Does anyone know what the official theory is for how lightning rods work? I've heard some different claims and I don't know which is correct.


Some people say that lightning rods help prevent lightning strikes by dissipating the voltage difference between the sky and the ground. Storm clouds mostly carry positive charges while the ground carries a negative charge.

The rods are pointed on the end specifically so that they act as electron emitters, though the emission is nowhere near rapid enough to generate a visible corona. The rods carry the negative potential up from the earth, spray it into the air, and help ionize the air and dissipate the charge-difference in the air that could lead to a lightning strike near the building.


And other people say that none of this happens. The rods are simply there to provide an easy route for the strike to follow, rather than along a path that leads right through the frame of the house, for example.

There is no dissipation or ion-streaming effect, and the rods don't need to be pointed at all... a cut-off flat or rounded stub is equally as effective.

Rods also don't do anything to reduce the frequency of strikes, and buildings with lightning rods are struck no more or less than structures without them.


So which of these is it? Is there any charge dissipation occurring, or is it only just an easy path to ground?



[This message has been edited by Scalar (edited 09-28-2005).]

wouldn't an ammeter confirm the flow of electrons? Is anyone brave enough to collect data both during good and bad weather? Where is Ben Franklin when you need him?

If you asked four engineers, you would get five different opinions on this topic.
There is lots of opinions on the web.

LOL!!!

I am leaning towards "The Other People's" descriptions, from "Scalar"'s post, as being more accurate.
"Other People" description would be the one starting with:
"And other people say that none of this happens."

Scott35

Do a google search and find out for your self. Ltg rods do not prevent a strike.

Lightning rods do not prevent a strike?

Okay... I disagree with that one. The most widely held theory is the equalization of charge between earth and sky. Now the debate begins as to whether the sky is pos or neg, or which is which, doesn't matter, it releaves the imbalance condition which causes a strike.

"Easy path for a strike to follow". Nah, we ain't got wire enough for this, too many amps for a pathway, well, more than once.

Good on ya George!.
Common thought would have it that lightning strikes the ground.
To a degree that is untrue.
From the highest point in a given area, high speed photograghy will show you that there is a "leader" that goes from the ground up to make contact with the Cloud-Ground contact.
It is that contact that gives the sound of the lightning strike.

The whole point ( no pun intended) of a lightning rod is to leach away electrons in a controlled manner to ground to pre-empt a strike proper, as conditions move toward an event? As George points out ( no pun intended) we ain't got enough wire to carry a stroke safely down a building more than once. They aren't, as far as I'm aware, continually renewing the LC on the Empire State Building, which is said to get struck many times per year. The point is, ( NPI), Good Ol' Ben did all the experimental work some 12 score and ten years ago. And he found, by experiment, that a point on the rod ( his was made of iron- Yankee parsimony? ) seemed to give better results. He had some kind of bell mechanism rigged up in his stairwell to indicate that some kind of electric current was flowing during thundery conditions, and we know this worked because there is a letter extant from his wife to Ben in England begging him for directions as to dismantling the bloody thing as it was driving her to distraction.

Alan

ps. Which could explain how he survived the crazy kite stunt.

[This message has been edited by Alan Belson (edited 09-29-2005).]

I think the rods are "pointed", so you can pound them into the ground.

(How I got ground rods out of this, I'll never know)

We are discussing air terminals, right?

Lightning will hit anywhere it wants to, we can't "as of yet", control where it will hit.

Lightning rods, and lightning protection systems, do not "attract" lightning, they provide an easy path to earth, so the strike can be disapatted.

Roger Block has some interesting books on this. Alot goes into it, alot more than I had ever thought.


Dnk...

[This message has been edited by Dnkldorf (edited 09-29-2005).]

Lightning rods are pointed because the electrical charge is most dense at the point.

For the same reason, ultra-high-voltage terminals are spherical, so the charge is evenly distributed.

Many new lightning rods are not pointed for safety reasons. Look here . Research shows no loss of effectiveness.
Don

Well, I'll go to the foot of our stairs! How can you fall onto a lightning conductor point and injure yourself??!!!
Absolute bull***t!

