rsd@sei.cmu.edu (Richard S D'Ippolito) (07/11/88)
In article <1988Jul8.045005.17867@utzoo.uucp> Henry Spencer writes: > [...] Lightning rods and the like are based on giving >it a more attractive place to go, not on stopping it from doing what it >wants. Ligthning rods work by _preventing_ the buildup of static charges in the vicinity of the pointed end(*). This reduces the formation of streamers radiating from the ground, which become the return path for the cloud-to-ground bolt when they reach the charged cloud. They do _not_ attract the bolt (God forbid!). Look at the size of that skinny pole and wire and imagine hundreds of thousands of amperes flowing through it... The normal path to the computer equipment is the same as it is for all other appliances -- through the power lines (and phone lines for a MODEM). Those utilities strive mightily to protect their investments with voltage-limiting sparkgaps and suppressors, but the resulting surges can still do in an electronic appliance. In many cases, an extra fifty-cents of protection is worth just about that, so it isn't worth the bother. I suggest commercial surge protectors on the lines, not in the equipment. Rich * Electric field theory will explain the reason for the sharp point.
ornitz@kodak.UUCP (07/12/88)
In article <6179@aw.sei.cmu.edu> rsd@ae.sei.cmu.edu.UUCP Rich D'Ippolito writes: >* Electric field theory will explain the reason for the sharp point. Several years ago, an article on the current status of lightning protection was published in the Journal of the Franklin Institute (naturally!) by scientists from one of the Arizona universities. In this article, it was shown that the sharp point is not optimum but instead an elliptical point should be used. This suggestion was backed up by simulation studies and the study of several thousand actual strikes on lightning rods. In virtually all cases, the rods were hit several inches to several feet _below_ the pointed tip. The article also debunks a number of myths about the "cone of protection" supposedly produced by a high lightning rod and also about the neutralization of space charges by lightning rods. If there is enough interest, I can dig up the date of the issue. Barry WA4VZQ ----------------- | ___ ________ | | | / / | | Dr. Barry L. Ornitz UUCP:...!rochester!kodak!ornitz | | / / | | Eastman Kodak Company | |< < K O D A K| | Eastman Chemicals Division Research Laboratories | | \ \ | | P. O. Box 1972 | |__\ \________| | Kingsport, TN 37662 615/229-4904 | | -----------------
logajan@ns.ns.com (John Logajan x3118) (07/13/88)
In article <1314@kodak.UUCP>, ornitz@kodak.UUCP (barry ornitz) writes: > > shown that the sharp point is not optimum but instead an elliptical point > should be used. > The article also debunks a number of myths about the "cone of protection" > and also about the neutralization of space charges I'm interested in this for reasons unrelated to the current discussion. Do you remember what was the basis for these statements? There are three general cases for the field around charged surfaces. 1.) a point charge has a field gradient inversely to the distance squared. 2.) a two dimensional (wire) charge has a gradient inversely to the distance. 3.) a three dimentional (plate) charge has a constant field for distances << than the diameter of the plate. Of course at a great enough distance all these cases begin to look like point charges. Anyhow, I would guess for a lightning rod you would want as steep a gradient as you could get. The combination of the straight part of the rod might tend to overwhelm the field around the point. Perhaps that is why an elliptical point works better. The tip then is further isolated from the more constant gradient around the straight section. I don't know about the cone, but there must be some shape of protection around the rod, i.e. some area where it is easier to go to the rod than to some other structure. I suspect that the rods do tend to neutralize the local space charges, but since there doesn't have to be a local space charge for lightning to hit, that factor wouldn't do that much good. - John M. Logajan @ Network Systems; 7600 Boone Ave; Brooklyn Park, MN 55428 - - {...rutgers!umn-cs, ...amdahl!bungia, ...uunet!rosevax!bungia} !ns!logajan -
abostick@gethen.UUCP (Alan Bostick) (07/14/88)
In article <6179@aw.sei.cmu.edu> rsd@ae.sei.cmu.edu.UUCP (Rich D'Ippolito) writes: >Ligthning rods work by _preventing_ the buildup of static charges in the >vicinity of the pointed end(*). This reduces the formation of streamers >radiating from the ground, which become the return path for the >cloud-to-ground bolt when they reach the charged cloud. They do _not_ >attract the bolt (God forbid!). Look at the size of that skinny pole and >wire and imagine hundreds of thousands of amperes flowing through it... > >The normal path to the computer equipment is the same as it is for all other >appliances -- through the power lines (and phone lines for a MODEM). Those >utilities strive mightily to protect their investments with voltage-limiting >sparkgaps and suppressors, but the resulting surges can still do in an >electronic appliance. In many cases, an extra fifty-cents of protection is >worth just about