Every 480V variable frequency drive I've had the pleasure of dissecting has had two DC capacitors in series across its DC bus, each rated for ~400VDC. UL 508c testing as executed on one of my recent products requires that all components whose failure to short could cause damage be evaluated. At the time we had a single DC bus cap rated for 1200V. When that cap was shorted for testing, the full DC bus was placed across two diodes, which caused flame to escape the box.
To solve this, we had to put two capacitors in series across our DC bus, each rated for the full DC bus voltage. Obviously, drive manufacturers don't do that. If any one capacitor in the drive shorted, you'd have a ~650VDC bus across a ~400VDC cap. So how do they pass the tests? Does the cap pull enough current that the fuses open before the cap ruptures? Is this predictable by the surge rating of the cap? Or perhaps the cap ruptures, but the box is designed in such a way as to contain the flame? Or do they simply not run this test, and I'm misunderstanding something?