Re: Does a MOV shunt current or equalize voltage?

On Aug 2, 8:31 pm, Jamie
  A MOV does not have the ability to fully suppress the incoming
surge with out destroying it self if it's severe. I think most of us can
understand that. The idea is to have a fusable link or protect release
circuit that will open in line with the MOV in this case so that it
don't get past that point. Yes, the MOV may get and most likely will
be destroyed in the process of a major hit how ever, in all cases I have
seen the series protection circuit like breakers or fuses have been
able to open only because the MOV was doing its job by putting
excessive load in the circuit between it and the device its trying to
open! Mainly the fuse or breaker.. Normally when a MOV gets
destroyed it is shorted permanently but it does make the incoming
circuit open as it should unless some one had done a hack repair job
or does not know what they are doing!

Jamie has posted about things he has seen. He does not post what
MOV manufacturers states even in datasheets. MOVs that fail as Jamies
has seen are grossly undersized - violate what MOV manufacturers
require. Jamie has seen what happens with grossly undersized plug-in
protectors that also do not even claim to provide sufficient
protection.

Properly sized 'whole house' protector earths a direct strike and
remains functional. Effective protectors that remain functional after
a direct strike also costs tens or 100 times less money per protected
appliance. But those who only know from what they have seen (ie " I
have seen the series protection circuit like breakers or fuses have
been able to open) did not first learn the science.

Those fuses or circuit breakers are emergency backup proetction
when the protector did not do what it is supposed to do. Surges are
done in microseconds. Those fuses or breakers open in milliseconds.
Those fuses or breakers are to stop a fire because the protector was
grossly undersized.

Jamie - go read some manufacturer datasheets. Note the charts so
that MOV protectors can be properly sized - do not fail
catastrophically. However plug-in protectors often gorssly
undersized their MOVs. The myths promoted by that exploding MOV then
get the naive to recommend more of those surge protectors.

An effective protector earths direct lightning strikes and remains
functional. An effective protector means the human does not know the
surge existed. But that would not promote sales of excessively
overpriced plug-in protectors.

Relevant are these scary pictures. If the MOVs are grossly
undersized, then the protector must fall back upon the emegency safety
circuit. Sometimes that safety circuit does not work. These scary
pictures result:
http://www.hanford.gov/rl/?page=556&parent=554
http://www.westwhitelandfire.com/Articles/Surge%20Protectors.pdf
http://www.ddxg.net/old/surge_protectors.htm
http://www.zerosurge.com/HTML/movs.html
http://tinyurl.com/3x73ol
http://www3.cw56.com/news/articles/local/BO63312/

Last thing anyone wants is MOVs that routinely fail on that first
surge; that must fall back on emergency safety circuits to avoid
fire. And yet many who have only seen grossly undersized plug-in
protectors feel that dangerous failure mode is acceptable and normal.

Datasheets are quite blunt about this. Catastrophics failure that
results in vaporization or shorted MOVs is not listed as acceptable
operation. MOVs must be sufficient sized so as to degrade - a 10%
voltage change. Those who did not read datasheets - who saw what
grossly undersized protectors do - would then post as Jamie has.
Those scary pictures are not acceptable - but can happen when the
protector is grossly undersized - completely unacceptable.
.