that, so it isn't worth the bother. I suggest commercial >surge protectors on the lines, not in the equipment. > >* Electric field theory will explain the reason for the sharp point. Because of the wire connecting the lightning rod to the the surface of the earth, the tip of the lightning rod is at the same potential as the ground. The static charge builds up not on the ground, but in the AIR, in the clouds in question, due to convective circulation between the base of the cloud and its top. Because of the sharp tip of the lightning rod, the electric field strength there is much greater than on the (more or less) flat ground. It becomes much easier for the insulating air to break down in the vicinity of the rod. Hence, the preferred path for the lightning is for it to strike the rod, and travel down the wire to the ground. Remember that those thousands of amperes are flowing for a small fraction of a second. If the wire is stout enough it will be able to sustain the flow for the needed time without melting. Alan Bostick ucbvax!unisoft!gethen!abostick
ornitz@kodak.UUCP (barry ornitz) (07/14/88)
In article <432@ns.ns.com> logajan@ns.ns.com (John Logajan x3118) writes: >In article <1314@kodak.UUCP>, ornitz@kodak.UUCP (barry ornitz) writes: >> shown that the sharp point is not optimum but instead an elliptical point >> should be used. >> The article also debunks a number of myths about the "cone of protection" >> and also about the neutralization of space charges > >I'm interested in this for reasons unrelated to the current discussion. >Do you remember what was the basis for these statements? >..... >Anyhow, I would guess for a lightning rod you would want as steep a gradient >as you could get. ... > >I don't know about the cone, but there must be some shape of protection around >the rod, i.e. some area where it is easier to go to the rod than to some other >structure. John, you really need to read the whole article since it is far, far too long to fully discuss here. The full reference is: Moore, C. B.: Improved Configurations of Lightning Rods and Air Terminals, "Journal of the Franklin Institute," Vol. 315, No. 1, January 1983, pp 61-85, Pergamon Press, Oxford. My memory was mistaken on Moore's address. He is with the Langmuir Laboratory of the New Mexico Institute of Mining and Technology. You have actually hit on the problem. The high gradient at the point causes breakdown of the air near the tip. The ion flow from the tip absorbs any increase in field strength above the ionization level. The point thus protects itself from further increases in field strength. The author of the article beleives the "cone of protection" exists, but it has a much smaller half-angle (probably around 22 degrees) than most people think. Some suggestions about the optimum rod are presented in the article that are quite useful. I would suggest that you find this article. I can photocopy it for you if your local library cannot find it (please send me a SASE). This article has both theory and experimental data sprinkled with some history. Even if Franklin got the sign wrong, most of his other work was right! :-) Barry WA4VZQ ----------------- | ___ ________ | | | / / | | Dr. Barry L. Ornitz UUCP:...!rochester!kodak!ornitz | | / / | | Eastman Kodak Company | |< < K O D A K| | Eastman Chemicals Division Research Laboratories | | \ \ | | P. O. Box 1972 | |__\ \________| | Kingsport, TN 37662 615/229-4904 | | -----------------
karn@thumper.bellcore.com (Phil R. Karn) (07/15/88)
> You have actually hit on the problem. The high gradient at the point causes > breakdown of the air near the tip. The ion flow from the tip absorbs any > increase in field strength above the ionization level. The point thus protects > itself from further increases in field strength. If by this the author meant that a lightning rod can actually PREVENT (or at least discourage) a lightning hit, then this is at least highly controversial. While doing a little library research on lightning, I found another textbook that attacks this belief as being false. The author stated that a lightning protection system can only lessen the damage caused by a lightning strike, not prevent it from happening. Although I am not an expert on lightning, this latter view seems to make much more sense. Another interesting tidbit from the book: the "furrows" seen on the outsides of trees that have been hit by lightning are not caused by the vaporization of water under the tree's surface, as is commonly believed, but rather by the explosive shock wave generated by an arc on the *outside* of the tree. Trees are rather poor conductors. When they are hit it is quite common for the lightning to find a better path to ground along the outside of the tree than within it. This is why it's so dangerous to stand under a tree during a lightning storm; if the tree gets hit, the lightning may well decide to jump back out of the tree and through you to ground. BTW, the reason I'm interested in this stuff is because a pine tree only about 10' from my tower (and about the same height as my antennas) has the characteristic spiral furrow of a lightning hit... Phil
ornitz@kodak.UUCP (barry ornitz) (07/15/88)
You really need to read the entire article.
