On Thu, Nov 27, 2014 at 9:12 PM, Bill Hawkins <[email protected]> wrote > > > So I wonder about this concept of a lightning "arrestor". The report > referenced by Arthur Dent is quite complete. It also says, at the bottom > of page two, "It is impossible to prevent damage from a direct lightning > strike ..." Why, then, do people sell lightning arrestors when they > wouldn't dream of selling hurricane arrestors?
Because the lightening arresters actually work. That is why they are sold and wide used. They do not and can not "stop" lightening. What they do is act as a switch. They sense the voltage on a conductor. Normally the conductor is allowed to have a signal voltage on it but if the voltage relative to ground goes high the arrester closes a switch and connects the conductor to ground. They don't stop lightening, they simply close to provide a path to ground. The tree could have been protected too. Let's say we could measure the voltage at the top of the tree and if it ever got high we'd connect the top of the tree to ground via a study copy cable. Then the current would have avoided the tree trunk and flowed harmlessly to ground. What is amazing is that we can build a simple switch that can work so fast. Many of them work by using some kind of gas that is non-conductive until it becomes ionized then the ionized gas connects the conductor to ground. Also the arrester never sees the full current of the strike. In a coaxial antenna cable MOST of the current is on the shield and this is shunted to ground directly and never goes into the arrester. And BTW we do build and use "hurricane arrestors". They are called storm shelters. A large concrete structure works pretty much like a lightening arrester, it deflects the effects of the storm from some small protected area. We should not argue that lightening protection is impossible because we have many thousand of examples of them working. Yes normal variations and statistics will eventually take out a system. But will it happen in your lifetime? OK one more analogy. Earthquakes. Why bother with robust building codes when we know that a big enough quake will destroy any building? It's because the "big one" is unlikely to occur during the building's lifetime but we know 100% that many smaller ones will occur. So we protect for the normal case These things obviously DO WORK. There are cell towers all over the Orlando Florida and they continue to operate. With a cell tower the FIRST line of defense is structural grounding. The steel structure is bonded to a copper grounding system. They get this down to about 6 ohms tower to ground and most of the current follows that path. The second line is surge protectors on ALL wires that come into the building. Again you do NOT "stop" lightening with an arrester, you use them to provide a easy path to ground.. -- Chris Albertson Redondo Beach, California _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
