> I have just started a discussion here at work about this and we are miffed
> on how one would do this. Could you please elaborate this to me or if you
> would like to save time you can fax it to me at 972/404-3419 and on the
> cover sheet address it to me.
>
>
> Thanks!
Tim, (and others who indicated an interest):
I don't have the paper at home, but I am sure I have it at work. However, I
won't be back at work until this coming Wednesday (one of the benefits of
shift work!). In the meantime, the Polypaser Corp. book, The Grounds for
Lightning and EMP Protection, Second Edition, by Roger R. Block contains a
good explanation in Chapter Five, Ground Impedance, pages 27-28, Measuring
Your Ground System. I'll attempt to paraphase it here (apologies to Mr.
Block!):
_______ ______
| | | |
_________________| vs |_____________| am |______________________
| | | | | |
| |______| |_____| |
| ______ |
| | | |
|________________| vm |_________________ |
| |______| | |
| | |
| | |
| E1 | E2 | E3
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++G++
| | |
| electrode | |
under test
vs - ac voltage source
am - ammeter
vm - voltmeter
G - ground level
E - electrode or ground rod
The initial spacing between E1, E2, E3 for E1 being a single ground rod
would be about 100 feet. If E1 is a ground system, initial spacing may be
as much as 1000 feet. Actual distance is increased or decreased depending
on the size of the system and the results of moving E3.
E3 is moved discrete intervals along a line between E1, E2, and E3 and the
voltage measurement is recorded or plotted at each position. When no change
in voltage measurement for movement of E3 occurs (the "flat" part of the
curve if voltage is being plotted), this value is then converted using
tables or formula supplied with the measuring instrument to the
"resistance" of the ground rod or system.
This is what Eric Woods, K6GV, more concisely stated in his e-mail on the
subject:
>
>Measuring the effectiveness of a ground rod involves finding out what its
>connectivity to a place called "Remote Earth" is. You do not measure ground
>values at DC. Too many stray amps of DC floating around in the earth. You
>use a test set that sends out a know AC freq. at a known current to a remote
>ground probe. You then take another ground probe and profile the voltage in
>the ground at ten foot intervals (or more, depending) from the ground rod or
>field you are concerned about.
>
>When the current/voltage calc. levels off at a certain distance, you assume
>you have reached remote earth and use that value for your ground field
>contact resistance.
Roger Block goes on to make the comment that the low frequency ac source
used in most instruments (60-90HZ) does not adequately take into account
the impedance that may be presented to a lightning strike, but continues by
referencing an IEEE paper demonstrating that gound systems typically show a
lower dynamic impedance during actual lightning strikes than low frequency
or DC measurements might indicate, due to the ground saturation that
occurs during a strike causing localize arcing and causing momentary low
impedance paths between ground masses (a larger dissipation area is
momentarily presented to the strike energy).
We use a specialized instrument for these measurements at the Libby
Hydroelectric Project; when I go back on shift I will check it out and post
more information if there is interest.
The bottom line is, there is more to it than just sticking one probe of the
DVM in the dirt and one on the ground rod to find out if you have a "good"
ground.
BTW, I recommend the Polyphaser book as a good source for getting up to
speed on lightning protection systems. It is written in non-technical
English, draws together a lot of information that you could go blind trying
to extract from textbooks and IEEE papers, and to the best of my knowledge,
is based on sound scientific principles and research. Yes, they do sell
what might appear to be expensive lightning protection equipment and
components. However, I can do a complete, by-the-book installation using
their (or I.C.E.'s or whatever commercial supplier) components, covering
tower, radio equipment, telephones and consumer electronics, service
entrances, etc. for about the same as the deductable on one claim on my
homeowner's insurance, or a fraction of the value of my home, its contents,
and the lives of my family. I'll jump off my (well-grounded) soap box now!
Hope this helps,
Steve Z KJ7CH
Libby, MT USA
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