IanM wrote:
Marty wrote:

Capt. JG wrote:

22:02 LIGHTENING PROTECTION SYSTEM: The lightening protection
system provides a "cone" of protection around the boat in the even of
a lightening storm. Grounding wires of #8 gauge copper connect all
chain plates and the mast step to the keel.



#8? Ha, Jon, I've seen the inside of an underground vault with the
walls spattered with copper after a 75KA short vaporized copper bus
bars 1/2'" thick by 4" wide. That's one hell of lot of #8 wires in
parallel. Imagine what happens with surge that may exceed 200KA?

I go along with others that have suggested that lightning protection
for a plastic boat is probably an exercise in futility.

Cheers
Martin

#8 is obsolete. #4 grounding wires are now called for. Also I'd lay
odds that the 75KA short lasted for several cycles of your 60Hz mains
before the breakers cleared it. It almost certainly had more than 1000
times the energy that a 100KA 1us lightning strike would dissipate in a
single #8 cable.

Doubtless you are correct, was probably 3 cycles; it's a suprisingly
common problem, circuit was a 600 amp 4.4Kv service, breakers that were
supposed to protect it were rated to break 10,000 amps. Problem was that
the supply was able to feed 75,000 amps, so the primary breakers tried
to trip but fried in the closed position, two cycles later the breakers
on the main feeder tripped. There are other problem with the lightening
strike, to start with duration is likely more than 1us, probably also
more than one stroke, usual pattern is several cycles, further 100KA is
likely a low estimate, some run higher than 500KA. Then just to
compound things, we're are talking voltages into the hundreds of
millions, and then we are generating plasmas, there are strange
electromagnetic effects, none of which are well understood or tractable
to simple modeling. Energy would be from 10 to 100 billion joules!
Your #4 is going to be toast. You need a cable thick as your arm to
guarantee that the cable wont fry, but with the other weird effects,
there is little guarantee that all of the energy will remain confined to
the path you are providing.

Cheers
Martin

If the protection system can prevent serious structural damage from 9
strikes out of 10 its obviously worth doing. If it reduces damage 50%
of the time so you dont have to abandon ship in the middle of a storm
its still worth doing if the cost is comparable to that of a liferaft.