[TowerTalk] lightning effects was: Ground Radials Insulated or Not

Jim Lux 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 gradient:
>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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