[TowerTalk] Mot R36 related to NUMBER of installed ground rods

[Grant Saviers]

A "ground resistance meter" (of course ;) ).  As for most instruments 
the US brand electricians meters are pricey, think $1k and up.  I found 
a clamp on style on ebay.  Clamp ons are super easy to use.

https://www.ebay.com/itm/UNI-T-UT273-Auto-Range-0-01-1000-Earth-Ground-Resistance-Clamp-Meter-2-2-LCD/262194748793?hash=item3d0c065d79:g:bq0AAOSwo3pWcVoC

Although I bought another brand at a lower price, and it works fine 
(looks to be the same instrument).  The UNI-T instrument line has good 
stuff at low prices.

The concept for measuring is a bit confusing at first.  An inductively 
coupled AC signal is used, something around 300Hz which is not a 
multiple of 50 or 60, and a tuned bridge to resolve the resistance.  
Resistance is measured from a reference ground to the rod just 
installed.  So two rods are installed to measure the series resistance R 
between them, and a starting assumption is each is R/2 to ground.    
Resistance to the "perfect" ground can't be measured, unless one is 
handy.  But as rods are added or some really good ground reference is 
available the resistance approaches the perfect reference.  Or if a 
mains bonding spot is a low resistance ground, then it can be the 
reference for the first rod.  In my case I could use a Ufer 250' long 
building footing as a reference and got to about 7 ohms with 9  8' x 
5/8" rods and the buried radials all Cadweld connected.  I declared 
victory since the QTH is in the lowest strike incidence in the USA.  
Motorola and others target 5 ohms and I suspect large towers even go for 
lower.

In designing the ground system IMO it is better to bring each radial 
string back to the central bonding point so each radial can be 
measured.   There was a 1.5:1 variation in  rod resistance, depending on 
the micro climate soil conditions.  I also found #2 buried wire for 
radials at 35' long was about equal to one 8' rod so am now convinced 
besides code it is a good idea.  I found the measurements were 
repeatable but varied with the season.

Grant KZ1W

On 11/3/2017 7:15 AM, john at kk9a.com wrote:
> How is ground resistance measured?
>
> John KK9A
>
> To:	towertalk at contesting.com
> Subject:	Re: [TowerTalk] Mot R36 related to NUMBER of installed ground rods
> From:	jimlux <jimlux at earthlink.net>
> Date:	Fri, 3 Nov 2017 05:52:59 -0700
>
>
>
> "The contractor shall install sufficient ground rods in accordance with
> document XYZ that the measured ground resistance is less than 5 ohms."
>
> At the end of the job, the customer goes out, measures the resistance,
> finds it's 4.2 ohms and says "you get paid"
>
>
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