Which material has high enough ohmage to resist lightning?

I don't know why I'm bothering to respond to this troll, but....

It's not a question of simple resistance in ohms. Look up "breakdown voltage" for further information.

Bob M.

Around a million? Not several millions? Cloud to ground strikes can be several miles long and range into the hundreds of millions and billions of volts, and tens to hundreds of thousands of amps.

Lots of electrical texts have charts for dielectric strength. I doubt that one insulator will be right for all applications. Dielectric strength is specified in volts per mil, so even poor insulators, if thick enough, can stand off high voltages.

What was your idea? Surround something with an insulator to protect it from lightning? Surround what?

Lightening is a fast rise time pulse(s). The insulator, even if it withstands the strike, will likely couple enough energy to the "protected" device to cause damage

All the practical systems of lightening protection I'm aware of, deal with directing the strike away from something, or bleeding off the space charge before a discharge can occur in that area.

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Ohhhh ! Not you again you brain dead nitwit.

Just try *thinking* about the stupidity of your question will you ?

Graham

I would think a vacuum would do it.

Sometimes it's better to "go with the flow" and "re-route" the lightning, rather than attempt to block it.

Consider surrounding the device to be protected with a grounded conductor and forget about insulators.

Chuck

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I thought that a vacuum had relatively weak (electrical) insulating properties, to say.. air for example?

Mark

A hard vacuum is very good. The lowest breakdown is at some low pressure, a small fraction of atmospheric as I recall; after that point, better vacs are better insulators. A perfect vacuum is a perfect insulator, and the only limit is when the field strength gets so high that ions are ripped out of any metal electrodes that are present, at vaguely around 1e8 v/m. Tomographic atom probes use this effect; see Imago.com.

An interesting effect is secondary emission in metallic surfaces, which can become a positive feedback effect for AC. See "Farnsworth multipactor."

John

Probably in the billions of volts. I was just guessing.

Okay

I was just curious

Anything.

Are you referring to damage caused by the EM waves emitted by the pulse?

True.

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Doesn't a vacuum has very *low* resistance? Or are you referring to the stuff around the vacuum? What is the insulation covering the vacuum made of?

Yes.

True.

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Insulators are dielectrics they will transfer energy just like a capacitor charging. Fast rise time is similar to high frequency - large high frequency component in the wave form. One plate is the ionized gases surrounding the insulator, the other the protected device.

And as you point out EMP

I was on a beach stringing a wire from my bike to a tent to run a reading light. A strike that was easily 10 miles away induced enough voltage in my wire to give me a shock.

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An ohmeter I would expect to read infinate resistance, but at the voltages we're talking about (ie involving lightning) isnt the breakdown voltage much lower than air?

It would be damn hard to manufacture vacuum tubes and CRTs if you were right.

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Vacuum can't break down; there's nothing to conduct with.

Electrodes in a vacuum can break down, namely emit electrons or ions from their surfaces, but that's different.

John

Once again, with feeling: do not confuse "insulating properties" with resistance ALONE. Think "breakdown voltage," which is something else altogether.

Bob M.

A vacuum has an extremely HIGH resistance, as long as you are talking about conduction in the normal sense. Again, go look up "breakdown voltage" and learn something for a change. Up to the breakdown point, I think you'll find that the I vs. V curve for a reasonable amount of separation in a vacuum is pretty damned flat...

Bob M.

Damn, John, now you're REALLY going to send Radium, et al, down a rathole.....

I'll get the popcorn...

Bob M.