While the tip is protected, the rod will generally be hit several inches to
several feet below the tip unless a better path is found in the vicinity.
The rod will _not_ prevent lightning strikes!
The full reference is: Moore, C. B.: Improved
Configurations of Lightning Rods and Air Terminals, "Journal of the Franklin
Institute," Vol. 315, No. 1, January 1983, pp 61-85, Pergamon Press, Oxford.
My memory was mistaken on Moore's address. He is with the Langmuir Laboratory
of the New Mexico Institute of Mining and Technology.
I was about 75 feet from a large tree hit by lightning once (in a building).
Not only was the tree split, but the root system exploded leaving numerous
furrows in the ground with the major roots exposed. Scary!
73 Barry WA4VZQ
-----------------
| ___ ________ |
| | / / | | Dr. Barry L. Ornitz UUCP:...!rochester!kodak!ornitz
| | / / | | Eastman Kodak Company
| |< < K O D A K| | Eastman Chemicals Division Research Laboratories
| | \ \ | | P. O. Box 1972
| |__\ \________| | Kingsport, TN 37662 615/229-4904
| |
-----------------travis@ihlpa.ATT.COM (Marlatte) (07/15/88)
Rich D'Ippolito correctly wrote:
"Ligthning[sic] rods work by _preventing_ the buildup of static
charges in the vicinity of the pointed end(*)."
In article <1041@gethen.UUCP>, Alan Bostick writes:
"Because of the sharp tip of the lightning rod, the electric
field strength there is much greater than on the (more or less)
flat ground. It becomes much easier for the insulating air to
break down in the vicinity of the rod. Hence, the preferred
path for the lightning is for it to strike the rod, and travel
down the wire to the ground.
"Remember that those thousands of amperes are flowing for a small
fraction of a second. If the wire is stout enough it will be
able to sustain the flow for the needed time without melting."
While it may be true that a stout wire could sustain a lightening
strike, that is not the intent. Rich is correct. A lightening rod will
prevent, or more accurately, minimize the build up of charge in the area
around the rod.
Alan's description is basically correct. But the point on the rod does
not attract or induce a strike, but prevents it by causing a slower
discharge. If lightening actually strikes a lightening rod on a building,
it is as good as striking the building. The wire is attached to the
building, the building is attached to the ground. The wire provides a
very good path, but the building offers a close second.
This is not a perfect solution. In severe thunderstorms, the
build up of charge can be very sudden. The presence of moisture, like
rain, can alter the affects of a lightening rod. Also, charge build up
can be quite isolated. This is why a tree within a close distance of the
lightening rod can get hit.
All of this can be seen in an experiment at home. Ready boys and girls,
here we go. You know how you can scuff your shoes on the carpet and get
a shock when you touch a door knob? Well this is SIMILAR to
lightening. If you approach the door knob with your hand or finger, the
static build up is sufficient to arc several centimeters. Now ask your
Mommies for a straight pin. You know, the kind she uses when she sews.
If you hold the straight pin, and approach the door knob holding the
point of the pin out, you will not get a shock.
--
Travis
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Travis Marlatte ihlpa!travis 312-416-4479 AT&T Bell Labshenry@utzoo.uucp (Henry Spencer) (07/19/88)
In article <6179@aw.sei.cmu.edu> rsd@ae.sei.cmu.edu.UUCP (Rich D'Ippolito) writes: >Ligthning rods work by _preventing_ the buildup of static charges in the >vicinity of the pointed end... There is a lot of religious argument about lightning rods and the precise mechanisms by which they work. However, it is a verifiable fact that they really do get hit by lightning, and that they do appear to have a strong tendency to protect the structure underneath from the worst of the bolt's effects. If you look at their history, in fact, such incidents were what first convinced people that they worked. Any prevention effect is a bonus. -- Anyone who buys Wisconsin cheese is| Henry Spencer at U of Toronto Zoology a traitor to mankind. --Pournelle |uunet!mnetor!utzoo! henry @zoo.toronto.edu