Metal Roof~Can you be electrocuted????? (AC, vacuum, insulated, installed)
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I am purchasing a home in Russellville, Ky which is a historical home, with a new metal roof. I have never had one of these...and know nothing about it.
I am wondering, when there is lightening, can you be electrocuted?????? I know it is probably a dumb question, but just wondered, since Ky. gets lots of rain and lightening.....and ice storms.
I don't think you have anything to worry about, never heard of anything like that happening. I've lived in several houses over the years with metal roofs. My roof will need replacing in the next 2 or 3 years and I will be going with a metal roof rather than a shingle roof like it currently has.
If the roof has a ground strap, you are safer under it that just about anywhere. The concept is that it forms part of a Faraday cage, where the electricity goes around the protected space.
FWIW, lighting is not "attracted" to metal. Lightning is miles long, and doesn't pick a spot in the last 100 feet. When lighting is in an area, the ground has a charge and the cloud has a charge. When the charges become localized, a number of false leaders (electrically charged fingers of air and ions) will reach up from points on the ground at the same time the bolt is trying to find an easy downward path. One or more of those false leaders will connect with the downward bolt to make the real leader, and the sudden increase in current will cook the air, making the current flow even easier, which then ionizes and blasts out the remaining air in the very small shaft. The superheated ionized air creates a near vacuum, and when the air rushes in to fill the void, you get a thunderclap.
A lightning rod is designed to make a false leader in a spot that it is safe for the lightning charge to go. In some cases, when the false leaders are numerous enough, they can "bleed off" enough of the ground charge to the air to reduce the ability of the main lighting bolt to form. Here is an example: Lightning Porcupine Static Discharge, Fulgurites, Fusion
If you have a metal roof, you want the vent pipes that pierce it to be plastic, and you want a nice big wire or metal strap leading directly (without any sharp bends) to a ground rod. Once the electricity finds the easy path to ground, it'll follow that route. If you want to add a porcupine on the roof or nearby pole, that would be a good thing.
Wow, thanks for all the good info. I will check into my roof further, and see if it is grounded.....good info to know about lightening and such. Appreciate you sharing all your knowledge.
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Harry - That's called "skin effect", right? The idea that electricity would only travel along the surface of a metal conductor. Its the same reason you are safe from a lightning strike in your car.
Question about lightning rods and grounding cable for your home's metal roof:
A lightning bolt would be in the hundreds of millions of volts (maybe more), would 9' of cable really make much difference in the strike's path?
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Quote:
Originally Posted by jimboburnsy
Harry - That's called "skin effect", right? The idea that electricity would only travel along the surface of a metal conductor. Its the same reason you are safe from a lightning strike in your car.
The rubber tires in a car insulate the car from ground. Without any ground, the lightening won't strike. Electricity tries to find the easiest path to ground. Ground straps and such provide an easier path than going through the house. But a car is not grounded at all. Thats why, when the weather is cold, you can get a nasty shock when you open your car door. You are gounded (feet on the ground), but the car could have built up electricity trying to find a way out.
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Quote:
Originally Posted by rubytue
The rubber tires in a car insulate the car from ground. Without any ground, the lightening won't strike. Electricity tries to find the easiest path to ground. Ground straps and such provide an easier path than going through the house. But a car is not grounded at all. Thats why, when the weather is cold, you can get a nasty shock when you open your car door. You are gounded (feet on the ground), but the car could have built up electricity trying to find a way out.
With the kind of voltages involved in a lightning strike, I'm not 100% certain that a car looks very insulated. Aircraft, as an example of a platform that is truly completely ungrounded, are sometimes involved in the path of a lightning strike. What (in theory anyway) provides a modicum of safety is the metal cage surrounding you. Linesmen wear hot suits (which I think are actually called "Faraday Suits") that have conductive fibers woven into the fabric to allow them to work on live transmission lines without injury based on the same theory that current will travel along the fibers of the suit rather than through the linesman's tissues.
jimboburnsy, the 9' of cable wouldn't make any difference in the sky path of the bolt, but would make a tremendous difference in the path between roof and ground. Lightning is a form of electricity that we aren't used to, so it can seem to do some strange things. Not only is it high voltage, but it is pulsed, partly AC and can be at a number of frequencies, which can change the characteristics. Electricity has a skin effect in general (tendency to flow along the boundary of a conductor rather than the interior). High voltages can overcome the resistance of some materials and allow lightning to use them as a conductor.
That brings up rubytue's comment about car tires. While the rubber in a tire is non-conductive by itself, a large portion of the carcass is carbon black, which gives the rubber better wearing and stiffness properties. Carbon isn't an insulator, and car tires are better described as resistors than insulators. Normal static, especially on a dry day, can't easily bridge the gaps and resistance. On a wet day, with a skin of water and the high voltage of lightning, that resistance is easily overcome.
If you think you car could be struck by lightning, not touching the metal of the interior and driving with only the right hand is probably a good idea. The bulk of the charge will go around the exterior (unless the car is fiberglass) but there can be transient currents throughout the body of it. Remember that the dry air inside a car is more of a resistance than the wet air outside, so part of the protection comes from being partly isolated by the air, seat cushion and clothes, and minimal contact with the frame itself.
If you are trying to make a path for lightning, you want a metal strap that has nice slow curves and no sharp turns, which cause the field around the turn to turn on itself and resist the flow. If you have a rooftop antenna or satellite dish, the installer will often make a small coil of the excess wire. There is a purpose to this - the coil acts as a choke to a major surge of lightning. Hopefully, as it is being resisted there, it will find an easier path through a ground strap or cable.
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Quote:
Originally Posted by harry chickpea
If you are trying to make a path for lightning, you want a metal strap that has nice slow curves and no sharp turns, which cause the field around the turn to turn on itself and resist the flow. If you have a rooftop antenna or satellite dish, the installer will often make a small coil of the excess wire. There is a purpose to this - the coil acts as a choke to a major surge of lightning. Hopefully, as it is being resisted there, it will find an easier path through a ground strap or cable.
That's very interesting. I remember asking the guy who installed our satellite why he was doing that and he didn't know. I also noted that the cable leading from old TV antenna that I subsequently removed had a tightly wound coil just outside the soffit penetration. Data assimilated.
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