[TowerTalk] lightning effects was: Ground Radials Insulated or Not
jimlux at earthlink.net
Tue Dec 7 16:56:06 EST 2004
At 03:14 PM 12/7/2004 -0500, Gary Schafer wrote:
>This thing is getting long and drifting off the subject of radials.
>Just a few notes on the other stuff:
>When I say don't pay too much attention to the NEC codes I am talking
>about what is and is not effective for the subject at hand.
>When the ground tries to absorb a large amount of energy via a lightning
>rod there are eddy currents that are set up in the ground. That increases
>the inductance and the impedance.
Inductance and impedance stay the same. I'll buy induced currents
(technically not, in this case, eddy currents, but still the same
mechanism... current induced by changing magnetic fields). However, just
like any other circuit, the medium doesn't change, it's electromagnetic
properties don't change, etc.
>When in the ground the energy wave travels much slower than it does in air.
Sure.. If the soil has an epsilon of 13 (much beloved by modelers as
"average soil") then the propagation speed is about 28% of that in free
space (why there's no real thing like a "tuned buried radial".. you
probably don't know the ground properties well enough to tune it, and it's
so lossy that something like "Q" doesn't really apply).
>Current carrying capacity of rods:
>The current carrying capacity of ground rods for a safety ground is
>irrelevant when discussing lightning.
Not totally irrelevant, especially since in many, many installations (and
certainly ones that meet code) the ground is the same. Sure, it may have
been installed to carry lightning, but the code requires it be
interconnected with the "greenwire" ground.
>Quality of lightning ground:
>A lightning ground that is "good enough to keep a building from burning
>down" is not what we are talking about here.
>A good lightning ground is one in which equipment is not damaged during a
>strike. That takes a little more than a "minimum requirement ground".
No ground you can make will save any equipment from a direct lightning
strike. The best lightning ground in the world, with a resistance of 0.1
ohm will still rise to thousands of volts, and there's not a heck of a lot
of equipment that will take that kind of overvoltage. What will save your
equipment is good system design which limits the maximum differential
voltage that your equipment will see. The IEEE Emerald book is full of
stuff on grounding sensitive electronic equipment.
>Voltage differences do occur at different points on the ground due to the
>resistance of the soil. They also occur because of time delays in the
>travel of the energy being dissipated in the earth.
This is true of any conductor carrying any non-DC current (or for that
matter, radiated electromagnetic energy). I think that if you do the
analysis (I haven't done it, so I'm speaking off the cuff here. It might
be in the IEEE grounding spec.) you'll find that surface voltage
differences across soil will be mostly due to resistive effects, not
transient propagation. In a small diameter conductor (i.e. a tower, a
lightning grounding conductor, etc.) the inductance will have a significant
effect, but in a bulk disspative medium (soil), the inductive effect will
be pretty small. In fact, this effect is used for geophysical prospecting:
called electromagnetic induction surveys, I think, that looks for an
induced voltage that is out of phase with the voltage resulting purely from
the resistive component (they use quadrature detection, etc.). I don't
recall the relative magnitude of the resistive and inductive component, but
I think the inductive component is something like 1000 times smaller.
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