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Above picture is that of a Belden surge protector with some kind of cloth covered over the MOV and thermal fuse for isolation to promote more conduction. I don't want to cover the whole thing with compound but something akin to cloth or other cover.. do you know what kind of cloth can't burn? or what more solid cover I can put that won't conduct and short the other elements? What do others use?

I'm designing a circuit and want to embed MOVs (Metal Oxide Varistors). I'd like to know two things:

  1. I'm thinking of adding a thermal fuse. I read that the basic purpose of them is for temporary overvoltage (TOV) event when they can open before the MOV gets so hot and at end of life stage (where the MCOV kept decreasing gradually from each surge until it conducts at normal voltage initiating thermal runaway). But how about in actual surges.. if the MOV is rated at 3kA and you have a surge that is, say, 50kA, would it make the MOV initiate thermal runaway or not? How big should surges be compared to the MOV surge current rating before thermal runaway (bursting into flame) can occur?

  2. Normal design is to put the thermal fuse beside the MOV. But does it depend on ambient temperature? If the room is cold with aircon.. can it prevent the thermal fuse from working?

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    \$\begingroup\$ You can buy MOV's with thermal cut-off built in. Ferraz-Shawmut is one manufacture. They are not cheap but are self contained. The more expensive ones have switch contact wires up top to indicate if it has tripped. They are not resettable. \$\endgroup\$ – Sparky256 Nov 8 '18 at 5:10
  • \$\begingroup\$ Most thermal fuses I have seen are permanent, not temporary. Once they open, they stay that way forever. They are not directly intended to protect against anything but over-heating. If you have a motor in your product and you are worried that it may overheat, you can put a thermal fuse on the motor housing, and run the motor power through that thermal fuse. In the event that the motor gets too hot, it will blow the fuse and permanently open the circuit. \$\endgroup\$ – mkeith Nov 8 '18 at 5:45
  • \$\begingroup\$ I already bought the MOV and thermal fuse. I just want to know if MOV can go into thermal runaway from strong single time surge and how sensitive can thermal fuse pick up the burning MOV and how big a factor is ambient temperature inside the circuit casing... so please answer the original question if anyone knows. \$\endgroup\$ – Jtl Nov 8 '18 at 6:12
  • \$\begingroup\$ Yes, the MOV can go into a thermal runaway, depending on the pulse characteristics, but to evaluate the ambient contribution to the thermal fuse response, you have to look at the fuse parameters and how much thermal conductivity is there between the MOV and fuse, which is dependent on layout, copper trace thickness, distance between components, etc. Ideally you want the fuse to trip before the MOV erupts in flames, and you may end up doing some testing to figure out if your design works as intended. \$\endgroup\$ – isdi Nov 8 '18 at 13:56
  • \$\begingroup\$ It should be possible to mount the thermal fuse in such a way that it tracks the MOV temperature fairly closely. The ambient temperature will not have a big effect as long as it is in a reasonable range, and as long as you put the MOV and fuse in good contact with each other. For example, mounted on the same large copper fill, and perhaps covered with some silicone or other mastic or encapsulant or potting compound. Copper is a great thermal conductor. \$\endgroup\$ – mkeith Nov 9 '18 at 5:23

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