[TowerTalk] lightning path vs current path

[jimlux]

This discussion of lightning, side flashes, etc, brings up some 
interesting points.. As a side note, about 15 years ago, I spent several 
years making large (many meter) sparks follow predictable paths (or, at 
least trying to do so) as a theatrical effect.

It's important to distinguish between what Roger (K8RI) was describing, 
the lighting coming out the side of the tower and striking some distance 
away, from the transient suppression/lightning protection thing, which 
is at short distances.


When talking about the large scale path that the lightning stroke takes, 
that's pretty much all determined well before the kiloamp currents and 
big di/dt (for which inductance is important) even start.  The big 
currents are in the return stroke, which basically just follows back 
along the path established by the original leader.  The path the 
*leader* takes is pretty random, going in 50-100 meter jumps, generally 
following the electric field, but with a significant random component.

If you think about a leader propagating down from the clouds above, when 
it reaches the top of the tower, it's still propagating, and there isn't 
much current in the tower base yet, because the transient hasn't 
propagated there.  The transient front moves down the tower, but the 
circuit properties below the leading edge don't have a lot to do with 
what happens.. it's more about the local electric field, when the local 
field at the surface gets high enough to exceed breakdown.  A very 
smooth uniform tower will tend NOT to have the leader move away, because 
there's no breakdown along the surface. A bumpy tower (or tree) will 
tend to branch off, because a localized field concentration can start 
the leader going a new direction.




The big current (and lightning damage) for which inductances are 
important, is the return stroke.  By then, the "circuit" is established, 
and now, inductances are important.