Lightning Rods

[Hunt Stoddard]

What do people think about lightning rods around here? A person I know actually saw ball lighting in their house after a strike. Are metal roofs supposed to be grounded? Are actual lightning rods better than a grounded roof?

[TomK]

A lightning rod is much better than a grounded metal roof. Charges tend to congregate at a pointy conductive point. Next time you fly, look at the aircraft's wings and see small metal spikes at the back end of the wings - they're there to dissipate the electric charge when flying through clouds. Then again, having a lightning rod does not guarantee that's where lightning will strike, but it improves the odds. I certainly wouldn't be standing near a lightning rod if there was a nearby thunderstorm though.

[Hunt Stoddard]

According to "the internet", having a lightning rod won't increase the chance your house is hit (which would kind of spoil the point of it).

I'm not exactly sure I buy it. I would think that having a low resistance path to ground WOULD increase the likelihood of getting your house struck, but then having the grounded rod would make the strike less destructive and dangerous.

So it would seem they're a double edged sword? Or what?

[geochem]

The lightening will follow the most conductive path to ground - without a lightening rod, that might be your power wiring, water pipes, internet cable (if you are still using copper) etc.; with a lightening rod, it will be the heavy conductor that is grounded (protecting your wiring, piping and everything connected to it) and preventing arcing inside the walls or under the house that could start a fire.

Long ago, on the mainland, I had occasion to replace the thermostat for the furnace - when I pulled it off the wall and drew some of the wire out that connected it to the furnace, I found several feet of the insulation blown off/melted. I suspect that it was the result of arcing over of a lightening strike (of which, over the years, we had several near our home).

Bottom Line: the purpose of the lightening rod is to reduce the likelihood of that happening.

[My 2 cents]

Would a grounded TV antennae work in a similar way?

[MarkP]

I have never understood how a lightning rod is supposed to work. Lightning can leap across miles of clear air, melting things and exploding trees when it does. I think that if you actually get directly hit, you're screwed. You would have to have a copper conductor the size of a tree to conduct the energy and even then it would be glowing red hot after the strike. However direct hits are rare. The extraneous EMP and stray electricity around the actual lightning covers a huge area though. That is what you can hope to mitigate.

I think.

[TomK]

"Would a grounded TV antennae work in a similar way?"

It might, but I wouldn't trust it. If it took a direct hit from lightning it probably wouldn't be able to handle the load and so the lightning goes elsewhere. Lightning rods help, but they don't guarantee your house won't be struck.

When I get the chance, I'll provide a link to some videos in Colorado from scientists studying lightning strikes. From memory, they did everything they could to attract lightning yet it still didn't always strike where it should have done. At least in theory.

Bottom line - a regular TV cable won't have the capability to safely disperse a lightning strike. I know, I've experienced it!

[Hunt Stoddard]

quote:

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Originally posted by MarkP

I have never understood how a lightning rod is supposed to work. Lightning can leap across miles of clear air, melting things and exploding trees when it does. I think that if you actually get directly hit, you're screwed. You would have to have a copper conductor the size of a tree to conduct the energy and even then it would be glowing red hot after the strike. However direct hits are rare. The extraneous EMP and stray electricity around the actual lightning covers a huge area though. That is what you can hope to mitigate.

I think.

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Kind of my thoughts as well. I found an average lightning strike current on the internet as 30,000 amps, so even if 0000 wire is used to ground a rod, which has resistance of .00245 ohms per 50 ft.


Power dissipated in the grounding wire would be I**2 x R = 2200 kilo watts.

That's energy expended per time, and it would be over in a fraction of a second, but still, imagine turning on a two million watt bulb inside your house. Not something I would want to do.

[TomK]

That's why lightning is so bright! And so dangerous.

[terracore]

I have a TV antenna on my roof at the end of a 10' steel pole. Is this a lightning rod? I shopped for a lightning surge protector to keep any strikes from traveling down the coax to my TV but there are so many products on the market for this I don't know where to begin. Any recommendations?