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I have a question about AC capacitor ratings. I've reviewed this post -

What is an AC voltage rating for a capacitor?

...but it only made things a little more confusing. I have a system wherein the applied mains voltage is 277Vrms/60Hz, but the EMI capacitors only have a rating of 250VAC (printed on the component and listed in the datasheet).

The caps are installed from line to ground, not daisy chained, so the full 277rms (391 peak) voltage is applied directly to each terminal of each cap. It's a power factor corrected device, switching at 40kHz, but if I understood the above linked post, that should only make the rating requirement higher, not lower.

Why don't these caps blow up immediately, or shortly after applying (what appears to be) a higher voltage than is specified? Some of these devices have been installed for years without issue.

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  • \$\begingroup\$ It would be helpful to put the full details of the capacitor into the question \$\endgroup\$ – user1844 Sep 21 '15 at 15:20
  • \$\begingroup\$ Most caps do not behave like old dipped tantalums and will not fiercly explode when run slightly over spec. For these voltage ranges the more important rating is that of the ripple current. \$\endgroup\$ – PlasmaHH Sep 21 '15 at 15:24
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250VAC is the regulatory/safety rating for the part, which means that if you use it in an application where the steady-state voltage across it is at or less than 250VAC, it's considered 'safe' by those safety certification bodies and won't require any special investigation by the safety people when it comes time to certify the end-product. (Sometimes also known as a recognized component - the funky backwards UR mark is on this part, which is the UL Recognized Component mark.)

The capacitor is a class-Y2 device which means the part is impulse tested (i.e. for a very short time) at 5000VDC. This doesn't mean the part can operate at 5000VDC, but it's a measure of the strength of the dielectric material. Y1 is "stronger" (tested at 8000VDC).

300V is the manufacturer's absolute maximum voltage for the part. If you exceed 300V, there's no guarantee of performance or survival.

I would look for 277 or 312V Y-capacitors - they should only be marginally larger (if at all) and would eliminate any doubts about suitability.

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  • \$\begingroup\$ Thanks for your reply. Sorry if I wasn't clear, but this isn't for something I'm designing or troubleshooting, this was just something I noticed in an old (over 10 years running) industrial product. Though 277 is nominal, it's a UPS and is subject to surges and spikes as well. I am just curious why a front-end capacitor would (seem to) be rated so low/close to the margin. I thought maybe something about the application (power factor corrected switching @ 40kHz) may explain it, but I wasn't able to find anything that cleared it up. "Safety" makes sense. In any case, I appreciate your reply. \$\endgroup\$ – Joe Electron Sep 21 '15 at 19:04
  • \$\begingroup\$ If we ignore the 277V business for the minute, the point about the 250VAC rating is it's telling you what kind of mains voltage it's suitable for use on - so the margin that one would normally apply in choosing a capacitor voltage rating has already been taken into account - i.e. it's actually a 1000V DC capacitor. \$\endgroup\$ – user1844 Sep 21 '15 at 21:07
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A capacitor connected to ground will (should!) be a 'Y' capacitor, which is specifically designed for that role, and what you're seeing may be its service rating rather than a normal capacitor voltage rating, which is a DC voltage anyway.

But it still sounds like it's under-rated.

What's the manuafacturer/part number of it and what's the application which is 277V RMS to ground?

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  • \$\begingroup\$ Thanks for the reply. Yes, it's a Y cap, manufactured by Vishay - mouser.com/ProductDetail/Vishay-Roederstein/F17104101000/…. What's strange is that the link shows that it's rated for 250VAC, but if you look at the datasheet it claims 300VAC. The softcopy datasheet I have also lists 250, so not sure why the inconsistency is there. This is for a UPS application. \$\endgroup\$ – Joe Electron Sep 21 '15 at 16:49

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