Alan

I always told that you needed to put in several air terminals and wires around the building. In effect you were trying to build a "Faraday Cage" around you so that the lightning would go around the building instead of going through it.

Harold

From someone who was brave (read "mad") enough to connect equipment to lightning rods with the purpose of studying the magnitudes and durations of strikes.

Lightning (Franklin) rods do attract lightning if within the step distance of the strike - if outside then the strike may well miss the tip of the rod.

Rods don't increase the frequency of strikes for an area, but most certainly increase the concentration towards the rods within an area.

Some charge dissipation does occur (and I have seen the corona off the tip of a rod), but not enough to prevent strikes (come on, it's just travelled a few thousand feet. ya really think a few feet of ion cloud is gonna make any difference?).

Conductors can be made beefy enough to withstand a full strike.

Clouds can be both negative (general) and positive (about 1 out of 10 average) with respect to Earth.

Points are better, spheres will have lower attraction ratios.

M.

Alan I have been on plenty of flat roofed commercial buildings that it would be simple to trip and fall onto the points.

At one large building we where adding an addition to the GCs safety people would cover all the rods in the morning and then uncover them over night.

I specify the blunt tip air terminals now a days. I read an interesting study that concluded similar results between blunt tip and pointy.
I've have problems on roof mounted cooling tower equipment (or many commercial roof parapets) when pointy terminals are used, because it always would catch my shirt when I walked by.

Iwire, Ron,
Yes, I buy those arguments in favor of a blunt tip in such locations as commercial premises flat roofing for safety. I was thinking of the traditional 'church spire' arrangement. Browsing the Net just now I came across some references to Mars. It's so dry that static charges are believed to be going to be a serious problem for manned landings. Terming this effect "lightning in reverse", NASA engineers have developed a multi-tined "reverse lightning" rod to fit to Rovers etc.. It consists of many very fine titanium wires arranged aerial-wise to dissipate charges into the thin martian atmosphere, (in effect turning the atmosphere locally into "Ground"), to protect astronauts and electronic kit from discharges. Significantly, the thin titanium wires are sharpened to a very fine point.

Alan

Well I went out and searched around about lightning rods and there are a lot of theories, not many of them agreeing with my physics teachers' explanation. But I did find one that seems to meet what I see as the right reason. And whoever said it was right, lightning comes up from the ground and not down from the clouds. A lightning rod is more for providing a safe path to the ground in which nothing will overheat rather then to prevent them. Try this. http://science.howstuffworks.com/lightning9.htm
M@T

GMIntern,
What does your Physics teacher say? I don't get to electricity until Chapter 16, and I'm just now teaching Chapter 4. So I want to know what to say in front of the class, as everything that I read is conflicting. So I present it to the students that way too!

Well Ron, the last time I went over that was in my physics class last year, and I dont remember everything. But here is everything I can recall. Lightning is obviously electriciy, Ben Franklin figured that one out for us a long time ago, maybe back when some of you guys were kids, haha, just kidding. But he figured out, quite dangerously I might add, that lightning is indeed something like a visible path of electrons from the ground to the sky. Most people think that it is something that comes from the clouds, but actually it isnt. Lightning comes up from the ground to the sky. Now here is where it starts to get a little fuzzy. From what I can remember, there is electrical charge in the clouds, which will create lightning between clouds, but the real force behind lightning striking the ground is because the clouds and earth have a different charge. The main reason that the lightning will move from the clouds to the ground is due to ionization of the air in between. Ionization isnt the loss of electrons or protons, but simply these same electrons moving farther away from each other, sort of like a Van de Graaf Generator. Now dont take my word on all of this, I am after all human and subject to mistakes. I will recommend http://science.howstuffworks.com/lightning.htm for this one. Experts in a field can usually tell you more about it then a student can. Hope it helps. And my advice for teaching is to make sure you know what you are saying first. Because unless you are really good at bsing your way through something, you need to know what you are talking about before you can talk about it. Everyone usually likes the teachers who teach them things.
M@T

GM, there are cloud to cloud strikes, and cloud to earth strikes. "Leaders" come from the clouds, and reach up from the earth to form a path.

