[time-nuts] thoughts on lightning arrestors

[Bob Camp]

Hi

Any form of protection (no matter what you are protecting against) is part of a system. The more complex the system, the more it costs and the larger it gets. The more details you cover in your system the more 9’s you get to put in the “99.9xxx% protection” statement. Is there an event out there that will be the 0.001% event - sure. Are we talking about lightning or redundant timing systems? - it really does not matter. The same principle applies to both. Are two GPSDO’s enough? Would adding a couple of Cs standards make it better? You are just adding more 9’s to the number. Put a structure over your house that looks like the one over your local power switching station and you add a few more 9’s on the lightning protection number. Same idea as adding a few Cs standards to your system. 

In both examples, the added gear will only add 9’s if it’s part of a properly designed system. Simply adding more of XXXX isn’t likely to help much compared to a careful design. People make timing setups that run pretty much forever and ever. A mountain top (or skyscraper top) communications setup can be designed to take multiple direct lightning hits an hour and keep right on going and do it for may years. There are *lots* of systems out there like that. All of it just takes a careful approach with good attention to the details. Yes there is the minor issue of having enough cash on hand to pay for what’s required ...

Bob
 
> On Nov 28, 2014, at 11:26 AM, Chris Albertson <albertson.chris at gmail.com> wrote:
> 
> 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
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