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Commercial building code in Ohio requires that metal roofs be grounded.
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>Should a metal roof be grounded? What is best method?
*If you live in an area where there is a lot of electrical storms, you may want to consider lightning rods on your roof with proper grounding cable to the ground.An old idea that works.Gabe
*Electricians around here in New England sometimes will bolt a piece of #6 copper wire to a metal roof and then run it to the nearest electrical panel, which of course is already grounded. It's not a requirement though.I would think that during an electrical storm there could be substantial inductive currents in a large metal roof that would seek a ground path through some metallic building system (like electrical, plumbing) Lee
*I don't think there is an electrical requirement that the roof be grounded since it is unlikely to become energized. If lightening protection is a concern this should be done properly and not just by a ground wire to the roof.
*The technique I mentioned of bonding was a requirement in the Mass. building code for a while (that the roof be bonded to the electrical system). I didn't mean to imply that it would substitute for a fully-fitted lightning protection system, though I guess that's how it read.Lee
*Yes, we are in Central Texas and we have lots of lightening strikes around here--usually the tallest structure will get it. Our concern was whether the metal roof might actually attract lighting.Sure appreciate the input we received.
*I am not aware of any code requirements in my area for steel roofs to be grounded. I have had customers concerned about lightning protection, and I am lucky enough to know a good lightning rod company in my area to refer them to. As Gabe says they are an old idea that still works.Just as an aside, I never sub-contract to him. I just refer and let the customer deal directly with him. Call me chicken, but that way if there ever is damage due to a strike, I don't get stuck in the middle of a liability action. What little mark-up I could make wouldn't be worth the hassle.John
*I have a lightning rod system on the metal roof of my barn. The main rod sits on the cupola, on a tripod. One ground wire goes down to the ridge on each side, runs under the ridge cap to the gable end where a second rod stands, then down into ground, not sure if it's a driven rod or water pipe. I'm not quite as worried about it since the %$$holes at Sprint were so kind to put up a 19 story tower across the street about 400 yards. That should do a real good job of attracting all the close lightning strikes.MD
*I'm in California where lightning rods are unheard of so what I have to say may be way off the mark.I can't understand why a person would ground their roof, creating a perfect route to ground for an extremely high energy electrical current. If the roof were not grounded, wouldn't it be less likely to become a conductor? And the suggestion to connect the roof to your electrical panel ground seems ludicrous. I mean, the house wiring is not sized to handle the current from a bolt of lightning! Erecting a nearby metal tower or rod a little taller than the highest point of the house seems much safer.I've heard the best place to be in a lightning storm is your car (without grounding straps}, since it's isolated from ground with rubber tires. This makes sense.common back you guys,Dave
*David,I've wondered about this too. But with regard to grounding a roof, It does pay to provide a safe path to ground for lightning, especially if the building is among the higher elevations around, and also if it has a metal roof. If indeed it becomes slightly more attractive to lightning, so be it. This means that less chance of lightning striking other nearby, ungrounded objects (like the roof of a house). Besides, lightning that close to be attracted would probably strike anyway, and at least this way, some protection is provided, and will greatly reduce damage or injuries.Also, when they mention connecting to the house panel ground, they are routing it directly to the ground, not through the house wiring. Many houses in our area have lighning protection installed on the box, connecting to the ground. It's not ludicrous.MD
*The theory behind lightning rods is that they actually discourage strikes by not letting the stucture build up the static charge to attract the lightning in the first place. Providing a lightning stike with an unimpeded path to ground is a secondary function. Dissipating the electrical charge is the primary function.John
*Thanks John,I didn't know that. Makes sense though.MD
*Hey Dave, I think most of your misconceptions were corrected in the other posts, but they missed one old wive's tale: your car is safer not because of rubber tires, but because of the "skin" effect of electrical current flow. Current doesn't flow uniformly through a cross-section of a conductor, but tends to flow through the outer edges, and this phenomenon generally increase as the voltage increases. The electrical charge doesn't penetrate the metal "cage" of the auto, rather it flows through the outer surfaces and then to ground, bypassing the occupant.Oh yeah, another one they missed is the concept that a conductor is rated for a specific amount of current flow regardless of the conditions. The ampacity of a conductor has a lot to do with it's resistance,which changes with temperature, and ability to dissipate heat. A wire that maxes out at a load of say 15 amps to run an electric heater for a couple of hours might very well be able to handle 5000 amps for 1/2 a cycle of 60hz AC current. Thus a #6 or #4 copper ground wire can handle some of the huge currents generated by lightning which occur in the nano-second range. I installed an 10,000 amp-rated lightning arrestor on a 400a service, and the arrestor conductors were 12-gauge. I'm no expert, just an electrician who finds lightning to be an interesting thing. Lee
*Commercial building code in Ohio requires that metal roofs be grounded.TNB
*Just out of curiosity, if the metal roof isi notgrounded, and lightning strikes it, what happens?Rich Beckman
*The lightning keeps going finding it's own way to the ground.
