[TowerTalk] Ground Rods

[David Robbins]

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:
http://ewh.ieee.org/soc/pes/lpdl/archive/lpdw.pdf
There are many other references, try to google for "weck lightning ground".



Dec 1, 2010 01:20:29 AM, JoeCoolDXer at msn.com 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 strike...??
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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