If you have a metal roof on a house, should it be grounded?
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If you have a metal roof on a house, should it be grounded?
If it wasn’t for the Bank Payments,
Interest, Taxes, Wages, and Fuel Costs,
I wouldn’t have to charge you!!
Insurance industry research shows that stout roofs, strong windows, and metal connectors can help protect houses from high wind, hail, and hurricanes.
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Replies
If the birds arn't dropping then it might be ok.
Anything that's grounded is more likely to be a lightning target. That's why lightning rods work- they take the brunt of the hit instead of the structure they're mounted to.
I am just curious about the differences between grounding an antenna which is on a roof, and a metal roof which is not grounded.
Is lightning attracted to metal objects, or just objects which are highest off the ground?
If you had a standing seam metal roof which was not grounded, and a lightning bolt hit the roof, what would happen?
If it wasn’t for the Bank Payments,
Interest, Taxes, Wages, and Fuel Costs,
I wouldn’t have to charge you!!
Lightning is not "attracted" to metal objects per se -- it is attracted to an electrical ground. (Or, more precisely, to any body with a different electrical charge. That's why you often see lightning passing between two clouds -- the atmosphere in those clouds are at different charged states.) If a particular metal object happens to be connected to a ground, the lightning will pass through that metal conductor to reach the electrical ground IF that's the path of least resistance.
All parts of a building that happens to be resting on the earth are, to some extent, "grounded". Maybe with a very high resistance, but grounded nonetheless. If the house with the standing seam roof happens to be in the path of least electrical resistance between the charged atmosphere and the electrical ground, the lightning will pass through it, just like it would pass through a asphalt-shingled roof under the right conditions.
Then you smell smoke and get a new PC. ;-)
You ground the antenna so the path of least electrical resistance is the grounding wire, not the house to which the antenna is attached. Since the antenna is tall and pointy, it stands a higher than average chance to be the tallest part of the the lightning's shortcut the ground, so the lightning will strike there first. The only question is, what makes up the last 30' of the lightning's path to earth.
As to the OP's Q, as another poster pointed out, the surroundings of the shed are likely more important to determining the need for lightning protection than is the material of which the roof is made. Farm houses & barns are traditionally protected because they sit in the middle of a field and are likely the highest point for a long ways around, meaning they stand a good chance to be the shortest path to the ground for an electrical charge building up overhead.
Mike HennessyPittsburgh, PA
PS:
Edited 10/30/2007 1:13 pm ET by MikeHennessy
Hmmmmm,
I always thought the lighting rod allowed charge to get to ground before there was enough of a difference for lightning to occur. I didn't think the lightning rod was there to attract lightning, but to prevent it.
There's disagreement on the topic, and then disagreement on the disagreement. IMO, there are probably several modes of operation, varying by the location, type of lightning rods, and nature of the storm.
If your view never changes you're following the wrong leader
They're there so charges can dissipate but if it's going to build up enough, you'll have lightning. It can come from above or go up from the bottom. depending on the charge. Better to have the strike hit a dedicated conductor than hit house wiring, plumbing and things like that. Lightning suppression cable can't be within 18" (IIRC) of any electrical wiring, high or low voltage. I have seen houses that were wired before the lightning guys showed up and they strapped their braided cable to the electrical wiring bundle going up a corner. The inspector made them change that. With millions of volts, arcing can occur but distance decreases the likelihood.
"I cut this piece four times and it's still too short."
I think all bets are off when it comes to direct strikes with or without a lightning rod. I get to sell replacement RTU/radio equipment when they get damaged by direct and indirect lightning strikes. When I see the summer storms rolling across our weather report radar I am morbidly thinking "replacement orders.....Cha-Ching"! There was one memorable hit on a 100' radio antenna tower (giant lightning rod) that was about 300' from a control room in a water treatment plant. The tower was "properly" grounded with a strap on three legs, only one strap survived the hit. The antenna cable (7/8" foam heliax with three ground kits on it) was blown to bits, it was the only conductor going to the tower from the RTU panel (had a polyphaser on this cable inside the panel which of course didn't survive) which was inside an adjacent building. The RTU equipment in the panel was fried. The DIN rail plate on the radio had its screw heads blown off and the radio fell off to the bottom of the panel. All the monitors in the control room were a nice calm shade of solid blue. The operator on duty that night was not nearly as calm as the blue color on the screens. The guy I deal with said when he got there to help bring things back up about 45 minutes later the poor guy was still shaking. They hadn't yet found the yagi antenna that was on the tower when I was on site a couple days after the event. I don't think it will hurt anything to ground the roof as was asked initially, I was just cautious about overselling the ability of the lighting rod.
