Lightning Strike


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Old 06-30-05, 12:36 PM
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Lightning Strike

Hi All,

We had a lighting strike at our house. 2nd one in less then 6 Months.

Details, Struck a tree about 150' from our house. Dug a 12" trench from the base of the tree for about 25' and stopped. Dug into the ground and completely melted 1.5" PVC conduit I use to bring power to my barn. (#6 THWN wire with ground in conduit). Actually the conduit does not exist for about 2' where the strike occurred. The barn is at a higher level then my house so all the run off water pulled into the now open conduit and dumped into my basement out of the junction box. It looked like a pipe burst in the basement. I was so surprised at the water pressure. Many devices damaged in the house along with all the outside motion lights. My well was struck as well.

Is there anything I can do to protect the house systems better? Why did the lightning seek out my conduit in the ground. Heck, the wires are all encased in PVC. Very strange. It's not like there are trenches going out in all directions from the tree. Just one with my conduit being the target.

Thanks
Bob
 
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Old 06-30-05, 12:39 PM
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I don't believe that there is anything you can do to protect against a lightning strike that close.

Thanks for the interesting story.
 
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Old 06-30-05, 02:03 PM
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Originally Posted by bob_m
My well was struck as well.

Is there anything I can do to protect the house systems better? Why did the lightning seek out my conduit in the ground. Heck, the wires are all encased in PVC. Very strange. It's not like there are trenches going out in all directions from the tree. Just one with my conduit being the target.
Lightning is so incredibly more powerful than anything man-made. Lightning can charge up the ground for hundreds of feet around an actual strike, so it wasn't so much that it sought out your well or conduit, but that those things got charged to the level of the rest of the ground and happened to conduct better than dirt. A lightning strike can easily be several hundred million volts, and at that potential PVC might as well be copper because the juice is going to go through.

As John said, there is nothing you can do about a strike that close other than be glad it wasn't any closer. For more general protection, consider installing a whole-house surge protector and using individual surge protectors on valuable devices. Those will help with lightning strikes at greater distances.
 
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Old 06-30-05, 05:12 PM
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Lightning can do some crazy things. It hit a neighbors maple tree next to his shed. It followed the tree roots and blew chunks of sod on the shed roof, threw a propane tank over, made the chains hanging in his tree for his hammock weld together. He had an extension cord touching the ground to power his bug zapper, it hit that and blew out the bug zapper, melted the power strip it was plugged into inside the shed and blew the inside breaker. His spouting elbows on the ground were both crimped shut too.

I wonder if it could have setoff that propane tank, the sod blowing off the tree roots is what threw it over.
 
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Old 06-30-05, 07:56 PM
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Don't have multiple grounds. A lightning strike conducts thousands if not millions of amps through the ground. And as Ohms law states, if you have current flow through a conductor (in this case the earth) you have a voltage drop. If you have multiple grounds (for instance, an earth ground at the barn and another at the house) you develop a potential rise between the two ends and any connected device sees the difference. Cooper Industries has some excellent articles on this if you can access them.
http://www.cooperpower.com/Library/T...df/line897.pdf
Scroll to the "Storm Trapper" article.
Another reference is http://www.cirronet.com/pdf/lightnin...protection.pdf

A lot of people think of lightning coming in on the power lines. In a lot of cases, a direct strike rises the ground potential above the line voltage and actually causes a surge from ground to the system voltage (not from hot to ground). GE has some excellent articles on this.

If you get a direct shot, your goose is cooked. This happened to me even with surge protection and did about $795 worth of damage (yes, my homeowners policy has an $800 deductible). But fortunately, most strikes are not right on top of you. Invest in a whole house surge arrester to be installed at the main panel. Then invest in individual units at anything of value (your PC, DVD, TV, well, etc.) Surge arresters work better the closer they are to the device they are trying to protect.
 
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Old 06-30-05, 09:12 PM
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Umm, in this situation you are essentially _required_ to have multiple grounds.

You are required to have grounding electrodes at both structures, and these grounding electrodes must be connected together. Increased lightning risk, perhaps, but there are presumably other reasons that makes the code require this grounding.

-Jon
 
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Old 07-01-05, 03:02 PM
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>but there are presumably other reasons that makes the code require this grounding.<

Now I am wondering if I should remove the ground Rod at the barn. Why does code require a ground for sperate bulidings? Assume we carry ground out to the building, as in my case.

In my strike, the ground wire inside the condiut seemed to be the target. It had the most damage.

Thanks Bob
 
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Old 07-01-05, 03:10 PM
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Do not remove the grounding rod at the barn.
 
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Old 07-01-05, 04:38 PM
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I agree with John. Do not remove the ground rod. The NEC articles are based on safety, not whim. There can be a significant difference in the the ground potentials across distances considering differences in the resistance of a grounding conductor and of the earth itself. The purpose of the separate ground rod for detached buildings is to eliminate (or mitigate) that difference in potential.

I work for a large phone company and we have a whole engineering department dedicated to grounding and bonding. Millions of $ a year go into keeping our stuff from getting zapped just here in the lightning capital of the world. It is a good investment.

Lightning is the explosive discharge of electrons. When lightning _strikes_ earth, it is actually the opposite. The electrons are discharged from the earth due to a build up of static electricity and a difference in potential with the atmosphere - or even another part of the earth. And when those electrons need to go, they leave any way they can. I'm guessing that your tree happened to be the point of departure at the time and the wiring was probably just in the way of nature.
 
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Old 07-01-05, 07:18 PM
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"In my strike, the ground wire inside the conduit seemed to be the target. It had the most damage."

This would indicate a difference in potential between the grounds. However, I also agree to not remove the grounds (and apologize if my previous post suggested violating code). Codes developed for personal safety don't necessarily work the best for electronic components. I'd always rather fry a DVD than my kid!

Also, MOST lightning does not hit that close and the judicious use of quality surge arresters will alleviate most of your problems. Remember, If lightning hits right on top of you, none of the above will help.
 
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Old 07-04-05, 05:51 PM
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How to power your detached garage without a ground

If you want to be able to power your detached garage or barn without a ground that connects back to the house, one way to do that is with a generator. You could use a non-metallic pipe to feed natural gas out to the detached building.

A more bizarre idea I thought of a while back was to power the building by pumping a non-conductive fluid through a pair of heavy duty plastic pipes, and drive a generator on the circulating flow.

The reason I have thought about these things is that I want to set up a ham radio shack detached from the house, with multiple towers going up perhaps 60 meters or more. The lightning risk will definitely be present there. Some things can be done to minimize it, or minimize the impact of a strike, but the risk will always be non-zero, and generally substantial in such a setup.

Running wires to the house adds to the risk. In addition to possibly spreading the damage from a strike, high electrical currents can develop underneath strongly charged storm cells passing over, even though no strike actually occurs. The longer your run, the more current that can be present due to a greater jump across the moving ground voltage gradient the wires make.

So my present plan is to forego all electrical connections between the house and the hamshack. I'll start with a propane generator, and maybe add natural gas. For telephone, I'll probably use VoIP over wireless LAN, and eventually lay in fiber optic (the transmitters could pose problems for the wireless LAN as much of my radio interest is the UHF/SHF bands). The circulating power fluid idea is a pipe dream, but something I do intend to investigate further. But I won't give up the propane generator in any case, because I would want to be able to operate the station even when utility power goes out.
 
 

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