On Wed, 28 Jan 2004 15:49:38 GMT, L. M. Rappaport
wrote:

On Wed, 28 Jan 2004 01:36:19 GMT, Gary Schafer
wrote (with possible editing):

You have been reading too many science fiction books. Just a little
common sense will tell you that most of your advice is bogus.

I'm not trying to start a flame, but as an engineer with many years
involving remote, mountain-top installations, I'd say his advice was
excellent, but I'm always willing to learn: what would you recommend?


Well for a start, I would not stand next to a copper flag pole or a
tree in a thunder storm. Side arcs can come just as easily from one as
the other.

Standing on the ground near a lightning strike can be deadly. Even if
you are not directly struck. As the strike current travels through the
earth there is a voltage gradient in the earth. In other words in a
distance of a foot or so there can be greatly different voltages on
the surface because of the high current flowing in the earth as the
charge spreads out from the strike point.

Lightning strikes can not be prevented. You can not "make anything
disappear from the competition for lightning".

There is what is called a "cone of protection" under something like a
mast with guys or shrouds. The lightning will hit those first before
what is underneath.

There is no such thing as bleeding of the charge. The earth can supply
charge much faster than it is possible to bleed it off.

A blunt air terminal is preferred over a sharp pointed one. A sharp
point will encourage streamers to form sooner. They can actually
attract a strike. This includes the toilet brushes mounted on the
mast.

The amount of voltage across an object associated with a lightning
strike is dependent on the amount of the current contained in the
strike.

Lightning is a constant current source.
A strike contains a given amount of current. Depending on how good the
conductor is that it strikes determines the amount of voltage
developed across it. The better the conductor the less voltage seen
across it. But the amount of current will be approximately the same.
In other words the voltage will rise high enough in order to produce
the strike current through the conductor. The higher resistance the
conductor the higher the voltage seen.

The reason for a low impedance ground conductor / system is to keep
the voltage rise to a minimum to prevent side flashes to other things.

The mast and all shrouds on a boat should be grounded. Each should
have their own run, as straight as possible, to the ground.
A common "ground buss" is not desirable as a considerable voltage rise
can also occur on the ground buss. It is better to have separate runs
to the ground for whatever you are grounding. And no 90 degree turns.

The mast and each shroud will share the current in a strike. Just
grounding the shrouds will not provide a sufficient path to ground.
Each has resistance, and worse, high inductance. This provides a high
impedance path for the sharp rise time lightning.

The high impedance path allows the voltage to rise very high at the
top of the mast. If the mast is not grounded at the bottom that will
allow the bottom of the mast to also rise high with voltage. It will
arc over to whatever is closer to ground in the boat.
Just grounding the mast provides the same problem with the shrouds if
they are not grounded. The mast also presents a high impedance to the
lightning even though it is quite large in surface area.

Large surface area conductors are desired for lightning current
carriers because of their lower inductance. It does not matter if they
are solid wire, stranded wire, braided strap or solid strap. The
amount of surface area is what is important to reduce inductance. The
only problem with braided strap is that it corrodes much faster than
solid strap.

Several wires or strap can be put in parallel for additional
inductance reduction


That should do for a start. :)

Regards
Gary