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Re: [TowerTalk] ground rod depth problem - and understanding ground rods

Subject: Re: [TowerTalk] ground rod depth problem - and understanding ground rods
From: K4SAV <>
Date: Wed, 14 Nov 2012 08:53:07 -0600
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My understanding of ground rod performance characteristics during a strike leaves a lot to be desired, and I can't find any information to answer those questions either. We have rules that specify distance between rods because of' ground saturation and the need to spread the charge over a larger area. I don't understand exactly what happens with the underground plasma that takes place around a rod during a strike, and what that does to the ground rod impedance, and how that affects ground saturation. I would guess that the impedance of that ground rod during a strike is a huge non-linear function, not even close to what you might measure with any instruments under normal conditions. Besides, if I had that information I could do an accurate model of a ground system instead of having to ballpark and conservatively estimate everything.

Then if you encase the ground rod in concrete, how does that effect the underground plasma and the rod impedance during a strike. Also what happens to the concrete. I would guess that it might explode if there were insufficient ground rods in the system. I wonder how many would be sufficient. If the impedance of the ground rod is much lower when encased in concrete, why don't the commercial cell tower companies use concrete around the rods? I wonder if they have tried it. Would concrete be better than packing the hole with bentonite? I know there is some information on Ufer grounds but those are just guidelines and really don't answer the details of how things work.

Lots of questions and nowhere to go for answers.

Jerry, K4SAV

On 11/14/2012 7:00 AM, Jim Lux wrote:
volume isn't the important metric.. surface area is.. a bar 20 feet long and 1x1 foot cross section is 82 square feet in cross section.

I guess, though, the top of the footing isn't usually buried, so probably 60 or so square feet..

Concrete is almost always higher conductivity than the soil surrounding it (unless you're using some exotic low conductivity concrete) because it's hygroscopic.

So instead of a contact area between conductor (rod) and soil (probably not even a square foot), you have a fairly good contact that can't be disturbed between wire and concrete, and then a very large contact area between soil and concrete, along with the "current spreading" from the concrete, so the current density at the concrete/soil interface is low.

In fact, for RF and transients, the *capacitive* coupling from the concrete to the soil is pretty good.


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