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Re: [TCML] Lightning?

On 6/19/12 1:40 AM, jhowson4@xxxxxxxxxxx wrote:
Hello all,
At the moderators discretion I have a quick unrelated question for you guys,

A wicked storm just rolled through here in Rochester, and I was standing near the window looking out trying to capture a few nice bolts on my cell phone camera. well one happened to hit extremely close by. Close enough that I physically felt a spark jump from the window or the blinds or something in that area to my chest. Perhaps2-3 inches long. It felt something like a nice big vandergraaff shock. Scared the heck outa me.


I am trying to figure the radius of effect for a lightning strike, how close does one have to be to the main event in order to experience induced charging and the like. I will go out and look for damage tomorrow, but nothings on fire that we can see so i don't know what we will find.
The other option would be that I just so happened to be a part of a failed leader stroke and the charge reached out to connect with the main but didn't, Hench the small none killing me spark. and the main event was a little further away that I though.
I tried poking around the internet but I cant find anything that specific with number, just general "they can cause induced charges on nearby objects"

Maybe one of you guys knows more specifics on the subject and could share your knowledge,
Contact me off list.

It's relevant to TCs, because the whole area of induced voltages and currents is how things get fried by TCs, when we don't want them to.

First off, there's no specific "radius" for a spark effect, whether from lightning or something else.

several aspects to be concerned about..
First, any time you have a current flowing in something, there's a voltage drop, partly due to resistance and partly due to reactance (inductance mostly).

For lightning and TC sparks, which have fast rise times, inductance in a current path can lead to surprisingly high voltages. Consider that lightning (at the end of the stroke) is basically like a constant current source, so the di/dt is driven externally (nothing you do is going to change it).

The voltage is L di/dt.. for a typical stroke the rise time is around 1 microsecond. For a TC, the rise time on a spark is nanoseconds. However, the lightning stroke is on the order of 30kA, while I doubt the TC current is that high.

In any case, you're looking at a di/dt in the 1E9-1E10 amps/second range.

Inductance for most conductor shapes is pretty close to 1 uH/meter, so you get several kV/meter for anything that lightning is flowing through.

(including the soil!)

That potential difference can induce a voltage in nearby objects, just as standing next to a Van deGraaff generator or TC can induce a voltage in you. This can either be a classical electrostatic induction effect, or, I think, more like a capacitive coupling thing... you have a dV/dt of several kV/microsecond, and with even a few picofarads, you can have significant current flow, which in turn can lead to charging of a counductive thing that's insulated, otherwise.

Once that "thing" gets enough charge on it, it can spark over to something else at a different potential, or to the air (just like off a TC topload).


Then, there's the magnetic field effects. That kiloamp current flow creates a hefty magnetic field, even if it's only a "one turn" coil. That rapidly changing field can induce voltages in a "victim loop" formed by, say, the grounding wires on some equipment and the cables interconnecting them. (imagine a computer and printer, each chassis grounded thru the plug, with a cable between them, also grounded to each chassis)

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