Lightning

[dmccarty]

There's a lot of confusion about this.

Systems to PREVENT lighning strikes (dissipators, etc.) are very dubious. Many experts think they are snake-oil.

Systems to PROTECT a structure from the effect of a lightning strike (grounded lightning rods) are well regarded, although there's still a lot of disagreement on how to best design them.

What is not confusing is that the insurance company ain't going to lower my premium if we install a system to protect my house from lighting.

Pat mentioned the 30 degree cone of protection which I have heard before. Since my house is pretty much surrounded by trees we might already have some protection though limited.

We have been having a decent number of T Storms lately but I have not seen any reports of houses being hit by lightning in the last month or so. In the past we have had storms with multiple house fires started by lightning from one storm. Kinda strange that some storms cause not one house fire but many and then other storms dont cause any.

Later,
Dan

 

[Toiyabe]

What you said is that a lightning rod works by preventing strikes by draining off static charge before it builds into a full-fledged strike. This is often called the "dissipation theory". It was Ben Franklin's original idea. The problem is that the available data doesn't show that protected strucutres are hit less often than unprotected structures. There are theory wonks on both sides, but the experimental data just isn't there.

Modern codes are built around the "diversion theory". According to this theory, you can't prevent a strike. Instead you should provide a safe path to ground for the strike. You do this by providing a lighning rod (or rods) that is much more likely to be hit than any other part of the structure, and then properly connect the rod to ground.

Available data does show that a properly designed lightning protection system reduces property damage in the event of a strike. But it does not show that they are struck less often.

Proponents of the dissipation theory prefer sharp tip lighning rods. They claim that these rods are better at leaking static charge into the atmosphere.

Proponents of the diversion theory prefer rounded tips (a la NFPA 780). They claim that rounded rods are more "preferred" by lighning strikes, so there's less of a chance that an unprotected part of the structure gets hit.

Nobody argues that lightning rods don't work. It is how they work that is the issue. If you think in terms of preventing a strike you may end up with a system that is inadequte to channeling the energy of a strike in the event that one occurs.

 

[Toiyabe]

What is not confusing is that the insurance company ain't going to lower my premium if we install a system to protect my house from lighting.

Your agent may be hung up on the word "prevent". Ask if they will lower your premium if you install a UL approved lighning protection system (UL | Lightning Protection Systems). I'd be surprised if they wont.

 

[patrick_g]

Nobody argues that lightning rods don't work. It is how they work that is the issue. If you think in terms of preventing a strike you may end up with a system that is inadequte to channeling the energy of a strike in the event that one occurs.

Now I get what you may have been getting at. Thanks for spelling it out. I think there is something to the dissipation idea BUT I am a firm believer in HD busses with no sharp turns and stood off from other surfaces that lead a strike to earth safely. I guess I am not firmly in either camp or maybe I AM FIRMLY IN BOTH CAMPS!

In my physics studies I have done a number of static electricity experiments and have seen first hand in the lab the difference between ball tipped and pointed rods. Little pointy tips do, in fact, tend to dissipate a charge much better than a ball and they do get melted away (sometimes leaving a more ball like shape) when they take a good hit.

A few years ago a new Mormon temple was built in San Diego. A statue of the prophet Moroni with a horn to his mouth was placed atop one of the tall spires. Not long after that lightning gave the horn quite a wallop. I think the statue was well grounded and there was no other damage.

Lets be results oriented and not get caught up in theoretical squabbles. If lightning protection does protect, then good. In the immediacy of the situation we should concern ourselves with the fact THAT the dog is biting us, not WHY the dog is biting us. After we have taken appropriate action THEN we can theorize and explore the cause.

Put me down on the side of HD ground wires installed with only large radii turns, direct routing, adequate stand off distance, good earth grounding, and anything else advised by proper authority. Sharp vs rounded is a lower priority and less significant part of the equation. Having both couldn't hurt. It might be interesting to see what happened to each if the only difference were the radius of curvature of the tip.

Electrostatic experiments using various static generators often use spherical shapes for storage because of the lowered electrical field strength and resulting lowered leakage via ionized air. Sharp points are used to decrease the voltage required to get a "brush" discharge.

Slightly off topic: a conductor shaped like half of a ******** with a low friction bearing at the balance point near the middle will spin like crazy if it is given accesses to a static charge (sit it on top of a Van de Graaf electrostatic generator.) The best action is had when the tips are the sharpest. The air is ionized by the electric field concentrated by the sharp points and repelled by the like charge. This is a typical "ion drive" demo. If you jam the rotating member and put your finger near it you can feel the electric "wind" coming off the point. (actual breeze, not an electric effect)

Pat

 

[Egon]

Just curious; what do the really tall buildings use for lightning protection?

 

[patrick_g]

Just curious; what do the really tall buildings use for lightning protection?

Prelubers, they aren't good for anything else!

Pat

 

[Egon]

Dargn Sugar Pat:

Here I was thinking of The CN tower in Toronto, Canada.

CN Tower is of course the highest lightning rod. They say, lightning strikes CN Tower average of 75 times a year.

What's with the preluber thing? A properly set up pre/post lube is just plain industry standard for large stationary engines!

 

[Toiyabe]

The big difference with very tall structures (especially ones that are away from other tall structures) is that they can also get struck in the sides. The "cone of protection" idea isn't strictly correct.

 

[patrick_g]

The big difference with very tall structures (especially ones that are away from other tall structures) is that they can also get struck in the sides. The "cone of protection" idea isn't strictly correct.

Which is it? Do the other structures supply protection from lightning coming down between buildings and hitting one in the sides. What is the difference between that and the cone of protection thingy? OK, maybe a lopsided difuse beat up polygonal zone of protection.

Pat

 

[patrick_g]

Dargn Sugar Pat:

Here I was thinking of The CN tower in Toronto, Canada.

Tall buildings getting hit by lightning started to be a really big thing in "modern times" with the Empire State Building. It is not a surprise to the architects and engineers that their tall buildings are going to take hits frequently so they allow for it. The conductive armature of the building is well bonded to ensure easy conduction of the current to an intentionally installed grounding system. Grounding and conductive paths are not left to chance but are designed in.

Pat