Lightning delay

[Suudy]

I have to agree, in principle, with Mr Goodman. However, it is a lot more complicated than that. My college degree was in electrical engineering, though I admit I don't work in a field now that applies any such knowledge to high voltage situations (I'm a digital guy working at 5V or less).

We spent plenty of time on examples of applying Gauss's law to conducting enclosures. The best example is our professor sitting in a large wire mesh cage, i.e. a Faraday cage, about 5' x 5'. A student was challeged to try and shock the professor using a Tesla coil. The Faraday cage dissipated any charge before it could reach our professor. This exhibited the fact that the electric field inside a conducting enclosure is 0. Electricity always flows along the field lines of an electric field, from high potential to low potential (i.e. voltage). No electric field, no flow of electricity, no getting shocked.

However, I will say that all the examples we worked with, the gaps in the mesh were so close that they could effectively be ignored. Thus, for all purposes of calculation, it was a solid conductor. In the lightning example above, the gaps between the conductors may be too large to ignore. In this case, there may be an electric field in and around the field that could make it dangerous. I agree with NOAA, that the probability of being struck by lightning is extremely low, but not low enough for me to be confident standing on the field in a lightning storm. And also, with the possible electric field in and around the field, the ground currents, even if lightning struck a tower, could be dangerous.

[Robert Goodman]

Originally Posted by waltjp

Oh, you mean if they enclosed the road - sort of like a domed stadium?

No. I'm saying that the way those tall metal masts surrounded the field and stands (Mitchel Atheletic Complex, Nassau County NY) was as good as the way the metal pipes, etc. in a bldg. surround its occupants in terms of lightning protection. The roof on a bldg. adds nothing, because it's not conductive, and has no pipes or wires running thru it -- unless there's an antenna.

Well, unfortunately, the road wasn't enclosed and neither was your football field.

If the ratio of the height of the surrounding conductive structures to the width of the road was like the field at Mitchel, the road would've been as well protected as a bldg.

Robert

[Robert Goodman]

Originally Posted by Suudy

I agree with NOAA, that the probability of being struck by lightning is extremely low, but not low enough for me to be confident standing on the field in a lightning storm. And also, with the possible electric field in and around the field, the ground currents, even if lightning struck a tower, could be dangerous.

But at least in the basement of a bldg., the ground currents problem would be the same as out in a field. If you can get off the ground floor, or into a vehicle with insulating tires (rubber), then you're safe from the ground currents from a nearby strike.

But with a ring of metal masts all pointing in the same direction, the potentials at the time of a strike on any of those masts will be such as to carry ground currents away from the ring, not thru the ring. The middle of that ring is the safest place to be.

Could it be better with wires overhead? Sure, then you'd really be in a cage. But most bldgs. don't really provide a Faraday cage. Fortunately for DC, it doesn't matter.

If you could see the height and spacing of those towers compared to the size of the field they surrounded, you'd agree.

Robert

[Suudy]

Originally Posted by Robert Goodman

But at least in the basement of a bldg., the ground currents problem would be the same as out in a field. If you can get off the ground floor, or into a vehicle with insulating tires (rubber), then you're safe from the ground currents from a nearby strike.

I agree with this. But getting off the ground floor protects you because when you go up a level, you are now protected by a fully enclosed cage (i.e. there is wiring and plumbing below, in the ceiling of the room below). But I am suggesting that the field with a ring of high towers may not provide the same protection as a Faraday cage.

Originally Posted by Robert Goodman

But with a ring of metal masts all pointing in the same direction, the potentials at the time of a strike on any of those masts will be such as to carry ground currents away from the ring, not thru the ring. The middle of that ring is the safest place to be.

Your assumption is that all the masts are connected electrically to form this ring. And that the connections are large enough to support the current of the lightning strike. If either of these are not true, then there is potential for the currents to flow from one mast to another, possibly through the center of the ring.

But, like I said, it is _highly_ improbable. But not high enough for me.

Originally Posted by Robert Goodman

Could it be better with wires overhead? Sure, then you'd really be in a cage. But most bldgs. don't really provide a Faraday cage. Fortunately for DC, it doesn't matter.

It doesn't matter if it is AC or DC. Electricity _always_ flows from high potential to low potential along electric field lines. In a Faraday cage, there is no electric field, thus no flow of electricity.

But I would argue that a building does come close to approximating a Faraday cage. There is wiring and plumbing in the ceiling. Thus each room (except possiblethe ground floor) are close approximations of Faraday cages.

But even in the ring case I'm not sure that the current would always flow away from the ring, unless the rings were connected electrically and there were connections below the playing field.

Originally Posted by Robert Goodman

If you could see the height and spacing of those towers compared to the size of the field they surrounded, you'd agree.

I don't know. Like I said, I agree in principle, but the reality of the situation is different. And besides, even if it was safe on the field, would it be safe for those near the towers? Such as the stands? Or the parking lot? There would have to be a pretty wide berth around those towers' bases to ensure safety.

[Robert Goodman]

Originally Posted by Suudy

Your assumption is that all the masts are connected electrically to form this ring.

No, I'm just assuming they're up in the same air, and that there's nothing "funny" (insulating) about the ground between them.

And besides, even if it was safe on the field, would it be safe for those near the towers? Such as the stands? Or the parking lot? There would have to be a pretty wide berth around those towers' bases to ensure safety.

That's what I noticed about that field. The stands were inside the ring of light towers. They built them really tall, too, which is good for lighting a game; less chance of looking into the lights when you look up.

Robert

[Suudy]

Originally Posted by Robert Goodman

No, I'm just assuming they're up in the same air, and that there's nothing "funny" (insulating) about the ground between them.

That's my point. The ground is the insulation and the conductor. An insulator is nothing more than high resistance. Get the potential (i.e. voltage) high enough, and all insulators become conductors. Thus during a lightning strike, the ground becomes that conductor.

In order for it to function as a Faraday cage, i.e. to eliminate the possibility of any electrical field being present within the ring, they would have to be electrically connected with a low resistance path. The ground would be the electrical connection, thus would be the path for any discharge. Anybody between the towers would be at risk. A lower resistance path, such as heavy gauge wires between the towers, would protect against these currents.