Electronic – Capacitor with a lower AC rating than the application


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.

Best Answer

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.