lightning rod question

Hello, I was wondering why there is a ball on lighting rods? example

It seems there is a metal one in this picture which has some arcing under it, Its hard to believe they serve no purpose.

Decoration. Most of these balls are colored glass.

-- Bert --

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Oh the irony--you're standing in the front yard when the lightning strikes the lightning rod... potentially saving your life. The thermal shock of the strike fractures the glass ball, half hurled striking your head killing you! Oh the irony.

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Still decoration. At one time (early 30's or late 20's ) there were rods made with a ball at the top based on some half baked idea that came into vogue (to raise the price, probably) that it would provide better shielding. To sum up- it didn't work. There have been, are, and will be a variety of devices made and sold to provide lightning shielding. Typically they don't do as well as simply sticking a bunch of old bedspring on the roof and grounding them well - but they are shinier and more expensive.

If you get killed by this sequence than it was definitely your time to go!

Hi,

The ball is usefull in spreading heat from a direct strike. A #8 wire can carry any known strike but it will bern at the point of the strike without more thermal mass.

Ray

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wouldn't the head produced be caused by the current through the wire, or is there another method of heating in lightning strikes i'm not aware of? Wouldn't this spread the heat through the region of the ball but not farther along the path of the wire? I hadn't considered the heat involved before, intresting.

The heat generated is much more in the air above the wire. The power generated causes the vapor in the air to turn to steam and the rapid expansion of the steam creates the thunder.

Ray

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What's lightning, life just gets darker with age.

Not exactly... thunder does not require the presence of water in any form. Like all sparks, the rapid expansion of the spark channel creates a shock wave that ultimately creates thunder. For lightning, the exact mechanisms are subject to some debate, but thunder simply does not require the presence of water in either vapor or liquid form.

-- Bert --

That would appear to be an "early streamer" type lightning rod. i.e. it is supposed to attract lightning better than a Franklin rod, by some means of ionizing the air around it.

There is a lot of debate as to the effectiveness of these devices, in my opinion they are no better than snake oil and the Australian standard at least is unsupportive of them (despite appreciable pressure from manufacturers of these devices).

V.

I didn't catch the original post but I thought that this may be a good time to mention that lightning rods are/were not designed for direct hits. The purpose is to keep the accumulated charge below a safe level. Tom

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--------------- Actually this is extremely questionable. Design that I know of is based on the probability of the stroke striking the rod rather than the protected area. Granted, in some situations, particularly with tall structures, such draining does occur (possibly increasing the chance of side flashes-another problem) but this is a bonus, not the basis for design. I am aware of a purveyor of lightning protection that claimed that his protection works on the basis of charge dissipation. I have seen no evidence that it does. I have no idea if he is still is in business.

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.

References: Moussa & Shrinivasta, "Shielding of Tall Structures Against Direct Lightning Strokes" Canadian Conference on Electrical and Computer Engineering, Vancouver, BC, 1988 Energy Systems Journal, Vol. 11, N0.1 1991 EPRI Transmission Reference Book, 345KV and Above. I believe that IEEE changed its standards to reflect the concepts involved in these references. Sorry that I do not have more recent references but I have been retired for some time.

I agree with Vermin on the lack of usefulness of the ball. There appears to be no rational basis for its use. Note that its thermal mass is generally pretty small and by the time it starts to dissipate heat, any damage is done.

A best source of information in this discussion are both posts from Don Kelly. Especially where he defines what makes a lightning rod effective: earth ground. We tend to rationalize only upon what we see. We see the lightning rod and assume IT is the protection. We then get all hyped over blunt rods verses pointed rods verses one with a ball. All irrelevant once the facts are considered. Since we don't see earth ground, then we tend to forget the most important component of a lightning protection system.

Don also cites another well regarded expert on the subject - Dr Abdul Mousa. Dr Mousa's IEEE reviewed papers contain good, scientific language. But to summarize into laymen's terms - Early Streamer Emission (ESE) protectors that are suppose to discharge the air are scams. Air terminals provide the best or more conductive path from cloud to earth. Which again demonstrates what makes lightning rods effective - quality of that earth ground system.

D> Thanks for the info, I'll see what else I can find. Probably the

Thanks for the info, I'll see what else I can find. Probably the foremost expert on lightning in this group is Mark Kinsler. I believe he did his doctorate on the subject. I have not seen posts from him in a long while. Tom

I'm no expert on this, but I do recall my undergrad Electromagnetic Fields instructor coming down on the draining side of this issue. His examples were opposite to yours: He pointed out that the size of the ground wires on the typical barn or home lightning rod was far too small to withstand a direct hit. He noted that tall buildings, which do sustain repeated hits, have massive ground conductors to handle the current.

That was probably the only meaningful thing I recalled from that class. All the rest was curls, dels, and other funny symbols. (That was over 35 years ago. A lot of synapses could have drained to ground since then!)

Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis

I agree with the #8 wire not seeming to be adequate. The coast guard / power squadron use the #8 wire and 60 degree protection cone as there rules but for my sail boat I use a welding cable size ? connected from the aluminum mast directly( no curves ) to the lead keel. I have observed a boat being directly hit by lighting about 100 feet away from me. The vhf antenna made a baby carriage size puff of orange smoke. No one was injured but the electronics were fried. I would like some explanation of ball lighting. I have known of a ball ( corona ) in the cockpit of a sail boat.

Ray

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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.