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Re: [TowerTalk] Ground Rods

Subject: Re: [TowerTalk] Ground Rods
From: David Robbins <>
Date: Wed, 01 Dec 2010 06:56:19 -0600 (CST)
List-post: <">>
normally  when measuring ground resistances a higher voltage source is used to 
overcome the drift problems that you are seeing.  There are also special 
methods used that require either a large remote ground or 3 rods spaced certain 
distances apart where a current is passed between the outer 2 and the voltage 
is measured between them to calculate the resistance and ground resistivity.  

where i work i have done full scale ground impulse testing for lightning 
protection of high voltage power lines, and i write software that uses those 
results and other related published works to predict ground performance during 
lightning strokes.  When struck by lightning the ground resistance decreases 
rapidly due to underground corona and streamers that effectively increase the 
diameter of the rods or radials.  as an example, a pair of 25.4mm by 4m rods in 
sand should measure around 75 ohms... when the tower is struck with about a 
10ka stroke about 7ka may make it to the tower ground which causes the ground 
to ionize and reduce the effective resistance to less than 30 ohms.

This is an older version of the software i work on, look for the slides talking 
about the Weck model for grounding:
There are many other references, try to google for "weck lightning ground".

Dec 1, 2010 01:20:29 AM, wrote:

A fun thing to do is measure the resistance between individual ground rods in a 
group, and between any rod and the tower, *before* connecting the ground rods 
together and/or to the tower base. In our case, 4 rods in clay soil, damp year 
around below a foot or two, and having only a good digital ohmeter (but no 
megohm meter), the indicated "readings" drifted continually from a minimum of 
like 1K ohms up to over 10K ohms, never did stabilize between any pair, must 
breakdown/change dramatically during a strike? Not exactly inspiring of 
confidence however...have heard mention of periodically salting the area around 
a tower to make the ground really "play."

Would sure be interesting to know how all the resistance/impedance is actually 
distributed (and changing) among the rods and tower base during a solid 
Don N7EF
----- Original Message ----- 
From: jimlux 
Yeah, and that's a problem, because the code requires 8 feet to be in
contact with the soil. [-SNIP-]

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