Lightning!

[pete_c]

Frederick, glad no one was hurt.

My sister had a tree hit by lightning about a week and a half ago. It was not a very large tree (less than 20 feet in height) and about 30 feet from the house. It didn't cause any electronic damage. The tree though had to be removed as the lightning split the tree in half.

In the 1970's I helped a friend install a CB antenna on the roof of his ranch style home. The antenna wasn't that high and was on a tripod. We were in the process of connecting it and had not connected the earth ground (lightning arrestor) when a storm started to brew (it was only grey clouds). We did get off the roof though and went into the house. It hadn't started to rain. The other end of the antenna cable was about 1-2 feet away from a 120VAC outlet. We sat there in his family room for a bit and all of sudden we started to see an arc from the antenna coaxial outlet to the 120VAC outlet. It happened a few times in the next 15-20 minutes. No rain outside; just very grey and only some lightning and thunder. The arcing to his 120VAC lines didn't cause any harm but there were really not too many "electronics" plugged in to his home at the time.

 

[fcwilt]

few times in the next 15-20 minutes. No rain outside; just very grey and only some lightning and thunder. The arcing to his 120VAC lines didn't cause any harm but there were really not too many "electronics" plugged in to his home at the time.

I gather there was no direct strike? The nearby lightning was inducing this voltage into the antenna and cable?

 

[pete_c]

Yes I think it was maybe the conditions and only nearby lightning which caused the sparks as I saw. I have never seen anything similiar to date since then.

The voltage though must have been pretty high and have to imagine that a direct strike has even much greater impact depending on path as mentioned above.

 

[apostolakisl]

That is scary. I can't imagine what I would think watching it. Do you try to unplug stuff, get the heck away, jump into a rubber ball? Must have been a bunch of static electricity. I have felt my hair tingle and stand up a bit in thunderstorms and know that people experience that a lot. As I understand, that type of static electricity is how lightening warning systems work.

 

[JeffVolp]

Earth is by definition ground and all lightening strikes have to end there. It is not only the best ground, it is the only ground. Everything that is properly grounded in your house is connected to Earth by a conductor (wire or metal pipe). It is very common for ligtening to hit a tree and travel down the tree and then jump off the tree at some point through air and hit something else nearby, like a house. I suppose it is possible by some bizaare soil conditions that you have a patch of Earth in your yard that is electrically isolated from the rest of the planet, but I doubt it.

What you say is partially true. The lightning bolt will end at the earth. However, where it goes when it hits the earth depends on soil resistance. Soil conductivity out here in the desert is very poor, and elaborate grounding systems are needed to achieve a good ground. A single ground rod will generally do the job back east where the soil is generally moist.

If the bolt encounters a lower resistance path as it is heading toward the water table, it is entirely possible that some of the energy will be diverted along that path. There have been numerous reports of damage caused by lightning getting into a home through underground wires or pipes. That is one of my big concerns here due to the irrigation control lines that run underground.

Jeff

 

[apostolakisl]

Earth is by definition ground and all lightening strikes have to end there. It is not only the best ground, it is the only ground. Everything that is properly grounded in your house is connected to Earth by a conductor (wire or metal pipe). It is very common for ligtening to hit a tree and travel down the tree and then jump off the tree at some point through air and hit something else nearby, like a house. I suppose it is possible by some bizaare soil conditions that you have a patch of Earth in your yard that is electrically isolated from the rest of the planet, but I doubt it.

What you say is partially true. The lightning bolt will end at the earth. However, where it goes when it hits the earth depends on soil resistance. Soil conductivity out here in the desert is very poor, and elaborate grounding systems are needed to achieve a good ground. A single ground rod will generally do the job back east where the soil is generally moist.

If the bolt encounters a lower resistance path as it is heading toward the water table, it is entirely possible that some of the energy will be diverted along that path. There have been numerous reports of damage caused by lightning getting into a home through underground wires or pipes. That is one of my big concerns here due to the irrigation control lines that run underground.

