"The Natural Philosopher" wrote in message ...

The protection partially comes from the conductor bleeding charge
away from the tower and lowering teh potentil difference around it
making it less likley to be a strike target.

Wrong. This is a myth, still perpetuated, it seems, by school physics
teachers who ought to know better. To quote from BS 6551: "A lightning
conductor is incapable of discharging a thundercloud without a lightning
flash." Lightning protection systems are there to take strikes and divert
the current safely to earth.

Once struck, even the thickest copper strap is likely to melt.

Utter twaddle. Lightning currents are high (98 percentile range 3 - 200 kA,
median value 28 kA), but the duration is very short (typically 100 us), so
the I-squared*dt integral is quite low. The temperature rise of the usual
25 x 3 mm copper conductor taking a 100 kA strike is only about 1 degree C.
The energy dissipated as heat is about 400 J per metre of the conductor's
length. Large conductors are used for reasons of mechanical strength and
robustness, to withstand both the normal knocks they'll get on the outside
of a building and the considerable magnetic forces arising during a strike.

The conductor(s) can carry strike current easily. The danger, and damage,
come when the current leaves the conductor and forms an arc - in a 'side
flash'. The peak power in the arc can reach 100 MW/m, heating the
surrounding air to 30,000 K and initiating a shock wave. It's the shock
wave that blows tiles off roofs, etc.

To avoid side-flashing you have to consider inductive (and mutual inductive)
effects. The rate of rise of current in a strike can reach 200 kA/us, and
hence will drop 200 kV across each microhenry of inductance - i.e. something
on the order of 200 kV per metre of conductor!

Finally the earth system design is not trivial. BS 6551 requires an earth
system resistance not exceeding 10 ohms. 200 kA through 10 ohms will drop 2
MV, so voltage gradients at the ground surface (between someone's feet, for
example) become an important consideration...

--
Andy