Jim Lux wrote in
:
....
The other problem is that for fast transients, skin effect means that
the AC resistance goes more as the diameter than as the cross
sectional area (hollow tubes work just as well as solid conductors).

The problem is that while we might characterise the raw excitation caused
by lightning, and use assumptions about the shape, rise and fall times
and peak field strength, the response of circuits (such as those that
include the down conductor) is quite different, and it is unsafe to
assume in the general case that skin effect is fully effective for all or
even most of the energy spectrum.

Perhaps that is why some of these standards tend to treat the conductor
as having a resistance equal to that implied by just the conductivity (or
resistivity) and CSA. It might be conservative, but then standards tend
to be so.

Having seen the results of fairly detailed EM modelling of EMP and
lightning excitation of major infrastructure, and the effects of some
small changes to the model, I wonder a bit about the effectiveness of
some measures... but over engineering probably saves the day in a lot of
cases.

The real danger with lightning protection is that a half baked approach
my give the implementor some comfort, but actually increase the risk of
adverse outcome.

The most thorough and consistent practice I have seen is that employed
here in mobile phone base stations. Sure, they are occasionally damaged
by lightning, but the vast majority of lightning incidents do not cause
permanent damage.

Owen