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## Re: [TowerTalk] Ground Rods

 To: , Re: [TowerTalk] Ground Rods "Tom Rauch" Tom Rauch Tue, 4 May 2004 14:23:55 -0400
 ```> Does it really matter to a multi-million volt lightning strike whether > the ground is 300 ohms or five? The difference would seem to be > inconsequential. If the current is 1mA, then the difference means nothing. If the current is 100 amperes, then the difference in voltage between your equiment and alternate paths like the power line system would be 100*300 = 30 kV and 100*5= 500volts I'll take the 500 volts any day, or better yet the couple volts my ground systems provide at that current. > This reminds me of the school of thought that says the connection to > your ground rods must be tight. As if lightning, having traveled > thousands of feet through the air, is going to be stopped by a gap of a > few thousands of an inch. What you will find is the lightning melts the wire at the gap or poor connection, and often blows it apart. A good example of how this works is a welding rod. The rod rapidly melts at the arc-point, and even penetrates deep into the target metal. At solid connections points, or if you stick the rod to the metal, heating is minor even when steady-state current is dozens of times higher than arc current. One of W4AN's yagis had a large hole through a heavy wall 3" boom from a lightning hit on an element. The hole was there because the element was insulated from the boom and there was a short arc path between the element and the boom. The #16 wires in my radial system take lightning hits all the time without damage. At the same time a large 7/8th inch coaxial cable had the shield totally blown away where it crossed a copper flashing for the radial buss. Had that been a solid bonded connection, there is little doubt the shield would be intact. You need a good ground and good connections. Make no mistake about it. It doesn't stop the hit, it stops the voltage rise and the power dissipated by reducing E*I . If E across a junction is zero, it won't heat. If the ground resistance is low, it stops much of the crap from getting into the equipment in the house from tower hits. You do, however, have to bond the power line ground directly to the station ground with a very low impedance connection to prevent high current in paths to and from the powerline from going through all the devices in the house. 73 Tom _______________________________________________ See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions and ask for Sherman, W2FLA. _______________________________________________ TowerTalk mailing list TowerTalk@contesting.com http://lists.contesting.com/mailman/listinfo/towertalk ```
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