Owen Duffy wrote:
...

Jim, a lot of interesting stuff with which I generally agree.

The approach that my reference took to rating the conductor for a
lightning discharge includes a safety factor (as you might expect), and
so will rate the conductor at lower I^2*t than finding the conditions to
melt the wire. In real life, you would want the conductor to withstand a
second strike or fault soon after, and you would want to allow some
tolerance for other variables, hence the safety factor. The approach is
to find the I^2*t that raises the conductor one third of the way from
ambient (323K) to melting point. The calculator you used might assume
resistivity is at 0°C , ambient is 0°C, and the material is raised to
melting point with no heat loss, and that would give a fusing current
close to double of the approach that I used.

BTW, we have half inch copper water pipe over here (we still do but it
has a nominal metric size) and it is half in od... whereas half inch
galvanised steel pipe is half inch nominal bore... actually about 5/8"
id. Don't you like consistency in the same field!
But they're not the same field.. the stuff made of copper is actually
"tubing" and the stuff made of steel is "pipe", and historically,
they've been measured differently.

Tubing is usually soldered/sweated/brazed into fittings with a
receptacle, so the OD is important, because even with different wall
thicknesses, the fittings are all the same.

Pipe is based on something else (King John's toe diameter or something)

Some years ago I did extensive modelling of a double exponential
excitation of structures and facilities (not lightning, faster than
lightning) and it was interesting how much the circuit configuration
affected the transformation of the excitation waveform to structure
current, including ringing. The same software could run a lightning
scenario, but that wasn't the main goal of the analysis so my experience
with the lightning scenario is more limited. So, as I said, the nature of
the current waveform is the big uncertainty and so measures are usually
quite conservative to cover that uncertainty.

There's some fascinating papers out there that use NEC to model response
to a nearby lightning stroke (a much more common occurance than a direct
hit). It's actually quite involved, since they model the traveling
impulse of the stroke.


Owen