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.
DON'T, please get advice
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.
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
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
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.
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.
In article , DKSanders writes
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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!...
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)
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 ;-)
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)
As in ICI Nobel Division?
--------------- The general consensus seems to be that this is not a wise thing to attempt. I concur.
Does your friend live atop a barren hill? Do they really need, or simply want, one of these things? Are they aware of the downside of having one?
A conductor will attract the very strikes they seek to avoid. Which can be quite dramatic, scary in fact.
I lived in South Africa for quite some time and I must say the electrical storms around Johannesburg are spectacular, and quite dangerous to boot. Those of us that lived in houses of bricks and mortar with tiled roofs had little to fear, not so for those that chose the picturesque thatch style. Storms, and fireworks, were a worry. While little could be done to combat fireworks much was done to combat lightning.
This meant having a dirty great metal pole or two sticking up way higher than the rooftops. Rather like giant tapered flagpoles. These flagpoles were situated a metre or two from the outer walls. And they were sufficiently high that the building being protected fell within a 60 degree cone as drawn from the point of the rod.
They did work, mostly. But they draw lightning like you can't believe. With one of those things you get to see, hear and smell all the effects, real loud and real quick. The thatch dwellers had little choice as the gasses given off by wet thatch render them at risk. Your friends perhaps do have a choice.
With a lightning conductor you can be reasonably sure that you will get struck. Again and again. So I think a risk assessment might be in order because the fright you get when you do get struck is not pleasant.
Just wondering aloud - there is an awful lot of instantaneous power transferred when lightning makes contact. I imagine that this would fry any puny lightning conductor on the first occasion.
Presumably lightning conductors are designed such that they can carry the enormous load the instant of a second?
PoP
Then there is the noise from explosive expansion of the ionised air and the stromng smell of ozone. B-)
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)
Correct. I have a graph somewhere of the current flow during a lightning strike. The vertical axis is calibrated in (units of) kiloAmperes. Fortunately, the horizontal axis is calibrated in (units of) milliseconds.
Friends who have had amateur radio antennae struck have had them vanish altogether, or had to sweep up the congealed droplets of metal afterwards.
I certainly wouldn't consider installing a conductor myself. It is entirely possible to make the situation worse.
My father had a 23' mast antenna on top of his 3 story house. When lightening struck, it vapourised the glass fuse in his radio equipment. It didn't just melt - it simply disappeared. The noise was terrifying, and the smell wasn't great, either. My mother refused to have the antenna reinstalled.
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...
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