wrote:
I think the UL requires only that the MOVs don't start a fire when
exposed to conditions which cause their break-down. They don't rate
their ability to function as "surge protectors".


UL evaluates surge suppressors for fire, electric shock and personal
injury hazards, and also measures and categorizes the devices for how
much voltage they can "clamp," thus preventing excess voltage from
passing through to electronic equipment. UL refers to this as a
"suppressed voltage rating," with ranges from 330V (volts) to 4000V.
Believe it or not, the lower the rating, the better the protection.

How can one find this rating for a particular device?

Whatever surge suppression protection you're looking for, make sure
the surge suppressor has been tested and Listed to the stringent
requirements of UL 1449, the Standard for Transient Voltage Surge
Suppressors.

http://www.ul.com/consumers/surge.html

"The unpredictable nature of surges makes it difficult to suppress them;
you never know when, how long or how powerful they will be. In some
cases, a surge may have a higher energy level than the device can
handle. When this happens, the surge suppressor may be damaged and lose
its ability to provide protection against future surges."

I'm happy to see that UL agrees! They don't seem to put any evaluation
of this parameter, unless the "suppressed voltage rating" includes the
Jules rating of the MOVs.

http://ulstandardsinfonet.ul.com/toc...=s&fn=1449.toc

Thanks for the reference.

--
Virg Wall, P.E.