[time-nuts] thoughts on lightning arrestors

[Chris Albertson]

On Thu, Nov 27, 2014 at 9:12 PM, Bill Hawkins <bill at iaxs.net> wrote
>
>
> So I wonder about this concept of a lightning "arrestor". The report
> referenced by Arthur Dent is quite complete. It also says, at the bottom
> of page two, "It is impossible to prevent damage from a direct lightning
> strike ..." Why, then, do people sell lightning arrestors when they
> wouldn't dream of selling hurricane arrestors?



Because the lightening arresters actually work.  That is why they are sold
and wide used.

They do not and can not "stop" lightening.  What they do is act as a
switch.  They sense the voltage on a conductor.  Normally the conductor is
allowed to  have a signal voltage on it but if the voltage relative to
ground goes high the arrester closes a switch and connects the conductor to
ground.   They don't stop lightening, they simply close to provide a path
to ground.

The tree could have been protected too.  Let's say we could measure the
voltage at the top of the tree and if it ever got high we'd connect the top
of the tree to ground via a study copy cable.  Then the current would have
avoided the tree trunk and flowed harmlessly to ground.  What is amazing is
that we can build a simple switch that can work so fast. Many of them work
by using some kind of gas that is non-conductive until it becomes ionized
then the ionized gas connects the conductor to ground.

Also the arrester never sees the full current of the strike.  In a coaxial
antenna cable MOST of the current is on the shield and this is shunted to
ground directly and never goes into the arrester.

And BTW we do build and use "hurricane arrestors".  They are called storm
shelters.  A large concrete structure works pretty much like a lightening
arrester, it deflects the effects of the storm from some small protected
area.

We should not argue that lightening protection is impossible because we
have many thousand of examples of them working.  Yes normal variations and
statistics will eventually take out a system.  But will it happen in your
lifetime?

OK one more analogy.  Earthquakes.  Why bother with robust building codes
when we know that a big enough quake will destroy any building?  It's
because the "big one" is unlikely to occur during the building's lifetime
but we know 100% that many smaller ones will occur.  So we protect for the
normal case

These things obviously DO WORK.  There are cell towers all over the Orlando
Florida and they continue to operate.  With a cell tower the FIRST line of
defense is structural grounding.  The steel structure is bonded to a copper
grounding system.  They get this down to about 6  ohms tower to ground and
most of the current follows that path.  The second line is surge protectors
on ALL wires that come into the building.

Again you do NOT "stop" lightening with an arrester, you use them to
provide a easy path to ground..
-- 

Chris Albertson
Redondo Beach, California