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I've inherited an old design, and a part used has gone out of production. This part is a PTC resettable fuse, used in a bootstrap circuit, so it's one-shot. When the PTC opens, it has ~700VDC across it, and stays that way indefinitely. Most PTCs I see are rated in terms of AC voltage. My first thought would be that if something can withstand (for example) 600VAC, it should also be able to withstand 850VDC. But I've seen some vague references to this possibly not being true.

So if I have a resettable fuse rated for 600VAC, can I use that in my application? For example, this one: http://datasheet.octopart.com/TR600-150F-EX-2-TE-Connectivity---Raychem-datasheet-11088140.pdf

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    \$\begingroup\$ If I were in your position, my first move would be looking through my rep's line cards, and finding someone that reps the manufacturer of the original part. They can probably find you a one-to-one replacement. \$\endgroup\$
    – Matt Young
    Commented Jan 20, 2014 at 17:33
  • \$\begingroup\$ If you look at page 10 they recommend that the "V\$_{MAX}\$" operating voltage for DC is 250 V BUT I suspect your design doesn't come near that voltage in non-fault conditions. \$\endgroup\$
    – Andy aka
    Commented Jan 20, 2014 at 17:44
  • \$\begingroup\$ Mains voltage being ~250v RMS means the peaks are ~1.4x higher, so if that applies across the board (it may not) you may get away with 850VDC on a 600VAC part. However, really you should track down a part that is explicitly rated for 850VDC or higher in its data-sheet. \$\endgroup\$
    – John U
    Commented Jan 20, 2014 at 19:14
  • \$\begingroup\$ What is the original component's part number? \$\endgroup\$
    – Adam Lawrence
    Commented Jan 21, 2014 at 3:27

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I heard back from TE Connectivity. They say no part they make is rated for 700VDC. So the answer is no, you can't assume that an AC voltage rating on a PTC translates to a DC voltage rating.

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