How would one go about to implement accurate sensing of metal oxide varistor wear level?

Even cheapo surge protects come with a LED to indicate protection status, but I presume this is not based on actual sensing, but most likely a "time fuse" that simply prompts the use to replace the device, regardless of the amount of wear.

I've read that MOVs mitigate power surges by absorbing some of the energy and redirecting another portion to ground. And that they burn out when overloaded. In other words, the amount of thermal energy they generate is proportional to the amount of wear.

So one possibility might be to try and measure how much heat is displaced into the ambience, but it sounds kinda complex and course, and presumably most of the time the wear is rather subtle.

Maybe something else? I am interested in accuracy of measurement, not in economic viability, obviously MOVs are fairly cheap and typically do not merit sensing sensing circuitry that will be much more expensive.

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    \$\begingroup\$ AFAIK a common failure mode for MOVs is to fail to a short circuit after they get 'worn out' by absorbing many surges. As such they're always connected inline with a fuse (probably a high-rating slow-blow type) so that when they do finally die they're disconnected from the circuit - and it's this state that the el-cheapo LED indicator is showing you. \$\endgroup\$ – brhans Apr 3 '18 at 16:48
  • \$\begingroup\$ I want to measure the actual wear, not whether or not it has reached a level of catastrophic failure. \$\endgroup\$ – dtech Apr 3 '18 at 16:50
  • \$\begingroup\$ Maybe an inline current sensor which you sample frequently (fast enough to catch surges) and integrate over time... \$\endgroup\$ – brhans Apr 3 '18 at 16:52
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    \$\begingroup\$ It's not trivial. If you can get a hold of the following IEEE paper it explains one approach: "Research on leakage current characteristic of metal oxide varistor" by Xuni Rao et. al. \$\endgroup\$ – John D Apr 3 '18 at 17:11
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    \$\begingroup\$ That wasn't an opinion. MOVs are connected in parallel to whatever they' protecting. Look at some MOV tutorials, like here and here. The MOV does nothing (aside from leakage) until the voltage across it's terminals rises to a specified level - after that it allows current to flow through itself. The energy it absorbs during a surge is a result of the current flowing through it once its voltage rating has been reached. \$\endgroup\$ – brhans Apr 3 '18 at 18:59

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