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  #1  
Old 09-05-2004, 12:20 PM
I_Know_Nothing I_Know_Nothing is offline
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Is it dangerous to drive over a fallen power line?, and random power questions.

I was watching footage from hurricane Frances and saw a downed power line laying across a street. When a truck came by, it drove off the road to avoid driving over the power line. Was this neccessary?

What about if a power line falls on the top of your car?

Is your car really one of the safest places to be in a lightening storm?

Why can't a city just shut down power stations when they know a hurricane is on the way? Wouldn't it be safer?
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  #2  
Old 09-05-2004, 12:31 PM
danceswithcats danceswithcats is offline
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Hey, there's a wire in the street. Is it telco, cable, or power company? Is it 120 volts to ground, or 33kV? Treat all downed power lines as you would a firearm-assume them to be loaded/energized until it can be proven otherwise.

Should a power line fall atop your car, you are safest inside the vehicle. Should you attempt to exit the vehicle, when your foot touches the ground, you're no longer a sparrow on the line, you've completed a circuit, and at that point it sucks to be you.

During a thunderstorm, I'd say that inside a building is safer than being in your car.

As far as shutting off all power, that won't preclude whatever damage the storm will inflict via wind, downed trees, and so forth. The economic impact would be greater-cash registers don't work, neither do fuel pumps, fresh foods will spoil, etc. than leaving the power on and letting Mother Nature roll on in.
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Old 09-05-2004, 12:42 PM
GorillaMan GorillaMan is offline
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Quote:
Originally Posted by danceswithcats
During a thunderstorm, I'd say that inside a building is safer than being in your car.
There's nowhere safer in a thunderstorm than inside a car (no everyday place, anyway). It forms a faraday cage, so the current from a llightening strike passes around the metal shell and down to the ground.

I've seen footage on a science TV programme, where a guy drove back and forth under a multi-kilovolt-charged electrical source and above a grounded one. Each time he drove under, 'lightening' struck the car, and flowed all over it. Only when it destroyed the car's electronic ignition did they have to cut the power.
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Old 09-05-2004, 12:46 PM
matt matt is offline
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A downed power line might well still be live, especially the lower voltage lines.

If a cable drops onto your car, your tires should insulate you from any electric shock risk by preventing the current from running through your car to ground.

Even if you have no tires and are running on the rims, the metal body of the car should route any current around you rather than through you. The power guys who work on the hi-voltage cables wear a protective suit of metal mesh, like chain mail. This seems very counter-intuitive since the metal is a conductor, but it routes any current around the body rather than through it.

If you run over a downed line however, the cable itself may be physically hot and damage your tires. Also, the weight of your car on the cable may improve its electrical contact with the ground causing a lot of rapid resistance heating right under your wheels. Do a good enough job of this and you could get a minor explosion - not car-launching quality, but probably enough to blow your tires.

Cars are indeed one of the safest places to be during a thunderstorm, for the same reasons mentioned above re a cable dropping onto your car. Although in a hurricane, I'd rather be in a cellar.

Shutting down the local power during a hurricane could be done, but there must be good reasons why it isn't. There's a lot of board members who will be much better able to answer that one than me - I'll wait for one of them to come along.
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Old 09-05-2004, 01:22 PM
danceswithcats danceswithcats is offline
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Quote:
Originally Posted by GorillaMan
There's nowhere safer in a thunderstorm than inside a car (no everyday place, anyway). It forms a faraday cage, so the current from a llightening strike passes around the metal shell and down to the ground.

I've seen footage on a science TV programme, where a guy drove back and forth under a multi-kilovolt-charged electrical source and above a grounded one. Each time he drove under, 'lightening' struck the car, and flowed all over it. Only when it destroyed the car's electronic ignition did they have to cut the power.
An automobile fails to satisfy the definition of a Faraday Cage, based upon my understanding. 'No apertures' and 'perfectly conducting material' do not apply to an auto. Doors, deck, and hood are not well bonded to the body. Hinges and latch or nader pin are the only electrically conductive connections-otherwise a rubber gasket exists to keep out rain and wind. The tires act as effective insulators between the vehicle body and ground.

An elemental Faraday cage can exist in a building with electrically bonded rebar but only if wall and floor assemblies are tied together in such fashion as to provide a low impedence path to ground.

If my understanding is flawed, please enlighten me.
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Old 09-05-2004, 01:32 PM
GorillaMan GorillaMan is offline
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Quote:
Originally Posted by danceswithcats
An automobile fails to satisfy the definition of a Faraday Cage, based upon my understanding. 'No apertures' and 'perfectly conducting material' do not apply to an auto. Doors, deck, and hood are not well bonded to the body. Hinges and latch or nader pin are the only electrically conductive connections-otherwise a rubber gasket exists to keep out rain and wind. The tires act as effective insulators between the vehicle body and ground.
OK, so it approximates a Faraday Cage. It certainly does so a lot better than most everyday structures (particularly buildings, given that the various wires through to their interior make lightening strikes inside buildings possible).

The ground clearance of the body of the car is only inches - the insulating effect of the tyres is irrelevant, as jumping that distance is no problem for lightening. The gaps in the conductivity you describe are also across very small distances. The structure car does ensure that the path of least resistance does not go through the interior.

(Incidentally, how would you build a true Farady Cage, using "perfectly conducting material"? At room temperature, of course...)
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  #7  
Old 09-05-2004, 02:08 PM
barbitu8 barbitu8 is offline
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Inside an automobile is very safe as long as you don't touch anything made of metal in it.
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  #8  
Old 09-05-2004, 02:28 PM
Carnac the Magnificent! Carnac the Magnificent! is offline
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Quote:
Originally Posted by barbitu8
Inside an automobile is very safe as long as you don't touch anything made of metal in it.

No time to provide a link, but within the past 2 months, I heard about someone being electrocuted from a fallen power line--while in his/her vehicle.

No other details, but I remember being surprised to hear the report.
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