Ed Price wrote:

I would think that lightning protection should begin with the safe
equalization of charges.

Oh, how we all wish! But think what that implies...

If one could prevent localization of charge, there wouldn't be anything
to discharge.

That would require control over the weather - and again, oh how we all
wish!

Failing that, if one could provide a controlled discharge mechanism,
that drains charge without a massive discharge channel, that would also
be good.

But again, we don't know how to do that. Starting from a weakly ionized
probe leader, lightning has a huge positive feedback mechanism. Once it
has started to go, it'll go all the way!

Failing that, you fall back to a point g defense; first dissuading
the creation of a conductive channel to the protected area,

If an ionized leader has made it all the way down from the cloud into
the region of the protected area, we don't know any way to tell it
"Wrong Way. Not In My Back Yard".

If the leader has come so close, you absolutely cannot stop what's
probably going to happen next. All you can do is do is to design the
protection system to make the best of it.

or, failing at that, providing a specific, perhaps sacrificial path for
the massive discharge.

At last, we've come down to earth. All that lightning protection can
realistically aim to do is providing a specific path. The whole aim of
lightning protection is to provide a safe discharge path *past* the
structure that's being protected, as opposed to a damaging path
*through* the structure.

A "sacrificial" path is not an option to design for. The lightning
conductor *must* hang in there for the whole duration of the stroke(s),
or else protection will be lost before it's all over.

To keep the original discussion in perspective, all this stuff about
terminals at the top end of the conductor is about trying to achieve
some kind of "come here" effect in literally the final few feet of the
entire lightning path (or tens of feet, if we're really lucky) to make
sure the leader attaches to the terminal and not somewhere else on the
structure.

The well known and most reliable way to do that is to make the terminal
higher than everything else, so it dominates the local electric field.
But that's still no guarantee that a leader won't come wandering down at
some distance off to the side, and then strike downward or even sideways
from there.


Bottom line: it's absolutely vital to be realistic about what lightning
protection can do - and also what it cannot do. A system designed out of
hopes and dreams will be the wrong system.


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek