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#21
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On Sun, 15 Aug 2004 02:26:15 +0000 (UTC), "Reg Edwards"
wrote: The only trouble with Richard (Clark) is his abuse of the English language. Glad to observe you don't challenge my technical language. Figured out mud yet? :-P 73's Richard Clark, KB7QHC |
#22
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Richard Clark wrote: On Sat, 14 Aug 2004 01:08:12 GMT, Dave Shrader SNIP h, but let's get extravagant and say 0.1%; then both strikes express all of 150 milli-watt-seconds of power. Richard, I'm not going to try to out calculate you. But, please tell the group what the junction temperature of any semiconductor device is at transient thermal failure at 0.1 and 1.0 useconds. Twenty years ago the USAF test data indicated that failure occurred at 0.5 microjoules!!! That's 300,000 times more sensitive than your numbers. Note, it is extremely difficult to really achieve a 0.001 ohm mechanical interconnect. Secondly, the major electronic failure mode is from the coupling of the magnetic field. A di/dt of 10^5/[1E-6] yields a value of 10^11 amperes/second. A one foot length of wire has a self inductance of approximately one nanohenry [1*10^[-9]] and the di/dt impact is ... 100 volts peak from one end of the wire to the next. Back in the olden days, twenty years ago, the action integral, that you calculated was sufficient to burn a 0.5 inch diameter hole right through titanium that was 0.1 inch thick. Note: the titanium alloy we used melts at slightly below 2000 degree F. The design issue is TRANSIENT THERMAL EFFECTS not average heating. At 1 microsecond the heat flow from the stressed area has not started. The USAF required adiabatic heating as the peak temperature for the starting condition for the transient thermal analysis. Restated, all the energy is converted to instantaneous heat and then the thermal stress analysis would be performed under that constraint. Conclusion, lightning is a highly stressful environment. |
#23
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On Sun, 15 Aug 2004 11:39:30 GMT, Dave Shrader
wrote: Richard, I'm not going to try to out calculate you. But, please tell the group what the junction temperature of any semiconductor device is at transient thermal failure at 0.1 and 1.0 useconds. That was covered in another posting. Twenty years ago the USAF test data indicated that failure occurred at 0.5 microjoules!!! That's 300,000 times more sensitive than your numbers. Current USAF lore holds, rightly, that it is the power supply that fulfills the failure, junction penetration energy is insufficient to accomplish this. Turn off the equipment, disconnect leads and NOTHING HAPPENS! Same pulse = no damage. Damage is not found in the miniscule power it is found in poor practices. Shielding is rather simple to accomplish, and taking care of front door and back door paths is not Rocket Surgery. It takes very little imagination to withstand these pulses; unfortunately it takes even less imagination to succumb. The stories of 1 idiot's plight make the news, not the 1 million survivors' success. Note, it is extremely difficult to really achieve a 0.001 ohm mechanical interconnect. For you perhaps, and certainly others who experience lightning's catastrophic failure, but 1mOhm is no big deal - measuring it is however. It is simple to enumerate poor examples, but that does not pass as careful accomplishment being impossible or unlikely. Secondly, the major electronic failure mode is from the coupling of the magnetic field. A di/dt of 10^5/[1E-6] yields a value of 10^11 amperes/second. A one foot length of wire has a self inductance of approximately one nanohenry [1*10^[-9]] and the di/dt impact is ... 100 volts peak from one end of the wire to the next. You are pencil whipping yourself, Dave. You are off by at least 1 order of magnitude in inductance. As current USAF lore to this matter reveals (and it is nigh on to 25 years stale), huge pulse coupled currents at low frequencies do a ****-poor job plain and simple. Saying coupling and making it happen are two different things (per current USAF lore) and trying to fit 1MHz into a 1 foot wire is loath to considerably less potential. This is called overplaying your hand, a direct strike is enough to argue. Back in the olden days, twenty years ago, the action integral, that you calculated was sufficient to burn a 0.5 inch diameter hole right through titanium that was 0.1 inch thick. Note: the titanium alloy we used melts at slightly below 2000 degree F. I've already commented to this specifically. The design issue is TRANSIENT THERMAL EFFECTS not average heating. At 1 microsecond the heat flow from the stressed area has not started. I've already commented to this specifically. The USAF required adiabatic heating as the peak temperature for the starting condition for the transient thermal analysis. Restated, all the energy is converted to instantaneous heat and then the thermal stress analysis would be performed under that constraint. Conclusion, lightning is a highly stressful environment. Stress also encompasses primitive fear and probably far more so than the actuality of failure. This is called the probability of big numbers. In a nation of 280 Million, any individual's once in a million experience like lightning striking them or near them happens 280 times a year (5 times a week or more) - it misses the other 279,999,720 times tho'. Thus such hair raising stories inordinately color the topic and are suitable for selling insurance and amulets. Hence the advertising and $$$$$$ suggested retail cost for "peace of mind." At that price, if only 0.01% of those 279,999,720 pushed their credit cards across the counter, that makes for a very profitable living second only to ENRON. Moral: "Don't use insulated towers." Risk: "How well can you reduce R?" 73's Richard Clark, KB7QHC |
#24
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All about grounding and lightning protection at PolyPhaser -- URL:
http://www.polyphaser.com/ppc_pen_home.asp -- One Watt To steal ideas from one person is plagiarism; to steal from many is research. -- Comedian Steven Wright "Private" wrote in message om... Hello, I am looking for some advice on if the ground system featured below is sufficent or should be upgraded. It consists of: - 3 ground rods 10' each around the tower (bonded together) - 2 ground plates (one outside, one in the shack, also bonded together) - lightning arrestors and/or feedthrough adapters - tower to mast ground - interior coax switch (not shown) I provided some pictures below: http://www.telusplanet.net/~homac/exteriorground01.JPG http://www.telusplanet.net/~homac/exteriorground02.JPG http://www.telusplanet.net/~homac/exteriorground02a.JPG http://www.telusplanet.net/~homac/exteriorground03.JPG http://www.telusplanet.net/~homac/exteriorground04.JPG http://www.telusplanet.net/~homac/interiorground01.JPG http://www.telusplanet.net/~homac/interiorground02.JPG I am looking for constructive feedback..... Thank-you.... Lloyd |
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