OK, OK, this isn't really directly related to the topic (not that that's required on Breaktime), but for some reason I thought of this regarding the danger of seeking shelter in the cave near the top of Yosemite's Half Dome: "In July of 1985, five hikers ascended up Half Dome late in the day and met with tragedy from two ferocious lightning strikes. Their story is documented in the book, Shattered Air. A True Account of
Catastrophe and Courage on Yosemite's Half Dome, by Bob Madgic (Burford Books). It recounts how the young men, full of enthusiasm and bravado, ignored nature's warnings and hiked up the famed cable trail right into the vortex of a fierce thunderstorm. They took shelter in the rock “cave” enclosure at the summit. Lightning struck the Dome twice, killing one of the hikers and causing a second to tumble
over the edge, out of the grasp of his best friend. Two survivors were gravely injured. Other hikers arrived at the scene and administered emergency medical treatment for over five hours deep into the
night. Finally, an air ambulance helicopter arrived in Yosemite Valley at 12:30 a.m., and in a race with the descending moon, made three dangerous trips to the top of Half Dome to bring the surviving
victims down from the summit.http://www.hikehalfdome.com/Accidents.html
I was living in the Bay Area when that happened and my recollection is: they were a group of inexperienced city kids from nearby San Jose or something. They went up to Yosemite unprepared, ignored the park service personnel's warnings to stay on the ground and then got whacked by this storm.
There was an uproar, with some people blaming the park service and complaining that it should never have been allowed.
Very unfortunate but completely preventable. My memory could be wrong but I distinctly remember thinking that it was a group of inexperienced people who were out of their depth and didn't know enough to realize it.
Ed
Edited 10/31/2007 7:02 am ET by edlee
Lighting rods don't attract or deter lighting. What the do is present the least resistance to groung. There for if lighting is headed your way it will usulaay go the path of least resistance. It won't attract more lighting to it. Jay
Lightning rods are low(er) resisitance and are designed so they will attract the lightning and at the same time reduce the size of the strike. Many small strikes instead of one big one.
As charges build up in the air above it, electrons start flowing from the earth up through the rod and into the air , or vice-versa depending on the polarity. This attracts and develops a path for the lightning which eventually will release like a spark once it has ionized enough air. The plasma created can be hotter than the surface of the sun!
The rods are pointy because it's been found to be the best physical shape that allows this to happen....it's easy for electrons to fly off the end.
I read that most people who study this stuff buy into the "rolling ball" model of where the lightning strikes: not necessarily the highest poin't around, but instead picture a giant ball rolling. It generally hits the taller objects first, but sometimes a shorter one that is closer to the underside of it.
Ed
Edited 10/31/2007 6:46 am ET by edlee
Answer #1: Why, did it stay out past curfew?
Answer #2: Yes. Required by code, I believe, and a good idea in any event.
I grounded mine. Not really concerned about lightning, the reason was to make sure that I'd trip a breaker if the roof came in contact with a hot line from such things as Christmas lights or extension cords and tools in use up there. Around here, those are far more likely events than a lightning hit. Based on that thinking, I'd ground any really large piece of metal in the building. I don't know if code says anything about it, the inspector wasn't concerned either way.
-- J.S.
I think code is primarily concerned that a fastener from the siding-roofing may penetrate a mispositioned power cable and cause the metal to become energized. Plus a half-dozen other ways that a large hunk of metal can become "hot", such as the ones you described.
If your view never changes you're following the wrong leader
Do you know of any specific text in the NEC that applies? I didn't find any, but I didn't look too hard.
-- J.S.
I've never found it in the NEC (heck, I couldn't even find how to wire a duplex outlet in the NEC), and it may instead be in building codes, but I've seen it mentioned several times, in contexts that implied the writer had a reasonable degree of knowledge on the topic.
If your view never changes you're following the wrong leader