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

To: "john@kk9a.com" <john@kk9a.com>, towertalk@contesting.com
Subject: Re: [TowerTalk] Mot R36 related to NUMBER of installed ground rods
From: Grant Saviers <grants2@pacbell.net>
Date: Fri, 3 Nov 2017 13:18:44 -0700
List-post: <mailto:towertalk@contesting.com>
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@kk9a.com wrote:
How is ground resistance measured?

John KK9A

To:     towertalk@contesting.com
Subject:        Re: [TowerTalk] Mot R36 related to NUMBER of installed ground 
rods
From:   jimlux <jimlux@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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