From the site you posted:

"""The leader that reaches the earth first reaps the rewards of the journey by providing a conductive path between the cloud and the earth. This leader is not the lightning strike; it only maps out the course that the strike will follow. The strike is the sudden, massive, flow of electrical current moving from the cloud to the ground. """

If lightning orginated at the earth and went to the cloud, I would put lightning protection on my nieghbors house.


Dnk......

Here are some more interesting links.
http://www.arrl.org/tis/info/lightning.html

Radio hams on how to protect high antennae.

http://en.wikipedia.org/wiki/Talk:Lightning_rod

The wikipedia article is hedging its bets. It claims that there are TWO modes of operation: gradual discharge, to prevent (not to attract) lightning, AND conduction, in case of a direct strike.

Anyone figuring out how to control or attract lightning can name their price and retire. The utility companies have been trying to figure it out for decades.

Dave

I think the only thing that's agreeabble is that no-one can agree on this.

But we spend money to figure out why frogs don't fly...

Go figure.....


Dnk.....

Is there a difference between the average ground rod used at single family dwelling and a lightning rod?

I agree with marcspages.

If I recall, the problem with most lightning rod systems is that they are too small relative to the distances and currents involved.

It is hard for a 1000' lightning rod to equalize the electrical potential across 30,000'.

Shockme,
A residential ground rod is >8' long and is buried in the ground.
An air terminal is 12"-48" and is mounted in various areas of the roof extending into the air.
My point regarding the Physics teacher is that there are various theories, and none are a slam dunk. I generally teach several concepts, and suggest that they become a physisist when they grow up

The modern theory, propounded by Isaac Asimov, is that the true energy pump for these voltage differentials is cosmic radiation.

The earth is constantly bathed in such radiation. It energizes the upper atmosphere somewhate in the manner of a capacitor.

When the weather is appropriate the concentration of energized air molecules are ripe for a triggered release.

Some cosmic radiation is especially ionic and penetrating. It is assumed that they are the source of the 'leader' strike. Unlike current in a wire this charge carrier drives down through the atmosphere as if it were a cloud chamber at the end of some partical accelerator.

This functions much the same as if you were to drop some #16 across high voltage mains bussing -- hot to hot. The wire would explode into a plasma arc, which would then be sustained.

Unlike mains bussing, the pool of energized ions is comparatively small while the energy per electron is way, way high.

As the strikes proceed the earth operates as a conductor -- replacing the discharged ions. Florida, with its high water table makes for an ideal mobile electron tank. Hence it is no surprise that it holds the American records for repetative lightning strikes in an area.

Q. How does lightning form?
A. Lightning, a flow of electrical current between the earth and storm clouds, occurs as varying charges of positive and negative polarity build up in the atmosphere during a storm and begins to ionize the air. As more air is ionized and the result is a conductive path to earth. A discharge or current sent rushing toward the earth. As this downward force nears the earth's surface, positive charges rise up to meet it from many objects at earth level. As the negatively charged stepped leader thrusts toward the ground readying to discharge its energy, its path is erratic. Nearing the earth, positive charges are attracted by it and begin to arc upward from roof edges, lighting poles, antennas, etc. When any of these two opposing charge systems meet, they create a closed circuit. As the path to the ground is completed, a flash is created. This can occur several times as the cloud will recharg at a rapid rate.

Q. What happens when a building is struck by lightning?
A. A lightning strike to an unprotected building can be catastrophic. Packing up to 100 million volts of electricity and a force comparable to that of a small nuclear reactor, lightning has the power to rip through roofs, explode walls of brick and concrete and ignite deadly fires. In addition to structural damage, lightning surges throughout power lines causing wire damage and destruction of valuable electronic equipment including computers, televisions, stereos, security systems, etc.

Q. How does a certified lightning protection system work?
A. A lightning protection system provides a designated path for the lightning current to travel. The system neither attracts nor repels a lightning strike, but simply intercepts and guides the current harmlessly to ground.