Interesting possibly switching supplies have internal HV, I would be hard pressed to to believe an ordinary PC trace spacing could be made to arc at 50V levels (see Paschen's law for air).
The standards are set with a lot of margin for error, and are spaced well beyond what you would get from a Paschen curve. I doubt they would have an intermediate voltage above the final bus voltage, but it's possible. I've never worked in an industry that requires active power factor correction, so it could happen there. I'd guess that the highest voltage is the 250Vac max on the incoming power lines, which is 350V peak.