I've been scrounging a few parts from old defunct AT SMPSes, and I found a few that have these Maida Z151-03UL transient voltage suppressor MOVs, one across each of the 470uF, 200V filter caps. I found the specs here:
Only a few of the PSes have them. Each of the MOVs, or is it TVSs, has a plastic sleeve slipped over it, and the top half is filled with white goop, looks like silicone grease, but hard like caulking compound.
The last MOV that failed on me failed violently, big BRRRZAP! and flash of light, and it filled the power strip with soot and smoke. It seems to me that putting a plastic sleeve over one of these is like adding kindling to a fire. The white goop may be silicone and fire resistant, but the clear plastic sleeve doesn't look flameproof. I'm gonna take one outside later this morning and apply a propane torch to it to see how flameproof it is.
My thinking is that if an excessive voltage is applied to a big electrolytic, it will start conducting like a resistance, and if the product of the power and time is great enough it will overheat, and if it gets hot enough it will vent electrolyte. But it's much bigger than the TVS, and it should be able to handle more overvoltage/overcurrent than one of these little 8 mm MOVs.
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