In my experience, MOV's have a limited life.
Some years ago, I was involved in testing MOV's to the
IEEE discharge spec (long enough ago that I no longer recall
the nomenclature). What we discovered was that the MOV
was sensitive to total charge dissipated. In addition, it
was sensitive to the direction of the current.
5 shots and they died, if they were all polarized the same. If they
alternated between + and -, however, lifetime was extended.
The study was far from exhaustive, but we were able to conclude
that a shot in one direction was not fully offset by a shot with
the opposite polarity, possibly due to impurities in the device
and the way they migrated during peak currents.
The problem with MOV's is that they just sit there...and you
don't know what's happened to them. Are they still good? How good?
As a result, my strategy for sparky-deflection is disconnecting
everything when not in use.
This obviously ignores those situations where 24/7 service is required,
or something approximating it.
I think I'd favor gas-discharge tubes with replaceable elements, and
periodic PM, rather than MOV's.
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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