When currents are discharged into the earth (by lightning or power line faults), there is a phenomenon known as "Ground Potential Rise". That is, as currents flow omni-directionally (earth is a giant resister, so current flows in many directions like a huge parallel circuit), the point nearest the discharge rises to a potential above "ground", with levels decaying exponentially outward.
Therefore, if all devices that require protection are bonded with low impedance cables (200mcm copper), the entire bonded community will at least remain at the same relative potential, even if elevated above ground. Many people use #6 solid copper for bonding, but that is only good for under 50 feet, then you should use serious cable like 200mcm. This is a tie-in to discussions concerning lightning strikes. Take for example a television station in Sarasota, FL that kept losing gear in lightning storms. The property was littered with parabolic satellite discs (6 to 10 footers), antennas, metal structures, etc. in a busy lightning season they replaced a lot of gear. Of course, anytime something tied to the phone lines got fried, they blamed us, so we went out and performed a ground survey (measured the value of ground rods or connections at every object). The results were that the telephone terminal was connected to the well casing, with a 2 Ohm ground measurement (fall of potential method). All other grounds were over 25 Ohms....several over 100 Ohms. We had them tie all objects, ground rods, etc together with 200mcm copper and they never again suffered equipment losses. Over many years, I could repeat this scenario in damage investigations......too many independent (unmeasured) grounds at a site. Just because you drive a ground rod does not mean you have a "good" ground (good meaning 25 Ohms or less, some sites require 5 Ohms or less). In my Florida days, it was pretty comment to see 100+ Ohm measurements on a single 8 foot ground rod. There are formulas for determining soil resistivity and the number/depth of rods to equal the required measure......I have those in a course book from a grounding class I took in the 1970's......it's about 5 inches thick! As someone else mentioned earlier, the "Cone of Protection" method is well documented in a book entitled "Telecommunication Electrical Protection" by AT&T Press (1985). It has a blue cover so was called "the blue book" in industry circles. It also covers "tent" and other shapes for overhead protection schemes. It has much good info in protecting cables entering power substations and generating plants as well. Good read. Sorry for the long post.....I'm a newbie so this may be info already covered. I'm on LinkedIn, give me a ping. Daniel B. Burch (TesCom Corp) Dallas, TX Sent from my iPad On Jun 25, 2014, at 10:55 PM, Alexander Pummer <[email protected]> wrote: Charley Wenzel made it very safe: here is http://www.techlib.com/electronics/lightning.html, I used cross-correlation to identify the "right electrical noise" Alex On 6/25/2014 8:22 PM, Brian Lloyd wrote: On Wed, Jun 25, 2014 at 4:42 PM, paul swed <[email protected]> wrote: I am currently using a 12AX7 as a ELF preamp and have for years. A note in the coldest part of winter preheat the tube low filament voltage. They tend to fracture. It sits 200 ft from the house as far away in the woods as possible. That said and back to the thread. At these frequencies tubes do work. The 12AX7 can be found on vlf.it and numbers of tubes will work. They run 12 V on the plate. They also stand up to nearby lightning very well. So Diddier now you have no excuse. I can't wait to implement your design on one of my stm boards. Not sure how to get this back on time-nuts topics Regards Try a 6DJ8 instead of the 12AX7. It has a higher GM and a LOT more bandwidth. What kind of risetime are we talking about for a lightning strike? And why not a loop antenna? That should provide plenty of signal but not destructive voltages. I know you are talking about measuring lightning strikes but if you get the impedance high enough, you can actually measure the earth's electric field. (It is about 200V/m if I recall properly.) Interestingly it is affected by the solar flux and solar wind. _______________________________________________ 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. _______________________________________________ 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.
