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PTC fuses are resettable, i.e. they go to high resistance when tripped and return to low resistance when they cool. Some examples would be e.g. Bel fuse 0ZCJ or Littelfuse Polyfuse series.

One thing the manufacturers are coy about is what is the failure mode of these components. They do give you means to determine the safe operating area but won't tell you how does it fail if it overheats.

Does it fail open like a regular fuse (good) or does it fail closed like a TVS diode (bad)?

Failing closed is obviously pretty lousy for a circuit protection component to work like, I'd say from the principle of operation it shouldn't short but does anyone have a manufacturer's reference or maybe a white paper about the failure mode?

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    \$\begingroup\$ Whoever voted to close did not read the question... \$\endgroup\$
    – JYelton
    Commented Dec 2, 2021 at 17:55
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    \$\begingroup\$ I found an interesting paper via University of Maryland, "Failure Precursors for Polymer Resettable Fuses," which my cursory reading of seems to indicate that although they defined one failure mode (#2) as "Fuse does not trip at fault current (≥ Itrip) at the specific ambient temperature." they do not seem to report any as exhibiting that mode. I don't think this answers your question but is nevertheless interesting. \$\endgroup\$
    – JYelton
    Commented Dec 2, 2021 at 19:25
  • \$\begingroup\$ From experience, one failure mode is they go up in flames. This is more likely for SMT parts. They do not like long term overloads and frequent tripping ‘wears’ them out. I suspect the polymer gets stressed. \$\endgroup\$
    – Kartman
    Commented Dec 2, 2021 at 20:53
  • \$\begingroup\$ @Kartman It's indicated the "wearing out" actually increases the "off" resistance so the idle resistance will creep up after each trip. Obviously this will increase the power dissipation so it'll become more likely to trip! \$\endgroup\$
    – Barleyman
    Commented Dec 6, 2021 at 17:59
  • \$\begingroup\$ Going up in flames doesn't unfortunately mean the component opens, e.g. in this case the principle of operation is that the polymer expands and conductive particles lose connectivity increasing resistance. Plausibly the failure method could be the polymer melting and thus the device would short. Of course if it physically cracks open the point is a moot. The manufacturers wont tell you. The max voltage given is actually determined by voltage higher than that exceeding power dissipation limit and cooking the chip, not that it'll arc or something. \$\endgroup\$
    – Barleyman
    Commented Dec 6, 2021 at 18:12

4 Answers 4

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I just had one that failed closed. It was in equilibrium state, so "blown state" and then suddenly smoke and it closed. Large current. Lots of smoke. Supply-voltage was about 31v while it was rated at 30. So make sure to keep well within rated specs, and if your application does not allow this, consider using a regular fuse.

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  • \$\begingroup\$ Interesting. Would have thought that the polymer burns off/explodes when in an overvoltage situation, leaving the circuit open. \$\endgroup\$
    – tobalt
    Commented Apr 4, 2023 at 9:17
  • \$\begingroup\$ Makes sense +1. Polymer=organic. When a carbon film resistor burns out it has the potential to become a glowing ceramic rod rather than opening up. Metal film less so IME. \$\endgroup\$
    – Spehro 'speff' Pefhany
    Commented Apr 4, 2023 at 11:19
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I could not find a manufacturer specifically providing details on the failure mode — I suspect it varies by device/form factor.

However, there was an article about how PTC resettable fuses work, but it's from an unreliable source. It had indicated that allowable failure modes were open or high resistance, but I would hold out for manufacturer-backed information.

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  • \$\begingroup\$ Yeah, the datasheets are pretty bad about explaining how these parts work. Eaton and Littelfuse give you nothing but Bel Fuse datasheet has some basic information including how to figure out the approximate trip resistance belfuse.com/resources/datasheets/circuitprotection/… \$\endgroup\$
    – Barleyman
    Commented Dec 6, 2021 at 18:06
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Over voltage events can lead to a failure mode of a short circuit. Over current events can lead to a failure mode of open circuit. Refer this mfg web site: https://www.tchqsz.com/info/what-are-the-failure-modes-of-ptc-45719316.html

TDK recommend making sure operating conditions are within the max voltage and max current ratings of the device....

All of which makes a PTC a fairly crude solution in cases where you can't predict the max voltage you are protecting against. Or the max fault current.

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One failure mode I've identified in the field is the internal resistance goes up considerably (e.g. an order of magnitude). This was probably due to the repeated tripping straining the polymer as Kartman identified. This would qualify as "failing closed", but not in the way you imagined. It still functioned, the circuit it was in just didn't perform as intended anymore. I'll sure reconsider whether to use them in a next design.

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