lightning strike

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zotdoc
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lightning strike

Post by zotdoc » Fri Feb 13, 2004 9:32 am

I have a forced air multifuel (wod,trash etc.)heating unit that was scratch built by a machine shop. It has a 12 inch steel chimney pipe and is located in my storm shelter, a large metal building with a concrete "bunker" inside. The "oven" sits on an 8" concrete pad, and is not grounded. The steel pipe will stick above the roof of the metal building for a foot or two. I haven't erected the pipe yet. Should I be worried about lightning hitting the pipe? do I need to take any special precautions? Any help you can give will be appreciated.

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Chris Smith
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Re: lightning strike

Post by Chris Smith » Fri Feb 13, 2004 9:47 am

Your concrete pad in it self is a good ground. Attach a Wire if Necessary to it?<p>Touch a 120v hot lead to it and watch the sparks fly? <p>The fact that your exhaust pipe is relatively short, means that unless you live on a high mountain top, lightning is less likely to strike a lower object, before it hits the top of your roof?

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Externet
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Re: lightning strike

Post by Externet » Fri Feb 13, 2004 11:28 am

Hi.
It would not be a bad idea to connect the metal chimney to a grounding rod in moist soil outside the bunker.
Miguel
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haklesup
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Re: lightning strike

Post by haklesup » Fri Feb 13, 2004 2:11 pm

Given that you'll be using the shelter during a storm when lightning is likely, grounding the pipe to a ground spike is worthwhile even if it is not absolutely necessary. <p>Assuming the stove is bolted to the pad and the cement is in intimate contact with the earth, it could provide an adequate ground. However, the resistance is still quite high, meaning that if you are toutching the stove and a better ground (like a water pipe) you would provide a better ground and the current would prefer your (at best the voltage and current is divided). Then again, if the stove is hot, you shouldn't be toutching it.<p>Anyhow, There's no doubt that a ground wire or a real lightning rod will provide better protection than none. You simply want to create a lower impedance path to earth than you would be. Human body impedance is about 3k Ohms and less if you are wet or unclothed.<p>By the way, the steel building will also provide excellent protection by being a fariday cage. Make an electrical connection between the smokestack and the building exterior and you probably don't need the ground wire unless there is any chance the building might blow off the bunker in a tornado.

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jwax
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Re: lightning strike

Post by jwax » Fri Feb 13, 2004 3:24 pm

Metal roof, metal chimney? Sounds like a lightning magnet to me! So, add a lightning rod, and copper cable as directly down to earth as possible. Concrete is a poor conductor! Do not rely on it for a "suitable ground"!
John

Dean Huster
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Re: lightning strike

Post by Dean Huster » Fri Feb 13, 2004 9:07 pm

Lightning is nothing but magic and is nothing short of totally unpredictable, with the possible exception of lone trees on golf courses and Little League games.<p>We lived on the highest hill for miles around in the NW Oklahoma City area (Piedmont, to be exact), and our house sat on the tippy top of that hill. Beautiful view of all of the OKC area from our back porch. We had ferocious lightning/thunder storms come through time and again where I'd swear that lightning bolts would come in through one bedroom window and go out the other, but in 10 years, the house was never struck by lighting. It did strike the barbed-wire fence (steel posts) that was around the acreage, but never the much taller house.<p>Then we moved into OKC itself. Our 5th wheel travel trailer was parked on the driveway, backed up next to the garage door, plugged into "shore power". That first year there, we took a lightning strike that hit the gutter, went down the downspout and then took a parallel path: (1) into the dirt where it blew out a cubic foot and splattered it all over the garage and back of the trailer; (2) into the trailer where it took out the CB transceiver, Norcold reefer and electric element in the water heater; and (3) into the house where it took out the electronic combo lock I'd built for the garage door, the door opener electronics (4 separate components), a telephone and an answering machine. Didn't touch any VCR, TV or the alarm system and we didn't have a computer at the time. Managed to repair the door opener, replaced the telephone stuff, repaired the Norcold, but never could find all the shorts in the combo lock or get the CB going again.<p>We'd originally moved to the city because the wife was afraid of living out in the sticks and it would be cheaper to live in town because of less driving mileage. Right. Restaurants and shopping became 'way too convenient, house and car insurance both went up, house payments doubled, and we were broke into twice. Wimmen!<p>Dean<p>[ February 14, 2004: Message edited by: Dean Huster ]</p>
Dean Huster, Electronics Curmudgeon
Contributing Editor emeritus, "Q & A", of the former "Poptronics" magazine (formerly "Popular Electronics" and "Electronics Now" magazines).

