Actually galvanized and copper plated ground rods should not be mixed in any ground system. Electrolysis will deplete the plating. All ground rods in any ground system are electrically connected to one another.
73 Gary k4FMX _____ From: [email protected] [mailto:[email protected]] On Behalf Of Glenn Little WB4UIV Sent: Tuesday, June 30, 2009 7:53 PM To: [email protected] Subject: Re: [Repeater-Builder] Re: DC Ground Lightning Protection / Concrete Electrode Another way to pass a ground cable through concrete is via a PVC pipe. The last thing that you want to do is run a ground cable that can carry lightning fault current through concrete without isolating the cable from the concrete. The fault current will rapidly heat the ground cable, causing it to expand, at the same time vaporizing the water trapped in the concrete. The result is a violent steam explosion. The results could be the total fragmentation of the concrete. MIL HDBK 419 is available for download. This is a military manual that addresses grounding. Another very respected guide is Motorola R-56. Galvanized ground rods should only be used at the guy anchor points. Copper plated ground rods should be used around the tower base and the building with all bonded together by exothermic welds 18 inches below grade. Ground rods are to be placed no closer than twice the length of the rod. Any closer and you are wasting your funds. YMMV. 73 Glenn WB4UIV At 10:07 AM 6/30/2009, you wrote: I'm going to disagree with the following posting: If the tower is bolted to galvanized pipe that is embedded in concrete of which a significant amount is in contact with soil, you have a "concrete-encased grounding electrode" which is hard to improve upon. It is not likely that a ground rod would be worthwhile, since damp concrete (concrete in intimate contact with soil at grade level) is a fairly good conductor, and such a footing or foundation has hundreds of times the surface area of a ground rod. I have read Ericsson specs for cellular tower installation in that disagrees with the previous statement. Standard concrete without conductive enhancing materials can crack, pop or crumble if subjected to a direct lightning strike if ground rods are not properly installed. The water contained within the concrete will vaporize instantly causing the concrete to fail. There are types of conductive concrete mixes or additives that can be used, but the most common practice is to use a ground rod from each leg with a copper wire bonded to each tower leg. Our mfg building at work is made from steel I-Beams into concrete. I have noticed each I-Beam has its own ground connection. The strap is bolted to the beam about 1" above the concrete, then disappears into the concrete, and suspect there is a ground rod going into the soil beneath the concrete piling, but thats just a theory, as I dident see it before the mud was poured. Ed N3SDO
