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Re: [TowerTalk] [Bulk] understanding ground resistance measurements

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Subject: Re: [TowerTalk] [Bulk] understanding ground resistance measurements
From: Grant Saviers <grants2@pacbell.net>
Date: Wed, 12 Aug 2015 10:29:23 -0700
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
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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