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Re: Cap AC voltage ratings
> Original Poster: Gary Lau 13-Mar-1999 2038 <lau-at-hdecad.ENET.dec-dot-com>
> I have begun making a new cap using strings of 16 Wima FKP-1
.015uF/1600VDC
> polypropylene caps. The thing that concerns me is voltage ratings.
> While the DC rating seems to be no problem (16 x 1600 = 25.6KV), I'm
> concerned with how much the mfgr derates the DC rating for AC. My caps
> are AC rated for only 500VAC each (16 x 500 = 8KV). I believe the two
> reasons for the derating are 1) to allow for the 1.414X RMS to peak
> conversion, and 2) to limit the temperature rise due to ESR dielectric
> heating. The graphs I have seen show a derating curve, being a function
> of applied frequency, and I would expect this. The unexpected thing
> however is that the curves do not converge to a simple derating of 1.4X
> at a sufficiently low frequency, but instead, never allow over the rated
> AC voltage, regardless of frequency.
>
Perhaps the derating accomodates some practical aspect of the designed
application. For instance, a device nominally rated to work on a 480V power
line application might require an actual withstand voltage of 2kV to
account for typical line disturbances and impulses.
There are also some regulatory issues with rating voltages. Go over a
certain number and you fall in a different class, even though the actual
material may be good enough for the higher class. For example, "low
voltage" computer wiring is technically rated at 50V for Class 2 circuits,
even though the actual breakdown voltage would be much above that. The
difference is the level of testing and the margins in a typical
application.
600VAC is a significant regulatory threshold in the National Electrical
Code, for instance, as is 2000VAC. Things with ratings higher than 600V
have to be built and tested much differently than those slightly below.