O/T Question - What are the purpose of lightning r

Link

Here's a link to a paper giving some background on lightning protection and the history of the technology.

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As I stated before, the sole purpose of a lightning rod is to guide the lightning current to ground via a "safe" path. The rod MUST be grounded in order to work, otherwise you'll end up with side flashes to who knows where on the structure. Note also that the lightning rod/ground wire does not have to survive the strike to do its job. Once the path is established, the current will continue to flow in the ionized channel even after the wire vaporizes. Obviously, you have to replace the system after this happens.

I don't want to come across as arrogant, but I'd sure like to see some reputable references providing data to support some of the mis-information being banted about here. If you can't back your statement up with references or data, then its nothing more than an opinion.

Keith

I have repaired a barn after a strike, and I sure do not believe a rod was intended to take a strike. The strike took on the rod on the west (top) side of a large barn. It went across the total length of the barn following the cable. It then went to the eve and towards the ground. The path was clear as the asphalt and wooden shingles looked like a mole had dug under to the sheathing. At the eves it took the corner post 8 X 10 in the barn and exploded it. There was no fire, although some charing in the path.

Right or wrong we were always told the rods took out the static electricity. It has been explained that if you hold a wire that is grounded, you can rub your feet on a carpet all day and never hear a snap of static electricity. It does make some sence, right or wrong.

Most of the older folks will not sleep at night if you remove the rods for roofing. I must say we seldom put them back on now days however it be, right or wrong.

With todays copper prices I kind of think it may be worth while to haul them away for scrap.

Regardless if they did not work, there must have been some prety good marketing skills involved.

What you saw is a perfect example of what happens when a lightning rod is not properly grounded, either because the ground wire has degraded over time, been broken at some time in the past, or was never installed properly in the first place. The lightning followed the conductor until it hit a high impedance point, then flashed to the structure where it continued on to ground. Exactly what it would be expected to do if the ground wire for the lightning rod was broken.

Lightning rods have absolutely nothing to do with controlling static. You analogy of holding a grounded wire is only partly correct. Having an electrical connection to ground does keep static charges from building up on your body - in fact, people who work on sensitivie electronics use a wrist strap for that very purpose. In the case of a lightning rod on top of a building, there's very little to create static charges, basically just triboelectric charging as a result of dust being blown against the rod or building. Accumulation of charge on a relatively small object such as a lightning rod would have a negligible effect on any lightning strike.

Keith

Those old Hawkeye bulbs and rods fetch a good price on e-bay, I've got one from our place, had to climb up a rickety roof to get it, it's a keepsake though.

• ALRCI

Keith

Lightning analysis uses a term called the "action integral", which is the current squared integrated over time, i.e., the i^2 part of the the equation above. This integral times the real part of the impedance (resistance) gives the total energy (typically in watt-seconds) deposited in the material.

Explaining all the nuances of lightning interaction with materials would require a textbook. If you want a more detailed explanation, do a seach on the web for MIL-STD-464, Electromagnetic Effects Requirements for Systems, and read the rationale for the lightning protection requirements.

Keith

Best regards
Gary

Having a lightning rod that is grounded increases the chance of a lightning strike slightly over not having it. But it is better to have a controlled discharge path to earth than to let lightning find its own way should a strike happen.

There is nothing that can be done to prevent a lightning strike. You can only take measures to direct it safely to ground should it happen.

All objects build up charge as a charged cloud approaches. The objects do not have to be metallic to build up a charge. That is why lightning can hit a wood roof just like it can a metal pole.

Some believe that a lightning rod will prevent a strike by bleeding off the charge in the surrounding area. It will not. The earth, where the rod is connected to, can supply many times the energy much faster than it can be bled off. A small ion cloud forms at the tip of the rod that would repel the lightning but the wind readily blows that ion cloud away. A better lightning rod is one with a blunt end rather than the traditional sharp point as the blunt end is less likely to attract a strike than a sharp point.

As a charged cloud moves over an area, streamers start to form from objects on or near the ground and kind of wave around in the air. Then the cloud sends out what is called a “step leaders” which is sort of like a feeler from the cloud.
The “step leader” (the predecessor to a strike) moves in approximately 150 foot steps through the air. When it gets near a streamer on the ground object it attaches and that forms a small plasma path from cloud to ground. The large strike current follows that low resistance plasma path.

Because the step leaders move in 150 foot steps (down, side ways etc.) that is why lightning doesn’t always hit the tallest object in an area. It all depends on how the step leader moves. How it moves is determined mostly by local conditions in the air. Anything that is taller than around 150 feet is just as likely to get somewhere on the side as it is to get hit at the top.

