[TowerTalk] understanding ground resistance measurements

[Roger D Johnson]

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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