Paul,
Thanks for the comments and for the link to the reference. This one and
the earlier referenced post re the Times Microwave tower grounding guide
have been very helpful.
As I consider "loops", it is clear that with my metal building,
"everything is a loop". Structural steel anchors ground to rebar, entry
panels ground to structural steel, base angles drill into the footings,
sheathing ties in, etc. and any attempt at a Ufer measurement is
measuring the many loops, a few milli-ohms. Now that I realize that any
paths in the building are a few mohms and probably unmeasurable
inductance, I will use the structure as my shack side lightning
protection ground reference. The coax entry is almost diagonally
opposite in the building to the service entrance, but now that doesn't
matter. If a strike elevates the building potential w.r.t. "remote
earth" everything will move together. I could add more ground rods to
the structure, but have faith in the Ufer properties of the perimeter
footing. (the interior slab is insulated).
The very low ohms pair of external shop ground rods were measured
individually, so must be tied together underground. When measured as a
pair at the exit from the shop I get 45 ohms. That makes sense, since
the inspector wanted them 5' apart and was ok with buried mechanical
clamps - yuk.
The meter came with a 5 ohm test loop, and I will make some more standards.
Based on the uniformity of the 40 to 50 ohm resistances of the 8' rods,
I will add four 20' ones 25' out from the new tower base with #2 in the
trench, Cadwelded. Hopefully, they hit the water table year round.
Thanks ,
Grant KZ1W
On 8/12/2015 11:02 AM, Paul Christensen wrote:
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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