Its off 400Vdc.
Do you think the 20pF between input and output will cause noise problems since the switching node is banging up and down from 400v to 0v in some 30ns...and the secondary will be grounded to that.....primary gnd'd to circuit gnd as expected.
Know someone who used a small mains transformer for this purpose and came to grief, due to the interwinding capacitance.