[cesarb]

What I've read is that their main failure mode is that, as they age, their trigger voltage gets lower and lower, and at some point the normal line voltage is enough to trigger them all the time. And when they overheat, either due to being triggered all the time or due to diverting a large surge, they fail open and no longer have any protective effect on the circuit. High quality surge suppressors would have fuses physically touching the MOVs, so that when a MOV overheats and fails, the fuse opens and cuts power to the now unprotected output.