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

Grant Saviers grants2 at pacbell.net
Wed Aug 12 22:52:07 EDT 2015


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 at contesting.com] On Behalf Of Grant
> Saviers
> Sent: Wednesday, August 12, 2015 1:29 PM
> To:towertalk at 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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