Lightning conductors

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Hi All, I have been asked to fit a lightning conductor to a friends house extension (a 'tower' like extra floor in the middle of the existing house!). How should this be done, any special considerations etc, where do you get the conductor from, do you have to use copper 'tape' like you see coming down from church spires, does it connect to a normal earth rod?
Any help or advice gratefully appreciated, thanks.
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DON'T, please get advice
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Thanks James, but I was hoping this request would result in said advice. If not then I'll follow the sentiment of your reply, thanks again.

the
down
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If
Lightning conductors can 'ask' for a strike where none would have occured. KiloAmps can flow !. 'Step potential' can occur..i.e. you could electrocute passers by if your earth plate/rods are ineffective.
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On Fri, 24 Oct 2003 22:46:34 +0000 (UTC), "DKSanders"
_DON'T_ !
You have to design these systems (it's not hard, but it's more complex than you might think). There's a BS (6651 ?) standard you have to comply with, if it's to be seen as a "competent" system.
Earthing the things to BS (7430 ?) is as complicated as putting in the conductor, if not more.
There's also a working at height problem, which often involves renting a tower / scaffolding or whatever (and you now need sign-off by a qualified rigger if you erect scaffolding).
I used to know some of this stuff (I was working on GSM base station configuration) and it's not as simple as you might hope. It's really not something I'd want to go through "for a friend" - it's the sort of job that's a pain to do, it will take you longer (and maybe cost more) than someone who does it regularly, and the comebacks if there's ever any sort of lightning strike really will put you in the position of having been personally responsible for an act of God. On the whole, I'd rather replumb their gas cooker.
I'd probably do my own. But there's no way I'd do someone else's.
-- Die Gotterspammerung - Junkmail of the Gods
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As others have said, it is a job for an expert. FYI, you don't use ordinary earthing rods, but large plates buried in the ground, and the flat copper strip is chosen because, for transmitting lightning, surface area is more important than cross-sectional area.
Colin Bignell
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nightjar

Yes. I forgot to mention the flat plate in the earth. Or morte likley a few square meters of wire mesh with a solid plate in the middle
That spreads the charge around a bit and avoids localised hot spots of high voltage.

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wrote:

I installed one on an explosives store back in the 60s. I used inch by eighth copper strip and buried it in a loop around the whole building. Any joins had to be copper riveted and soldered. It must have worked okay since I never heard of any big bangs there until they shut the mine in the 90s.

-- Alan G "The corporate life [of society] must be subservient to the lives of the parts instead of the lives of the parts being subservient to the corporate life." (Herbert Spencer)
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Excellent! And it kept the elephants away too, right? ;-)
(No nastiness intended, Alan - jsut that it's hard to do realistic tests of a lightning conductor installation; hence the need you and others have pointed out to follow well-researched design codes. 'Course, you could always just slap something together based on misreading something in one manufacturer's brochure, but no-one in this NG would do such a thing. At least, no-one in *my* view of this NG. Ah, the peace of a small but perfectly-formed killfile ;-)
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On 25 Oct 2003 17:27:40 GMT, snipped-for-privacy@hp.com wrote:

And the tigers

You find out it doesn't work then it's too late. I reckon this one must have worked okay or you would have heard the bang 10 miles away. Possibly have debris rained on you 10 miles away too. There was a lot of stuff from the Nobel works in that store.

We put this one in according to instructions from the chief engineer based on regulations in mines and quarries legislation. I haven't the faintest idea how to design one. At the same time we installed an alarm system too. Until then the place only had a padlock on it. More innocent days then.

I have a large kill file but I inhabit a political group too. I mostly lurk in this one.
-- Alan G "The corporate life [of society] must be subservient to the lives of the parts instead of the lives of the parts being subservient to the corporate life." (Herbert Spencer)
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As in ICI Nobel Division?
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On 25 Oct 2003 19:47:38 GMT, snipped-for-privacy@hp.com wrote:

Yes. I think they were up in Scotland.
-- Alan G "The corporate life [of society] must be subservient to the lives of the parts instead of the lives of the parts being subservient to the corporate life." (Herbert Spencer)
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I believe so - memory augmented by Google suggests both Ayrshire and Dumfries. Presumably they put the plants in areas of relatively low population density ;-)
Always seemed a touch amusing to have the Nobel Peace Prize instituted to assuage old Alfred's guilt at inventing TNT, while ICI (which I believe bought up his business interests) kept the name going with the more original association!
Stefek (son of an ICI engineer)
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wrote:

The Stevenston ICI site (Ayrshire) is concerned with explosives manufacture. Dunno if that's its sole output though. This ICI explosives site is built on the coast with lots of sand dunes surrounding it (the folklore being that with sand surrounding it German bombs were less likely to explode when dropped onto sand during World War two).
Mungo
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DKSanders wrote:

