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## [TowerTalk] Disconnecting cables and lightning (questions --

 To: [TowerTalk] Disconnecting cables and lightning (questions -- w8ji.tom@MCIONE.com (Tom Rauch) Fri, 05 Jun 1998 09:43:31 +0000
 ```To: > Date: Thu, 04 Jun 1998 15:51:17 -0400 > From: Bill Coleman AA4LR Hi Bill, > Well, I'm not a lightning expert, There might very well be no such expert, because lightning is so esoteric. Roy Lewallen would call this a "high-Q" thread, because it could ring on and on. Wes Stewert calls it a tar-baby. I think this is a high-Q tar baby. ;-) There are some things that are not debatable, however, because they can be confirmed by repeatable experiment. > A grounded conductor aloft assumes the same potential as the ground it is > connected to. It therefore becomes difficult for it to build up a charge > and make it a potential target. Actually not according to physics. Charges DO move up in the conductor. The problem is we can't really measure those charges by normal means we are accustomed to in conventional circuits, and they move up in the conductor even if the conductor is an insulator!!! Earlier I gave the example of a charged paper cone. That's a common physics experiment. That charges all collect on the outside of the cone. If you use a string to turn the paper cone inside out, the charges all move to the outside again even though the cone is an insulator. > An ungrounded conductor aloft assumes whatever potential it wants. If it > is bombarded with rain, it picks up the charge of the rain. Wind friction > can also generate a sizable static charge. It may pick up enough of a > charge to become a target. Here's how I view the problem, I hope it makes sense. Imagine the great potential difference between the earth and cloud required to start an arc. It certainly is much more potential than a few dry cells, hi hi. Now how on earth (pun intended) can that conductor "collect" enough charge to arc over to a cloud thousands of feet away without arcing over to earth LONG before that point? The charges physicists speak of "collecting" do so because of the force on charges due to the electric field, not because of something like water into a bucket. We can't "pour" those charges anywhere because more charges will quickly fill the void EVEN when we talk about charge in an insulator! > Of course, lightning is dumb, and it doesn't understand theory - it > strikes where and when it wants.... Things can sometimes be too complex for a simple "one-fits-all" explanation. You can bet there are a lot of things going on that control getting a hit or not, but changing the potential of a conductor tower with reference to earth would make no difference at all. Draw it on paper. Lets say the cloud is charged to +5000 volts with reference to earth. A conductor connected to earth would have a potential difference of 5000 volts. If we let that tower float to 2500 volts, the potential difference to the cloud and to earth would EACH be 2500 volts, for a total path potential difference of 5000 volts. The only way to lessen the charge is to transport charges BACK to the cloud that were given up, and that requires some direct path for ions or charges. 1.) I don't disagee that adding a static brush MIGHT reduce the potential for a hit, because the corona from the brush would make the object appear fatter and blunter. But the very same thing could be accomplished by making the object fatter and blunter WITHOUT the noise generated by corona. 2.) I don't disagree a wider copper flashing has less inductance than a narrower "something else", but I disagree with the contention that the flat shape is always better because it is flat. A one inch wide flat conductor has about the same inductance as one inch diameter conductor of the same length. I don't rush out to buy copper flashing UNLESS I need a whole lot of area. Three inch wide copper flashing is better than a 3/4 inch copper pipe, but I simply elected NOT to mess with three inch wide copper flashing running all around my house because I can disconnect the antennas when storms approach. I am curious about determining some "rule of thumb" for the performance of various lengths and materials for broadband grounds. This morning there was a loud boom. Marsha was standing in the carport watching the storm, and tells me lightning hit directly on my 200 foot tower. This PC was also on and running. The only damage was to a digital wall clock sitting on the refrigerator and plugged into a separate outlet. SK clock. Does that prove anything? Not a thing except nothing is greatly wrong with my system. One or two data points mean nothing in a random event. Certainly everything survived without expensive gizmo's with what I consider proper grounding, but who knows about the next time. With another 200-300 foot tower going up on the highest point for several miles, I'll probably enjoy that next time soon enough. 73, Tom W8JI w8ji.tom@MCIONE.com -- FAQ on WWW: http://www.contesting.com/towertalkfaq.html Submissions: towertalk@contesting.com Administrative requests: towertalk-REQUEST@contesting.com Problems: owner-towertalk@contesting.com Search: http://www.contesting.com/km9p/search.htm ```
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