[Petrochemicals]

Yes, I know, but why would it be a steel frame, it's not a high-rise, and it's not a large span, it's just a traditional building like the vast majority of the others built in that era. And equally importantly, if a steel frame's protecting the building why put a lightning conductor on it.

To stop it popping through the roof across the ceiling and down into the darling children.

All the lightning conductors I ever recall seeing are flat copper strips. Braiding it won't make any significant difference to the skin effect because the strands aren't insulated from each other, which is why radio engineers use Litz wire to reduce skin effect. (At MF/HF, above that the usual ploy is to use silver plated copper.)

I agree, but the skin effect is misleading subject, it is only AC related. Highly unlikely that  1cm squareof copper is rated for the conduction of 100,000 amps, by memory I think 10mm^2 is rated to about 300amps, more likely a guide to lower the resistance, the electricity will probably mostly pass around the metal, I doubt very much a thin skin of aluminium is actually conducting such current 

use tinned copper wire

So non molecular binded metal still acts as one piece?

[Bored chemist]

I think 10mm^2 is rated to about 300amps

For how long?
It's important to remember it only has to carry the current briefly, and it doesn't matter much if it gets damaged.

but the skin effect is misleading subject, it is only AC related

The electrons can't know if the steep rise of voltage they see will be followed by another series of rises (and falls) in an AC circuit, or be a one-off like a surge.
The skin effect unquestionably is important for transients.

I doubt very much a thin skin of aluminium is actually conducting such current 

Reality  isn't too bothered what you believe.

[Bored chemist]

Digressing a bit, a novice mistake for radio engineers is to use tinned copper wire, in which most of the current flows through the lossy plating instead of the copper, or bare copper wire, which has the same effect by the time it's oxidised.

By that logic, the worst possible idea would be to use copper covered by a layer of air- which is very lossy.

The skin depth is proportional to the square root of the resistivity.
So, for  bad conductors like copper oxide, there isn't (usually) enough thickness of material to carry the current and some is "forced" into the copper below it.
Tin is a "good enough, bad conductor" to have an effect.


Silver plated copper is pretty so they use that a lot.

[paul cotter]

"tinned copper" is commonly used rf equipment up to 900mhz, in my quite extensive experience, not having worked with any higher frequencies.

[Bored chemist]

rf equipment up to 900mhz

I'd be impressed by a dipole antenna at that frequency.
I think this would be close
https://what-if.xkcd.com/157/

[paul cotter]

Hi BC, I don't quite understand your last remark. A typical aerial for that frequency is very like a uhf tv aerial but somewhat smaller. The active element in such a yagi is a dipole several inches long and made of aluminium.

[Bored chemist]

Hi BC, I don't quite understand your last remark. A typical aerial for that frequency is very like a uhf tv aerial but somewhat smaller. The active element in such a yagi is a dipole several inches long and made of aluminium.

The shift key is important.
Nine hundred millihertz is impressive for all the wrong reasons.

[paul cotter]

Ok, understood. I'll try to be more precise in future.