[time-nuts] Loran, GPS, Lightning, Timing
daniel.burch at ieee.org
Fri Jun 27 13:47:39 EDT 2014
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
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
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)
Sent from my iPad
On Jun 25, 2014, at 10:55 PM, Alexander Pummer <alexpcs at ieee.org> wrote:
Charley Wenzel made it very safe: here is
I used cross-correlation to identify the "right electrical noise"
On 6/25/2014 8:22 PM, Brian Lloyd wrote:
On Wed, Jun 25, 2014 at 4:42 PM, paul swed <paulswedb at gmail.com> 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
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.
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