*Here's a funny fact about cars and lightning: Tires contain enough carbon-black (this is added to make them black, natural rubber is not jet black until after it has been colored) in the rubber to make them conductive! They conduct well enough that 'grounding straps', besides looking ridiculous, and being useless (imagine grounding your house by laying the strap on the ground for one or two inches), are not needed.If you get a shock as you exit your car look at the upholstery on your seats, the materials they are made of generate static electricity as you slide over them to create quite a wallop sometimes (depends on the material your clothes are made of too). This has been looked into by the auto makers and someone even invented a carbon-fiber impregnated fabric that would disipate the static (but it cost too much)!If you care about this (maybe you have vinyl or leather in your truck so no problem) you can get another brand of shoes - depending on the sole material some 'ground' you better than others (but this might raise other issues...) so that when your foot touches the ground the static goes out that way before you touch the metal door to push it closed. Another countermeasure would be to hold on to something metal (a chrome plated inside handle or the door frame) to 'ground' yourself while sliding - exiting (but sometimes you can still feel the current tingling its way through your fingers, better but not perfect).Hey, just remembered seeing a photo of a car being hit by lightning (they had this lightning generator set up to test things like that) and the car did ground directly from the metal bumper to the ground, but that was not due to any insulation from the tires, just that it was the shortest way! (kinda dates the photo too, remember chrome bumpers?)As they say,"This Roman Meal Bakery thought you'd like to know"
*I haven's seen grounding straps on a car since I last saw a curb feeler.
*Ha Ha Ryan, took the words right out of my hand!
*You don't live near Detroit!My favorite is the 'roller skate' cars - slammed with the widest offset rims possible. Can't imagine what condition the suspension must be in after a few years of that kind of abuse. Usually Escorts and other little cars subjected to this, so they are pretty much throw-aways after 4 years anyway, I guess, so what the heck.Anyway, that kind of thing is why I mentioned 'grounding straps' is all. (silly car stuff)
*Running a wire from a roof to an electrical panel seems like a bad idea to me. If the roof takes a lightning hit, then every plug-in and light switch in the house has lightning delivered right to it. A 3 prong grounded tool that was plugged in would be about like holding onto the metal roof. I know that the ground cable from the elictrical panel should deliver most of the current to the ground. But what if it's undersized, broken , or poorly connected ?
*Yeah Ron, I was thinking the same thing. Grounding may make sense but Attaching the lightning magnet right to the TV doesn't sound like a good idea.
*The National Electric Code section 250-106 requires bonding between a lightning protection system and the building grounding electrode system. If the roof has lightning protection it is required to be bonded at some point. This keeps both systems at an equal potential. Lee
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Ed ; A correcttions to your corrections: Frequency, not current determines where most of the current is carried. Low frequency is carried throughout the bulk of the conductor (hence stranded and solid 12-gauge has the same ampacity at 60 Hertz because it has the same amount of copper). High frequency is carried primarily on the surface. Lighting is multi-frequency. some of it very high.