Many years ago, when I was responsible for the incomming quality of a major US corporation, I was encountering many failures of an off-line bu;k linear supply. After much investigation, I gixovered the the AC Hipot test was causing failures within the supplies. All off-line supplies, linear or switching must pass the hipot test. Here the both AC inputs are shorted, and the all the output teminals are shorted together. No ptoblem - right. NOT.

In the AC Hipot test a 50-60 AC voltage is applied to the AC input. The current is measured on the output terminals. During the AC test, the capacitive coupling of the transformer (plus any primary gnd to secondary ground stabilization capacitor) createds an AC current acoss the dielectric boundary. This, then induces an AC voltage drop across any device in series from the secondary to the output terminal(s). This stresses the protection diodes on any IC, or any low voltage MOSFET, which could lead to falure,
The DC Hipot test uses a DC voltage equivalent to the AC peak voltage. By using DC, the AC current over the transformer capacitanves are eliminated ant the low voltage secondary semiconductors are not stressed. One caution, the ramp-up and down periods must be longer than 1 second, otherwise the dv/dt on the AC line will cause transient currents in the secondary.