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

Subject: Re: [TowerTalk] lightning effects was: Ground Radials Insulated or Not
From: Jim Lux <>
Date: Tue, 07 Dec 2004 13:56:06 -0800
List-post: <>
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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