Grant,
You're right to be skeptical of clamp-on readings that appear too good to be
true. When the reading seems unusually low, it's often because the clamp-on
induction coil is pulsing an AC current into a loop within an
earth-terminated loop. This results in a division of current that will show
a reading lower than the real earth resistance. When possible, try and map
out a diagram of the ground points before testing.
Another error is the assumption that a clamp-on earth tester can measure the
earth resistance of an unconnected ground rod driven into the earth. That
cannot yield a meaningful result despite what some YouTube videos show.
See p. 23 of 38:
http://www.aemc.com/techinfo/techworkbooks/Ground_Resistance_Testers/950-WKB
K-GROUND.pdf
It's also a good idea to have a calibration loop. My Fluke test loop
validates at 100, 50, 25, 1, and 0.1 ohms. This is especially helpful when
testing in a dirty environment or if the tester is accidentally dropped. I
use the test loop before every group of readings. Even small layers of
dirt in the clamp will affect readings.
Paul, W9AC
-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of Grant
Saviers
Sent: Wednesday, August 12, 2015 1:29 PM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] [Bulk] understanding ground resistance measurements
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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