Long, fat, and pointed, and as conductive as you can make it.
The protection partially comes from the conductor bleeding charge away from the tower and lowering teh potentil difference around it making it less likley to be a strike target.
Once struck, even the thickest copper strap is likely to melt. Strikes can take out power lines easily.
Just hope that MOST of the energy goes down it before it deoes.
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"The Natural Philosopher" wrote in message

Wrong. This is a myth, still perpetuated, it seems, by school physics teachers who ought to know better. To quote from BS 6551: "A lightning conductor is incapable of discharging a thundercloud without a lightning flash." Lightning protection systems are there to take strikes and divert the current safely to earth.

Utter twaddle. Lightning currents are high (98 percentile range 3 - 200 kA, median value 28 kA), but the duration is very short (typically 100 us), so the I-squared*dt integral is quite low. The temperature rise of the usual 25 x 3 mm copper conductor taking a 100 kA strike is only about 1 degree C. The energy dissipated as heat is about 400 J per metre of the conductor's length. Large conductors are used for reasons of mechanical strength and robustness, to withstand both the normal knocks they'll get on the outside of a building and the considerable magnetic forces arising during a strike.
The conductor(s) can carry strike current easily. The danger, and damage, come when the current leaves the conductor and forms an arc - in a 'side flash'. The peak power in the arc can reach 100 MW/m, heating the surrounding air to 30,000 K and initiating a shock wave. It's the shock wave that blows tiles off roofs, etc.
To avoid side-flashing you have to consider inductive (and mutual inductive) effects. The rate of rise of current in a strike can reach 200 kA/us, and hence will drop 200 kV across each microhenry of inductance - i.e. something on the order of 200 kV per metre of conductor!
Finally the earth system design is not trivial. BS 6551 requires an earth system resistance not exceeding 10 ohms. 200 kA through 10 ohms will drop 2 MV, so voltage gradients at the ground surface (between someone's feet, for example) become an important consideration...
--
Andy



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"Andy Wade" wrote | The conductor(s) can carry strike current easily. The danger, and | damage, come when the current leaves the conductor and forms an arc | - in a 'side flash'. The peak power in the arc can reach 100 MW/m, | heating the surrounding air to 30,000 K and initiating a shock wave. | It's the shock wave that blows tiles off roofs, etc ... | Finally the earth system design is not trivial. BS 6551 requires an | earth system resistance not exceeding 10 ohms. 200 kA through 10 | ohms will drop 2 MV, so voltage gradients at the ground surface | (between someone's feet, for example) become an important consideration...
Memo to self: Do not pee against a church tower during a thunderstorm.
Owain
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On Sun, 26 Oct 2003 13:27:37 -0000, "Andy Wade"

And also because of the skin effect. The current flow isn't using the centre of that strip, even at a mere 1/8" thick. -- Die Gotterspammerung - Junkmail of the Gods
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"Andy Dingley" wrote in message

True.
--
Andy



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Have a look at http://www.furse.com/home.htm these are the lightning conductor people in the UK and this is an interesting website. In fact you can download their risk calculator, you have to give them some info to get it.
However you can take the advice "dont touch with a barge pole" or you perhaps can come to some agreement with your friend not to be held liable if it all goes wrong!..
Most of the parts are available from Newey and Eyre the Air terminals they may have to get in, the copper tape 1 x 1/8" copper they normally stock and the extensible earth rods and couplers they normally keep too. FWIW we maintain some radio base stations and after a job a few years ago we had quite a bit of this stuff left over so I fitted our gaff up with a system, according to the strike risk calc we were in the "at risk" grade. We have quite a bit of electronic gear about too.
Back in June one morning this year there was a bad storm and there came an intense flash and explosion which must have meant that the strike was very close as the whole house shook!. Last month I had to upgrade our TV aerial and whilst up on the roof I noticed a burn mark on the Air rod which definitely was not there when I originally put it up so it seems to have done some good!.
Of course you have to be very careful when installing to make sure the routeing is as straight and short as possible and than you have the ground system just so to avoid any danger from step grade potential which means that there is a very large distribution of voltage potential around where your earth rod is, but there are ways to overcome this etc. Your earth system is a far better one than that required for electrical earth systems normally about 8 or so earth rods 1500 mm long are hammered deep in the ground or you could use an earth mat but this will have to go in at some depth. FWIW we managed 10 rods before we hit something solid!
But I somehow doubt your friend would stump up for a steeplejack to do this so it will probably not get done, so if it does get struck the discharge will go and damage all and sundry!...
--
Tony Sayer


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