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

To: K4SAV <>
Subject: Re: [TowerTalk] ground rod depth problem - and understanding ground rods
From: Grant Saviers <>
Date: Wed, 14 Nov 2012 07:40:48 -0800
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In a recent new building I put a Ufer ground in the perimeter footing which yielded about 1000 sq ft of concrete in contact with ground. Since the slab is over a vapor barrier and insulation, there is no good electrical contact. My towers have Ufer's also, about 250 sq ft of concrete in soil contact. You can do the arithmetic about how many ground rods and buried connection wires that will take to equal the soil contact area. My to do project list has on it measurement of the conductivity among all three.

There has been thread discussion in the past about how concrete reacts to strike currents. My logic is that if enough rebar is well connected to the Ufer ground point then the energy is spread through a lot of volume, current density is less and the concrete is not damaged. A bad situation might be rebar spaced small distances so the current density has hot spots which can boil the water in the concrete with potentially damaging consequences. However, most any rebar design for structural purposes has plenty of steel tied together to distribute the current. A single ground rod in concrete in comparison might be a bad idea.

I think current UBC recommends or requires Ufers. My inspector had the electrician add two rods at the service entrance, but I think they are useless in comparison. I'll measure the conductivity between them and the Ufers.

One other data point is my new primary transformer by Puget Sound Energy has a Ufer ground point on the buried concrete vault and no ground rods. I'd guess there are about 8 sq ft of concrete in contact with the soil. I think you will see Ufers on most power line and cell tower bases.

On the web, one company in the midwest advertises drilled ground field systems, e.g. 30 to 60' depth as the solution to manage very frequent strikes on very tall BC towers. A pipe cased well is one alternative if you have one.

One other observation from the top of a 3500' ME mountain from a hike there. It is all bare granite at the top. Many wires were just strung on the surface to get the area increased. I didn't stick around to observe them in action.

Grant KZ1W

On 11/14/2012 6:53 AM, K4SAV wrote:
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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