Jeff

OK. I can buy that. If your house is sitting on a pile of silica sand that is dry, you are essentially sitting on a great electrical insulator (effectively glass) isolating your house from Earth. Effectively all of the bare metal in the ground is as though it were not bare but rather insulated and could therefore carry electricity from place to place. I have never lived anywhere where soil conditions were such. I have lived in Ohio, Michigan, Iowa, Florida, and Texas. Perhaps the word "unusual" soil conditions rather than the word "bizarre" might be more appropriate. Of course for the pipes and whatnot to carry the electricity into the house that must mean that the house is along the path to some other plot of Earth that can accept the electicity. Maybe it would come in the pipes (not grounded depiste being burried) to get to the metal stuff in your house that is grounded (deep grounding rod?). It would seem that the proper thing to do in that situation would be to provide a proper ground for those pipes and other conductors just before they enter your house, even though they are underground. The same as I currently do for anything entering my home above ground.

I have read tons about lightening and am well aware that is still a subject of great mystery. Superhigh voltage does have a mind of its own. Even if you ask the engineers who design and build the high voltage transmission lines, they will admit that the science doesn't fully explain all of the observations and that is a much more testable/observable environment than lightening.

In the end, the electrons are just trying to find the easiest path to a place where they can cohabitate with protons and be nice happy and have a sum neutral charge. Just try to be sure you and your stuff aren't on that path!

 

[JeffVolp]

If your house is sitting on a pile of silica sand that is dry, you are essentially sitting on a great electrical insulator (effectively glass) isolating your house from Earth.

Actually, our house sits on a lava flow about 30 feet thick, with sandstone below that. Out of curiosity I just measured the resistance of the lava. Even with the probe tips only 1/8" apart, it still read infinity on the megohm scale. That probably explains why we have no local electrical ground for the electrical system. The only ground reference for the house is through the utility feed. (The water line is plastic due to the corrosive nature of the local soil.)

Much of the ground out here is just broken down sandstone. It also reads infinity when dry. Since we can go months without any measurable rain, it is bone dry most of the year.

Jeff

 

[apostolakisl]

If your house is sitting on a pile of silica sand that is dry, you are essentially sitting on a great electrical insulator (effectively glass) isolating your house from Earth.

Actually, our house sits on a lava flow about 30 feet thick, with sandstone below that. Out of curiosity I just measured the resistance of the lava. Even with the probe tips only 1/8" apart, it still read infinity on the megohm scale. That probably explains why we have no local electrical ground for the electrical system. The only ground reference for the house is through the utility feed. (The water line is plastic due to the corrosive nature of the local soil.)

Much of the ground out here is just broken down sandstone. It also reads infinity when dry. Since we can go months without any measurable rain, it is bone dry most of the year.

Jeff

Living on lava is probably pretty unusual. I guess the conduction would depend on what the magma was made of. If it had iron it might be a good conductor, but I guess not your lava. My old house in Tx had a 6ft rod and a utility ground which were wired together and also to the house plumbing. I am actually not sure about my new house as it is all hidden off in the woods but I don't think I have a utility ground. I should go look. I did spike 3ft rods where my cable tv comes into the house. I also have a whole house surge supressor where the electricity enteres the house. The water is not metal and the phone is via my HAI GSM adapter. I do have my sprinkler control wires coming in without ground. I don't know of any way that you could ground them as they don't have metal shielding that is groundable. It is plugged into my Elk panel so it could be expensive if there were a surge. How do people ground sprinkler control lines? Maybe they could be fused?

 

[pete_c]

That is scary. I can't imagine what I would think watching it. Do you try to unplug stuff, get the heck away, jump into a rubber ball?

We just told my friends wife (and kids) to get out of the family room. We sat about 20 feet away on a couch; had a beer and watched the light show.

 

[cornutt]

The red clay that we have around here can reach a pretty high resistance if there's a long period of dry weather. If it dries out enough, the first couple of rains afterward just run off the top, and it takes a while for it to hydrate again. Local code requires ground rods to be at least 12' to ensure that they reach wet soil.

Then there's Florida. Well-grounded soil is easy to reach... but the salt in the ground water plays hell with grounding systems. I was involved some years ago with a spacecraft processing facility that got hit by lightning one weekend and a bunch of space flight hardware was damaged. Investigation revealed that every one of the building's ground rods had been corroded down to a nub -- the only part left was the part you could see above ground!