[TowerTalk] Rotator Lightning Protection

[Roger (K8RI) on TT]

Some things to remember when disconnecting antennas  and stations. 
Never, ever, leave the coax laying on the floor.  Best is to bring it in 
through a grounded plate which is a Common point ground (CPG) and ground 
the center conductor as well at that point.

When disconnected coax is laying on the shack floor, you have a very 
long antenna which includes the feed line stuck up into the storm with 
one end not far from equipment and possibly your feet.

It's a very dangerous practice!   Either ground the coax (shield and 
center) conductor, or water proof it (plastic bag, rubber band ) and put 
it outside with the end of the coax pointed up.  Under the house is 
almost as bad as on the floor.

I'll repeat what I've seen over many thousands and a wide range of 
Varistor  sizes.  There is no guarantee that MOVs will fail shorted.  
Their life is measured in Joule capacity 
https://en.wikipedia.org/wiki/Joule .  When properly sized, they may 
live a long life with many small events, or one larger event may do the 
job. MOVs, or a Metal Oxide Varistor is made up of many tiny metal oxide 
grains of different types forming, diodes randomly packed.  
https://en.wikipedia.org/wiki/Varistor

In NORMAL  use where they protect from small voltage spikes like we see 
on power lines in day to day  operation.  Each event degrades the 
varistor (again see the link to Varistor) each non catastrophic event 
lowers the clamping voltage a bit depending on the energy of the event 
(in Joules).  Non typical events are induced voltage spikes from nearby 
lightening strikes (or direct hits). NOTE a typical lightening strike a 
mile distant can induce as much as 1000 volts per meter in a piece of 
wire.  Following the inverse square law, a half mile = 4 KV, a quarter 
mile = 16 Kv, etc. I goes up to very large numbers from there on. With 
each halving you square the previous number. Those are for "typical" 
strikes. Many strikes can be far greater and then there are the "super 
strikes".  Any single event larger than the ultimate rating of the 
varistor will likely destroy the varistor.  Events close to the ultimate 
rating are likely to cause an open circuit failure and may leave the 
varistor in tact. The same is true for varistors that are near the end 
of life, that experience a single event that exceeds the remaining capacity.

One thing lightening is not, is consistent.  Watch the video in the link 
with 1 second slowed to 6 minutes.  It's the insert on the left side of 
the page.  Open it to full screen.  Of the many cycles there is one 
"long duration" event
https://duckduckgo.com/?q=Lightening+super+strikes&ia=videos&iai=W9xzU0xjlhE 
or do your own search.  There are a number of slow motion lightening 
strike videos on "The Net"

When using varistors, plan on a safety margin.  There is no such thing 
as 100% protection, particularly from direct hits, but following "good 
grounding practices" and using proven safe practices and protective 
equipment will minimize the likelihood of taking damage.  There comes a 
point where it costs a lot to make additional small gains and it quickly 
becomes cost prohibitive to try for more.  OTOH you can, for a 
relatively reasonable cost build a system that will reduce the odds of 
taking significant damage to a rarity. Even with the odds in your favor 
you may safely go for years or get wiped out with the next storm.

73

Roger  (K8RI)

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