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Author Topic: ground-rods in limestone country  (Read 1259 times)
KM3K
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Posts: 295




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« on: April 07, 2009, 07:03:50 PM »

Chances are very good that I'll hit limestone (I saw it when the basement was being dug-out) before I even get 4' of rod into the ground.
So, I envision lots of 4' rods; questions are:
1. What should be the spacing between those rods?
2. Presuming I'd connect them all to just one rod, what size should be the inter-connect wires?
BTW, I didn't have these concerns with my attic-antenna but then the attic-antenna was not effective in getting out.
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K9KJM
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« Reply #1 on: April 07, 2009, 09:30:18 PM »

General rule of thumb is to space ground rods about twice the distance apart as the depth in normal soil.

So if you only get them in 4 feet deep, Space them about 8 or so feet apart. (Thats what we have to do a lot of in this area, Bedrock is close!)

#6 bare copper is the minimum gauge to use for interconnecting them.

For some good info about lightning protection see:
http://members.cox.net/pc-usa/station/ground0.htm

For some tips on how to save money grounding pick up a May 2009 copy of Popular Communications magazine.

(All of this grounding with driven rods is good for your lightning protection system. Using balanced type antennas, It will do little to no good for your signal.  If you need a ground system for antennas, The "Radial" wires between your ground rods will help.)
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KB9CRY
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« Reply #2 on: April 08, 2009, 05:24:46 AM »

1. What should be the spacing between those rods?
2x their driven distance into the ground.

2. Presuming I'd connect them all to just one rod, what size should be the inter-connect wires?

#6 minimum
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N9DG
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« Reply #3 on: April 08, 2009, 10:33:35 AM »

You may also want to consider driving longer ground rods at an angle. That's what I have to do here because I hit quartzite bedrock from anywhere between 1.5 to 2'. For my antenna grounds what I try to do is drive them at an angle such that when lightning hits the antenna the current flows on the feedline travels down the feedline doesn't have to make a closed angle bend (a closed angle is an impedance bump) from the grounding conductor to the angle driven rods themselves. I have multiple ground rods per system like this. Then of course bond these antenna system ground rods to the electric service grounding system per NEC.

BTW this is what NEC 2008 says about ground rods:

"250.53(G) Rod and Pipe Electrodes. The electrode shall be installed such that at least 2.44 m (8 ft) of length is in contact with the soil. It shall be driven to a depth of not less than 2.44 m (8 ft) except that, where rock bottom is encountered, the electrode shall be driven at an oblique angle not to exceed 45 degrees from the vertical or, where rock bottom is encountered at an angle up to 45 degrees, the electrode shall be permitted to be buried in a trench that is at least 750 mm (30 in.) deep. The upper end of the electrode shall be flush with or below ground level unless the aboveground end and the grounding electrode conductor attachment are protected against physical damage as specified in 250.10."
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K0WA
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« Reply #4 on: April 20, 2009, 08:43:22 AM »

This is a very hard nut to crack if you have quite a bit of limestone.  In the Flint Hills of Kansas, we have large limestone deposits that they still mine.  At a commercial FM station on top of one of the limestone hills (Yes there hills in KS), we had to use a backhoe and dig down 10 to 15 feet to get the ground rods in.  In some cases, there are special systems that use Copper tubing about 3" in diameter that goes down about 5 feet and then turns in a "J."  Then there is salt (some sore of bromide which I cannot remember) poured into the tube and 5 gallons of water are poured in as well.  The salt leaches out in little holes in the bottom of the device and creates a salt ground system.  About twice a year, 5 gallons of water are poured into the pipe that is attached to the leg of the tower.  Also, many ground rods can be added...driven in at an angle if you need to...t get the desired grounding.  Before we did this, the lightening strikes were horrendous but after we installed the ground system and more rods it all stopped.

K0Wa
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KV7W
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« Reply #5 on: May 28, 2009, 09:40:48 PM »

GIS "chemical ground"
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KM3K
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Posts: 295




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« Reply #6 on: May 29, 2009, 01:25:52 PM »

GIS "chemical ground" Huh
Help; even after googling on 'GIS "chemical ground"', I still don't understand what it means.
73
Jerry, KM3K
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K9KJM
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« Reply #7 on: May 30, 2009, 12:32:28 AM »

There are a number of ways to "Enhance" grounding.  For commercial tower sites here we have been using Harger enhancement material:  

http://www.harger.com/products/grdcmp/grdeleac/uegem/ueem.cfm

Problem with that stuff is that it is very expensive! There are much lower cost alternatives, The bentonite clay, Some types of fly ash, Copper pipes drilled full of holes and filled with rock salt, etc etc.

Using a "UFer ground" is a good one too if you will be doing any concrete work.

A Google search for "enhanced grounds" should get you some ideas.
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KV7W
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Posts: 136




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« Reply #8 on: May 30, 2009, 08:35:59 AM »

"Help; even after googling on 'GIS "chemical ground"', I still don't understand what it means."

The whole idea of grounding is to give lightning a good path. When the soil isn't conductive, a ground rod in the soil is an open circuit. It doesn't matter how many ground rods are there if they're all open. The NEC figures ground rod spacing over average ground, (whatever that is).

If your soil is not very conductive, sometimes it's easier to change the conductivity of the soil. Minerals and water in the soil make a good ground. The very basic chemical ground is a copper pipe with leach holes, filled with salts, (to increase the conductivity of the surounding soil), and buried in a hole backfilled with bentonite, (to hold water).

Sometimes with really hard soils it's easier to put in a few chemical grounds than a whole bunch of rods. The basic chemical ground is very simple. When you start doing big dollar commercial work the engineering drives the price up, but the basic ground could be, and is, over engineered and installed by many hams.
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