I would like to add a few comments (and acorrection) to the
posting by Steve Z KJ7CH. Pretty good job going from memory!
Any responses should come directly to me as I'm not a subscriber
to this reflector, but it looks like I ought to be!
What Steve is describing is called the 3-stake ground resistance
measurement. There is also the 4-stake measurement, which is used
to ascertain the depth at which the first conductive soil layer
occurs and its value. This obviously is done prior to installing
the ground system!
Please refer to the drawing in the original posting. E1 is the
electrode under test and it should be the last (furthest out)
ground rod in the system, otherwise the sphere of influence and
presence of the ground system will give you erroneous readings
which will usually be on the low side. Hopefully you are not
dealing with buried utility lines in the vicinity, as they will
affect the veracity of your readings too. Without going into it,
the ground testers I'm familiar with have methodologies to
minimize the impact of this situation.
E2 and E3 are NOT part of the ground system under test. They are
the probes provided by the earth tester manufacturer. One
manufacturer is J W Biddle of Blue Bell, Pa and their product is
trademarked as the Megger. E3 is placed 100' past E1 and E2 is
placed about 62' past E1. Again, both of these probes (and their
connecting cable) should go out past the ground system so the
readings aren't compromised. Make a reading. Move E2 a few feet
either side of the initial point and make a reading. Go both a
few feet closer and further from E1. Make several readings. When
the readings start significantly departing (both higher [as E2
approaches E3] and lower [as E2 approaches E1]) from the initial
readings, you are finished. The readings (you can average them)
obtained prior to the marked increase/decrease is the value of
your ground system.
The Megger uses 100 Hz. As previously mentioned, there is NO
correlation between this reading and the impedance of the ground
system at the "important" lightning frequencies (10-100 kHz and
up to about 1 MHz; energy components exist into the GHz range,
although once past the 50 MHz point they are pretty low in
level--but the idea is to provide a low impedance path to ground
for ALL the energy). As a general rule, a ground system measuring
5-Ohms or less should do a good job if a low-inductance ground
was installed. As a real-life example (and without revealing the
customer), I did a site survey where the ground measured less
than 5-Ohms, but equipment damage was still occurring. The ground
grid was composed of #6AWG wire. In a nutshell, this wire was
just too inductive at lightning frequencies to be a good ground
despite the low measurement. If wire must be used, #00AWG
(commonly written as 2/0) is the minimum although you can "get
away" with #2AWG. However, 1.5"-wide copper strap of 26-ga
thickness (0.0159") has even less inductance than 2/0. In
addition, it has much more surface area.
If I can leave you with two buzzwords (concepts) that need to be
taken into account when dealing with grounding, they would be
"inductance" (impedance) and "surface area." The ground system
must have low inductance (impedance) to work properly and its
surface area must equal or (preferably) exceed the circumferences
of ALL the cables, waveguides, etc. going to it. It is not
uncommon (and I saw it at that aforementioned site) to see
several 1-7/8" Heliax(TM) cables going to a ground plate and
having a #6 (and if we're "lucky", a #2) wire going to the ground
system. Once again, the beauty of copper strap is (1) low
inductance and (2) surface area, as both sides count! So a
1.5"-wide strap has 3" of surface area.
The voltage drop formula to keep in mind is Vd=L (di/dt). For
those who have forgotten, di/dt is the change in current over the
corresponding change in time. Using average values, di would be
18,000 Amperes and dt would be 2 microseconds. A typical 150'
tower would have a L of 40 microHenries. Doing the math, there
would be 360 kV between the top of the tower and the bottom!
I hope I haven't made this more complex than necessary. In
actuality, describing the process takes longer than actually
doing it.
Of course, when all is said and done, take the most important
step of all and install a #1 Iron golf club at the top of your
tower. As Lee Trevino once said, "Not even God can hit a #1
Iron!" If your neighbors are unhappy with your tower(s), perhaps
you can point out that you are performing a public service since
the lightning strikes will be more likely to hit your tower than
their home, and they should be grateful rather than resentful of
their presence. :-)
Hope this helps!
Sincerely and 73,
Bob Wanderer AA0CY
Senior Applications Engineer
PolyPhaser Corporation
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