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What is the temperature value of a TCO (thermal cutoff fuse) which should be chosen in order to protect a metal oxide varistor (MOV)?
(I mean cut the current to the varistor, before a catastrophic failure.)

Lets say for example: a 7D391(220vac)varistor. what is the range of temperature (+/-) that a MOV start to fail(burn)?
And do all MOV size (7D,10D,14D...) and all MOV rated voltage burn, more or less, at the same range of temperature?

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    \$\begingroup\$ the acceptable temperature range of any component will be given in its data sheet. But to be honest, you are trying to protect a thermal protection device with a thermal protection device: This screams XY Problem, and I'll happily retract my "vote to close" the moment you explain (with a circuit schematic, if at all possible) why you'd want to do that. \$\endgroup\$ Commented Sep 28, 2019 at 17:27
  • \$\begingroup\$ Considering you have 23 questions, of which only 8 have positive votes: maybe it's generally worth adding more context to your questions. \$\endgroup\$ Commented Sep 28, 2019 at 17:29
  • \$\begingroup\$ I was trying to delete this question and ask a simple one,but i have noticed that you block me to ask new question.what a pity:( So my simple question is:what is the temperature(+/-)that the mov is starting to burn,while its operation surface tem. is 125°C? \$\endgroup\$
    – xchcui
    Commented Sep 29, 2019 at 12:00
  • \$\begingroup\$ Not quite sure what you mean with "blocking". Anyways, you can't ask new questions in comments. \$\endgroup\$ Commented Sep 29, 2019 at 12:32
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    \$\begingroup\$ @MarcusMüller I would just say that I know exactly why the OP is asking this question, and with a bit of Googling, it appears that putting a TCO in series with an MOV is a normal thing to do. It's there to stop the MOV catching fire if it fails short circuit. So the TCO should only operate if the MOV is outside its normal operating range. \$\endgroup\$
    – Simon B
    Commented Oct 3, 2019 at 23:14

1 Answer 1

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It's not really going to be possible to select the correct TCO operating temperature without knowing how the MOV and the TCO are connected together thermally.

It may come down to performing experiments and seeing what's the lowest rating TCO that won't fail in normal operation, but will fail when the MOV overheats.

Manufacturers such as Littelfuse now manufacture MOVs with a built-in TCO to eliminate this problem. There may be one that suits your needs.

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  • \$\begingroup\$ I was going to connect the MOV body to the TCO body,rather than connecting the built in TCO+MOV and you were right,it is for cutoff current to MOV before it catching fire.This was the reason whyi asked what is the temp. which the MOV start burning.So,if the maximum allowed operating temp.of the MOV is 125°C and the TCO takes time(few seconds)until it actually sense the heat and cut-off the current,what do you think should be the right cutoff temp. of the MOV?less than 125°C,equal to 125°C or maybe higher than that,since the combustion temp. of the MOV is ,anyway,much much higher than 125°C. \$\endgroup\$
    – xchcui
    Commented Oct 4, 2019 at 13:42
  • \$\begingroup\$ I would like to add that i understood that the TCO mustn't design above the MOV limits:125°C(as,MarcusMüller,mentioned),but wat the TCO temperature should be?equal to the max temp of the MOV(125°C)or less? \$\endgroup\$
    – xchcui
    Commented Oct 5, 2019 at 12:03
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    \$\begingroup\$ A starting point would be the maximum operating temperature of the MOV. But as the manufacturers seem to offer no guidance, I wouldn't want to ship anything without actually testing it in a laboratory. Taking the advice of some random person on the internet is not much of a safety case, for something that could potentially catch fire. \$\endgroup\$
    – Simon B
    Commented Oct 5, 2019 at 20:30
  • \$\begingroup\$ Yes,this is the problem,i don't find any guidance in TCO'S datasheet etc.My intention was to improve the circuit that has already fast fuse,but i guess that i will stay with the fast fuse protection and leave the TCO fuse for other tasks.Thanks. \$\endgroup\$
    – xchcui
    Commented Oct 6, 2019 at 13:26

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