Sitting in the living room having an after dinner glass of wine with wife and a friend and Bam!! Really came out of nowhere, it was raining but hadn’t heard thunder or seen lightning during the storm. Anyway, it was close but not a direct hit. 15 minutes later my wife smells something burning. Investigated and down in our basement/crawl there was something smoke. Turns out it was the rigid/foil faced insulation I had wedged up between the joists as insulation for radiant tubing. It was out by the time I looked but had singed about 3 feet of the blue covering that faced out. We ripped it out to be sure, but there was no real damage. Fire dept. came and checked for hot spots and we were good.
What I can’t figure out is how it got in. Electrical system was fine, lights didn’t even flicker. Right below the damaged insulation was a cold water pipe. As it passed below 4 joists in a row, it left some burns along the bottom of those joists and eventually made it to the polyisocyanate. Don’t understand how or why it did damage in the middle of a plumbing run. Any ideas? We’re on a public water system.
Another lesson is that the polyisocyanate is flammable. There is no furnace/boiler down there and its just plumbing and a few wires in the basement. I thought it was a low fire risk………. stupid!
Replies
First off, understand that the incident you had was kind of a one-in-a-million event -- the same lightning strike could occur again 100 times with no damage, or could cause something entirely different.
Next you need to understand about "induction". Induction is what happens in a pole transformer when they convert from, say, 14,400 volts to 120 volts. The "secondary" coil in the transformer that feeds 120 volts might have a thousand turns in it. The primary, in order to convert down, would have a thousand turns times 14400/120 turns, or 120,000 turns. This accomplishes the conversion of voltage down and also current up (both by a factor of 120).
Thus lightning doesn't really need to hit anything to cause problems. When lightning strikes near a metalic object (might be, eg, the "mast" for your incoming power) there is induction between the lightning bolt and the metalic object. Since the "turns ratio" between the two is 1:1, there is no (theoretical) voltage reduction, nor is there any theoretical current increase. But a lighning bolt is maybe 50,000 amps to begin with, and even though the transfer between the bolt and the metalic object is less than perfect there may be a few thousand amps flowing very briefly (a few tens of microseconds).
In your case the lightning may have actually struck the water pipe, or only close to it. But in either event there were thousands of amps flowing in the pipe for maybe 50 microseconds. This current then ran close to another metalic object -- the foil-faced insulation. Aluminum is a fair conductor, but its electrical properties are slightly weird when subjected to electro-magnetic fields with a very rapid risetime (which is what you have with lightning). A very high current was induced in the foil facing, and that current instantaneously heated the foam and set it to burning. (The water pipe likely only got warmer by a few degrees.)
Had the foil not been quite so near the water pipe, likely nothing would have happened. Inductive effects diminish with the square of distance.
Great explanation
What's the chance that a ground strike could result in current coming inside on a buried metal pipe and causing what the OP described?
"What's the chance that a
"What's the chance that a ground strike could result in current coming inside on a buried metal pipe and causing what the OP described?"
Lightning jumps from the clouds to the ground. If sometimes passes through things on the way to the ground. But it can't terminate inside a house. It always goes to the ground.
It takes the path of least resistance. Sometimes that involves travelling through metal parts of a house in order to get to the ground. But the destination is always the ground. So it couldn't come from the ground through a pipe and back into the house.
So it couldn't come from the ground through a pipe and back into the house.
The ground is not a perfect conductor (far from it). And there's a lot of juice in a lightning bolt. Even if only 10% of the strike gets conducted through the pipe that's more than enough to start a fire. Like the fire I mentioned where the lightning struck a tree and was conducted into the house on a brass propane pipe (the cause thoroughly investigated by the local arson inspector and the insurance company), lightning can follow a pipe for considerable distance underground.
Though there's also the chance, of course, of lightning hitting, say, a cast iron vent pipe on the roof and getting conducted into the house that way, then out on the water pipe. Same difference, ultimately -- either way the pipe (or any other metal object) can conduct lightning on a destructive path through the structure.
Thanks for the reply.
Current jumping from a water pipe to the foil faced insulation sounds feasable to me and is a likely explanation. We were kind of lucky in that it just flashed over a 3' section of insulation and went out on its own; could have been worse.
As for the one in a million. This is the second time in the last 4 years we've been hit. First time was a bit more direct and it fried a few components, switches, receptacles etc. So, it seems we've picked a good location up here on top of the hill with an old stone quarry nearby and our basement carved out of native stone. So, I'm left with preparing for another strike that may or may not ever come.
Putting in that foil faced was a real pita, given that half of my basement is crawl, the other half not too much better. I'd just as soon leave it in. It is about 3-4 inches above the bottom of the joists and any plumbing. What would you think about nailing up a layer of flashing above the few plumbing runs. Would this just serve as another conductor and make it worse, or would the higher ignition pt. make it much safer? How about some rubber pipe insulation? Just trying to think of easy solutions as if this ever happened again, I'd feel terrible if I hadn't tried to remedy it.
Also, I've been told that grounding water systems is no longer recommended (mine is not). Whats the feeling on that? I've has 2 inspectors tell me 2 different things. As of now, my only grounds are in my electrical system via rods outside.
Thanks
Note that it wasn't current "jumping" from the pipe to the foil, it was induction -- the current through the pipe induced current in the foil the same way an inductioin stovetop heats a metal pot. No "sparks" needed to fly between the two, and insulation between them will do no good -- what you need is distance. I'm guessing there was less than an inch between the two, and doubling that distance would cut the induced current by a factor of 4.
You could put heaver metal (with no flammable surfaces in contact) between the two to absorb the heat, but it would be a little tricky to get right. Or remove the foil-faced insulation near the pipe and use insulation without a metalic face (and otherwise non-conducting).
In terms of grounding the pipe, the place to ground it would be on entry to the area. This would reduce the tendency to conduct strikes into the house.