[TowerTalk] [Bulk] understanding ground resistance measurements

Grant Saviers grants2 at pacbell.net
Wed Aug 12 13:29:23 EDT 2015


I meant to Save not Send this post, but am seeking advice about proper 
use and pitfalls of these types of measurements, as well as any comments 
about what they mean.

Thanks,

Grant

On 8/12/2015 10:18 AM, 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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