lightning rod question

I will always read what you have to say William. I kind of thought that what I said may cause a "storm". I recall having correspondence with Mark Kinsler when he began his research in 1996 or so. He often said then and it still seems true now that there is no clear answer. There are still things much greater than ourselves to discuss however. Best Regards, Tom

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----------- The use of a #8 conductor is a bit iffy- even for home use. In addition the placing of a rod at each end of a gable roof is also very iffy. Even in the old days, a cone of protection was assumed. 30 degree cone good, 45 -not bad. The problem is that this left most of the roof unprotected by the rod system. Fortunately the probability of any house sized area being hit is pretty low. Another thing that was once written into codes was the use of odd sizes of conductors- e.g.#5- only available through the purveyors if lightning rods. However, from all that I have read on the subject and some design work on the basis of known principles (i.e. Moussa's work and EPRI). Look at your instructor's contention: say #8 was used - about 0.0006 ohms/ft- lets make it 0.01 ohms/ft to allow for skin effect and whatever. Now consider a typical stroke- about 35kA peak- Assume that it peaks in about 1 .5 microseconds and lasts for 100 microseconds for an average current of less than 20kA for 100 microseconds. Average power =4x10^6 watts--WOW! but energy per foot of conductor is 400 Joules. Not all that great. The point of the rod may be gone but the rest should be OK. What might happen is that there is a poor contact somewhere and a high energy dissipation at that contact- blowing it to ratshit.

As r w_tom indicated grounding and the soundness of connections is important. Thank you, w_Tom No point in diverting 35kA into a 10 ohm rod where the down conductor is 2 feet away from your backside on the john. EM theory comes in to play more when the stroke has occured- but little more than consideration of travelling waves and their reflections need be considered from a protection point of view. . The effect of a reflection can seriously burn your butt.

That's focusing on the cloud-discharging capabilities of -one- rod, obviously miniscule.

Would be the -combined- effect of (1) thousands of lightning rods in a moderate-sized community, plus (2) hundreds of miles of transmission line, telephone line, supporting cable, etc. suspended between hundreds of tall poles and tall towers, plus (3) tall lamp posts, tall antennas, etc. have a significant cloud discharging effect?

Order-of-magnitude estimate on this? I'm wondering, since I've seen several ground/tree strikes while out in rural areas, but only one direct strike to a tree in the city, and I'm an urban dweller.

Myxococcus xanthus

What about the sharp points on the edges of millions of tree leaves? At the magnitude of currents involved in corona discharge (not strikes) the resistance of copper and trees is insignificantly different.

--------- Unlikely- A large conifer forest has thousands of tips each of which is grounded( poorly but this isn't of imprtance until there is a strike) and there are records of thousands of strokes in such a forest in one storm. As for the towers, transmission lines, etc- these are targets. Tall buildings will produce their own "lightning shadows" or protected areas(which is what a lightning rod does). The effect of a strike will depend on their grounding systems. Ever had a transformer fail during a lightning storm in your urban area? Ever had the lights go out or flicker?

Back in engineering school one of the professors was famous for studying lightning. I recall him saying #10 wire was perfectly adequate, that we had to remember the wave form. A lightning stoke is a damped sinusoid, it is not like trying to pass 30,000 Amperes DC through a wire.

Something he never mentioned, but I have run into since, is Ufer grounding. It certainly gives low ohmic measurements. Do we know how well it works in real life, i.e. actual lightning strikes?

That comment doesn't make much sense to me. What's waveform got to do with it? What matters is power dissipation.

Or I guess that's what you are suggesting: the pulse is short enough that much higher instantaneous powers can be tolerated than at steady state. Why didn't you just come out and say that!? ;-)

COMMENT:

Yes. The waveform is "damped sinusoid." In this case it's the first descriptive word that's important, not the second .

SBH

Not obvious, since some books (and teachers) still insist that lightning rods protect your home by discharging the clouds.

Yes, but we need to remember that the ions produced by things on the ground would only reduce the net charge in the clouds if they could *fill* the empty space between the cloud and the ground.

Trees and buildings might be producing charged air, which makes the air more conductive, but this conductive layer of air would be like a film of insignificant thickness... because we compare it to the SEVERAL MILES of air between the cloud and the ground. I'm imagining that, at most, the charged air might move upwards at a foot per second. As the charged clouds arrive overhead and the e-fields become strong, the sharp objects on the ground would only have time to produce a layer of conductive air a few hundred feet thick. The layer will also be blowing sideways, so we shouldn't imagine that one lightning rod would make a cloud above itself. Rather imagine a smoke stack with a plume travelling downwind.

Lightning rods aren't going to have a large effect on the storm. A lightning bolt (a plasma streamer) is triggered up in the clouds, and then grows longer, sometimes growing downwards. Suppose it becomes several miles long and is approaching the ground. Could some ionized air hovering over the buildings have any effect? Sure. The movable charges will act as a resistive "coating" which makes the lightning think that the Earth lacks buildings. The e-fields which steer the growing plasma streamer would be altered by the conductive air, so they would not respond to trees and buildings as much as if there was no meters-thick layer of ions.

But is this what we want? The layer of charged air would keep the lightning path random, so the streamer wouldn't be guided to a safe attachment upon a nice thick copper ground-wire. Maybe it's a good thing that any wind would blow away the ions, letting the streamer "see" your lightning rod poking upwards.

Now the region BELOW a lightning rod... that's a different issue. If an incoming plasma streamer approaches a protected building, then the e-field around the building grows so intense that the lighting rod or even the ground wires will launch their own plasma streamers up to intercept the incoming streamer. A lighting rod acts like a "Scud Missle Launcher" which shoots down any incoming lightning bolts, forcing the incoming lightning to follow a trail leading back to the ground wire. With luck, the *TOP* of the lightning rod will launch the streamer, which prevents lightning strikes upon anything below the tip of the rod. The farther away you stand from the vertical ground wire, the more chance there is that the lightning rod wouldn't emit a streamer that intercepts any lightning bolts aimed for your head.

and

--------------- What you clipped may be of more importance than what you included. " .....and there are records of thousands of strokes in such a forest in one storm. As for the towers, transmission lines, etc- these are targets. Tall buildings will produce their own "lightning shadows" or protected areas(which is what a lightning rod does). The effect of a strike will depend on their grounding systems. "

This contraindicates the "discharge" effect.

Also ,in a previous comment: "Generally the source of the charge is several miles overhead -the rod doesn't get seen by it. When a leader gets near a rod then it may be a preferred target for the next step and if it is, then the main stroke will be to the rod-if not something else gets hit. Design is based on it being such a target for higher current strokes (but not necessarily lower current strokes). This is true for protective systems for transmission lines as well as structures. Catch the damaging strokes know that some of the little ones will get by."

In other words, the above is in agreement with what you have said quite well.

flicker?

Note that design doesn't count on this streamer and the protection zone is determined by the downcoming leader's strike distance which is related (empirically) to the main stroke current . The protected range is given by the "rolling ball approach. Basically a ball of radius equal to strike distance and just touching ground (or the local ground plane) and the rod tip. The shadow of protection is outside this radius for currents equal to or greater than the design current. Weaker strokes can get by. It's a saw off- you get what you pay for (and hope that the statistics are reasonably correct).

The purpose of the ball is to stop the windvane falling down. It serves no electrical function.

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