When properly sized, MOVs degrade with each transient. The
manufacturer's chart has charts for number of transients, size
of transient, and number of joules. More joules means the MOV
life expectancy increases exponentally. However if MOV is
grossly undersized, then conditions cited by www.rbs2.com
cause an MOV to operate beyond what the manufacturer intended
- become a fire hazard.

A minimally sized MOV may degrade on the first transient. On
that chart is a curve for one transient. However there is no
curve for zero. No MOV should be so grossly undersized to
completely fail on the first transient. MOVs should only
degrade - end up on the curve for 1, 10, 1000, or 10,000.
Properly sized MOVs remain functional after every transient;
must only degrade. Most manufacturers define 'degrade' as a
10% change in its operational voltage. Manufacturers don't
define vaporization as acceptable. Vaporization is when a
human grossly undersizes the protector.

clifto wrote:
Wrong URL. I thought this was the one I read that suggested that every
"hit" degrades a MOV. Instead, start with the article at
http://www.cob.org/fire/safety/surge.htm:

"All MOVs gradually deteriorate with use."

"The problem found with some MOV components is that over a 18 to 24
month period the materials used begin to break down. As the MOV
ages, its operating characteristics change and it can become more
sensitive and dissipate more heat."

...and I'll try to find the URL to the article I really meant to post.