The manufacturers of MOVs publish MOV surge degradation curves, sometimes called "lifetime ratings", or "pulse ratings". These curves show the number of surges that the manufacturer says their MOVs should survive at various surge levels.
Mysteriously, when these very same MOVs are installed in a powerline surge suppression product, it has been claimed that the MOVs do not degrade, can last 1,000 surges, etc. But curiously, the surge suppressor manufacturer seems unwilling to subject their product to an independent test lab for verification of their outrageous claims. Don't believe them. Demand to see independent proof!
2) There is no practical way to know how many surges an MOV has already intercepted and how large those surges were.
3) There is absolutely no practical way to determine how much degradation has taken place in an MOV, or how much surge life is left in the MOV. If anyone states otherwise, demand to see proof! Verify that proof with an independent, reliable source.
4) Since there is no way to tell how much life is left in an MOV, the ONLY conclusion that can be drawn, is that continued use of such a product is simply a gamble.
5) MOVs have proven to be so unpredictable, unsafe and ineffective that UL (Underwriters Laboratories, Inc.), CSA (Canadian Standards Association), and the Federal Government are all issuing strengthened specifications relating to powerline surge suppressors, all to become effective in 1996.
1. Harris Semiconductor Transient Voltage Suppression Devices
data book DB450.1