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Joseph
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Re: lightning strike

Post by Joseph » Fri Feb 13, 2004 9:41 pm

I would go with the lightning rod. That way you may avoid roof or chimney damage even though there is a chance that the EMP would get to electronic items.

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dacflyer
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Re: lightning strike

Post by dacflyer » Sat Feb 14, 2004 9:12 pm

ok, i know the so called rules...
grounding directs lightning etc..
but i have found that grounding actually has drawn lightning..had 3 antennas obliterated cause of grounding... this last antenna i have isn't grounded.. and has survived 8 yrs now...<p>i wouldn't ground the building and stove..unless you plan to be in it :p
but otherwise...its just metal...
but as for a house and electronics..by all means ground them.

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MicroRem
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Re: lightning strike

Post by MicroRem » Sat Feb 14, 2004 9:41 pm

POTENTIAL is what attracts a strike, and a chimney, grounded or ungrounded has the same POTENTIAL. One way or another, the lightning will get to ground. A heavy guage wire connected to a good copper grounding rod allows the energy top get to ground with less collateral damage. As a US Coast Guard Chief in upstate Newy York (Lake Ontario)I saw more than one sailboat get a hole blown in the bottom by a lightning strike when the mast was left "ungrounded" in hopes of avoiding a strike. Sailboats with grounded masts usually had damaged elctronics but intact hulls.<p>Tom

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Re: lightning strike

Post by bodgy » Sun Feb 15, 2004 1:32 am

<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>Originally posted by usteservice:
(Lake Ontario)I saw more than one sailboat get a hole blown in the bottom by a lightning strike when the mast was left "ungrounded" in hopes of avoiding a strike. Sailboats with grounded masts usually had damaged elctronics but intact hulls.<p>Tom<hr></blockquote><p>Now that's an interesting thought, how do you ground a boat that is in the water?<p>Leave it potentially floating perhaps? :) <p>Colin
On a clear disk you can seek forever.

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MicroRem
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Re: lightning strike

Post by MicroRem » Sun Feb 15, 2004 7:18 pm

hmmm, as I recall when your draft exceeds the depth of the water, you are grounded, ususlly followed by a haul out and repair.... :confused:

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jollyrgr
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Re: lightning strike

Post by jollyrgr » Wed Feb 18, 2004 4:31 pm

This is an interesting question. I have a fireplace that is a modular unit. In other words the fireplace was manufactured and is basically a metal box. The side walls and floor are cast ceramic of some sort. A wooden frame with foam insulation, covered by vinyl siding forms the chase for the metal chimney. There is no ground anywhere on this that I can see. The chimney is a double wall metal pipe that goes above the roof line. Should this be grounded? <p>I cannot see any obvious wire or grounding stake anywhere near by on the outside of the chase. Nor is there a ground wire inside the house leading to a cold water pipe or the electric service box. So what does the NEC say about this?
No trees were harmed in the creation of this message. But billions of electrons, photons, and electromagnetic waves were terribly inconvenienced!

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Joseph
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Re: lightning strike

Post by Joseph » Wed Feb 18, 2004 7:25 pm

With metal jutting above the house, I would think that if, being a high point, that it might tend to accumulate charge and attract a strike, in which case it may pass and arc through combustibles. I see the reason for your concern. I guess I am more cautious now after being a victim myself.

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haklesup
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Re: lightning strike

Post by haklesup » Thu Feb 19, 2004 1:34 pm

I don't think that NEC has anything to say about structural protection from lightning except to how it pertains to protecting the active electrical distribution system like the breaker box and service connections.<p>In general, only the local building code would require lightning protection and probably only for public buildings. Check with your local city hall to see if there is a requirement for new construction in your town.<p>National Fire Protection Association has plenty to say on lightning protection in its publication NFPA 780. (nfpa.org for $33) (there was a link to the draft of the 2004 revision but it didn't work for me)NFPA 780<p>I could not look up the details since both publications are for sale and the content is not on the web. For the price, it might be a worthwhile purchase since you live in a storm prone area and are designing/building a protective structure.<p>You should be able to find a copy at the library or visit your local fire marshal for advice, he should have a copy. (Try the link, it might work when you try)

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