Regards
Gary

Keith

I have absolutely no fear of my shed will be hit by lightening. Pole building with a full metal skin; metal roof touches the metal facia which in turn touchs the soffet which is in contact with the sides and that touches the ground.
Forms an excellent Faraday Shield killing a cell phone signal in the building and out about 20 feet. It also drains the charge out of the air during thunderstorms--you ask how I know that--I have experienced it. Drove in with the hiar standing up on the back of neck walked up close to the building and it just went away.
That draining of the charge prevents the strike; however should a strike occur it will ride the building skin into the ground.

24-Aug-2001

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Dear Cecil:

Everyone learns about lightning rods in grade school. There is the heavy-duty kind on skyscrapers that takes the brunt of bolts and conducts them safely to ground, and the delicate, sharply pointed ones that protect residences by bleeding off electric charge and preventing strikes. Having been well schooled in skepticism by our beloved Cecil, I tried to find a scientific study showing that rods actually ward off lightning, before plunking down dollars. No luck, except for manufacturers' marketing propaganda. I've seen a scale-model village successfully protected by tiny lightning rods from a "lightning" machine. But models don't always scale up to the real world. Do houses with lightning rods actually get hit less often than houses without? --John Glenn, N.T.A. (Not The Astronaut), San Francisco
Cecil replies:

You're thinking: What a stupid question. Everybody knows lightning rods work. People wouldn't have put them on buildings for more than 250 years (lightning rods were initially proposed by Benjamin Franklin in 1749) if there weren't a sound scientific basis for them, would they?

Well . . . maybe they would, maybe they wouldn't. We need to have a little talk.

Current U.S. lightning protection standards are embodied in a document published by the National Fire Protection Association known as NFPA 780. First issued in 1904 and updated periodically since, NFPA 780 codifies the traditional lightning rod installation, in which sharpened metal "air terminals" known as Franklin rods are connected to an earth ground by means of heavy conductors. A lightning bolt strikes a Franklin rod and is carried harmlessly to earth by the grounding apparatus, sparing lives and property. Though technically voluntary, NFPA 780 has been adopted by many local jurisdictions and government agencies and is the de facto national code.

Some years ago manufacturers of a new type of lightning rod using what's called early streamer emission (ESE) technology began agitating for a new NFPA code for their product, which supposedly requires fewer air terminals that can be placed farther apart than the traditional Franklin rods. An ESE terminal, described as looking like "a chrome cantaloupe with a chrome Frisbee around it and a little sharp prong on top and mysterious portholes around the sides," uses radioactive material to send a stream of ions aloft. This supposedly provides a path for lightning and guides it harmlessly to ground.

Most research to date shows that ESEs work no better than Franklin rods and fails to justify the claim that fewer terminals can be used. In 1993 the NFPA declined to approve proposed NFPA 781, which would have set ESE standards and presumably given the ESE industry a shot in the arm. ESE makers sued, claiming NFPA 781 had just as much scientific backing as NFPA 780. In a settlement the NFPA agreed to have ESE technology reevaluated by an outside panel.

The panel confirmed that there was no scientific basis for NFPA 781. But guess what, it said. There's no scientific basis for NFPA 780 (traditional lightning rods) either.

NFPA membership voted to approve an updated 780 anyway, but the NFPA standards council overruled them and announced its intention to rescind 780 unless somebody could give them good reason not to. While the NFPA wouldn't come right out and admit it when I called, I suspect the threat of another lawsuit was in the back of everybody's mind.

Proponents of traditional lightning rods freaked. A report from the Federal Interagency Lightning Protection User Group said, in essence, Come on, everybody knows this stuff works--if you rescind NFPA 780 it'll be back to the anything-goes days of the 19th century, when lightning rod con artists abounded. But there was an unmistakable air of desperation to the group's plea. Much of the evidence was from sources like the Iowa state fire marshal's annual report from the 1920s, which talked about barn fires. After 1950 research largely ceased; people just assumed traditional lightning rods worked.

They probably do work, but maybe not as well as was once believed. Critics of 780 point out that lightning remains a largely unpredictable phenomenon and that rods merely improve your chances rather than offering guaranteed protection, as the old code assumes. (Another beef: the committee in charge of 780 includes too many lightning rod manufacturers, who have a vested interest in maintaining the status quo.)

The NFPA standards council deferred a final decision "to give proponents of NFPA 780 an adequate opportunity to set forth the claimed substantiation for traditional lightning protection systems." Comments are currently being accepted; the group will decide what to do next at its October meeting. I'm betting the sparks will fly.

John T