
three #4 connections from Ufer rebar to power panels 0.034, 0.009, 0.009
ohms
two exterior 8' ground rods to power panel 0.023, 0.024ohms
Intepretation: these all seem way too low, particularly the two exterior
ground rod measurements.
Ufer rebar out of foundation at radios
measures >1000 ohms, instrument overload. Not good as equipment is bonded
to the #4 wire so there is a return path through the AC earth wire
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