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I have transferred the PCB of the leviton surge protection inside a metal enclosure (to avoid burning the case if the MOV burns). But the problem is the inside is much cooler now with the opening and metal heat sinking properties. This is compared to the original sealed plastic enclosure.

So the thermal fuse would take longer to open, and the MOV burning even more? If it burns more, it can burn the wirings and short circuit them.

Therefore I think I must put insulation inside that can prevent heat from escaping.. or trap the heat. Do you know what material I should cover the inside of the metal so heat won't get outside (besides covering the hole of course)?

This is assuming the MOV transfer of heat to the thermal fuse is dependent on ambient temperature, what do you think ?

1 Answer 1


With a part that is likely to get hot I would use fiber or ceramic spacers , plastic can melt, and rubber can burn. In power distribution systems fiber and ceramic standoffs is how we mount buss bars.

  • See updated picture above. If I use plastic spacer over the PCB.. how can the spacer melt? If it's hot enough to melt, won't this melt the PCB too?
    – Samzun
    Commented Nov 6, 2018 at 22:51
  • The same way when those MOV's fail they melt entire enclosures made out of plastic. I am not saying they will melt but as I read it you were trying to be safer than the original listed assembly. With a small air space the thermal safety would open sooner than a large area.i have seen PCB's burned up from component failures but they are compressed fiberglass layers and it takes a lot of heat.
    – Ed Beal
    Commented Nov 6, 2018 at 22:56
  • You are right. With my metal enclosure the area is much bigger so thermal fuse may not open and the MOV can burn more. Well. If the plastic spacer melted and the PCB falls to the bottom.. I want an insulation. But prefer material I can get locally. If I put rubber or cardboard at bottom, it may burn as you say. What are other materials that can't burn I can put at bottom? I don't have access to fiber and ceramic standoffs (whatever this is).
    – Samzun
    Commented Nov 6, 2018 at 23:06
  • Glass beads like kids make bracelets out of, plastic may be ok I was trying to follow your lead on safety thinking. Other ideas Old toaster plastic feet or parts that plastic is bakealight not spelled correctly but a high temp plastic. Possibly a couple of bumps made out of high temp silicone or red RTV used as gasket material used on car motors. in the old days my grandfather's time they used to use granite to mount electrical equipment I can't think of anything else at the moment.
    – Ed Beal
    Commented Nov 6, 2018 at 23:29
  • What is the connection between how large the space is and how the thermal fuse can open? It senses the temperature of the burning MOV and stop the current at certain temperature. If the metal enclosure is bigger (mine is). Would it delay the thermal fuse from opening since there is ample ambient air that the original plastic casing doesn't have? I don't want my new metal enclosure to have worse performance than original.
    – Samzun
    Commented Nov 7, 2018 at 0:10

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