[TowerTalk] MOV's

[Jim Jarvis]

I did some consulting work for a manufacturer of surge
protection equipment, several years ago.

We evaluated MOV's and their failure mechanisms, using
Keytek surge generators (whom I represented at the time).

Whether we WANT them to fail shorted or not, they fail
either shorted OR open, depending upon the total number
of Joules they have absorbed, the direction of the currents
which flowed, and what the latest peak does to them.  They
do it without asking us permission.

My conclusion at the end of the project is that an uncontrolled
variable in the MOV production makes it impossible to predict in
advance whether they are predisposed to fail in one mode or another.
Simply exciting them differently did NOT produce different results.

Yes, MOV's operationally clip peaks.  They just don't do it forever,
and they have no way of telling you that they just died, unless you
get lucky and they short and blow a fuse.  That's maybe half the time.

ANSI/IEEE C62.11 sets the standard.  There's another for the
energy waveform of the test excitation, which I forget, and
google isn't yielding quickly.

With respect to my personal protection, I stand by my statement:
An MOV based solution is at best a short term solution.  You'll
never know when 'short term' is over, until your equipment is fried.
Hence, I believe it's no solution at all, unless you're selling cheap
consumer crap to the unwary/uneducated.

In which case, the variable of interest is return on investment, not Joules
accumulated.

N2EA
jimjarvis at ieee.org