AC mains on DC ground
Assuming the power supply is designed to provide galvanic isolation between the AC input and DC output, ask yourself this: "When measuring the potential difference (the voltage) between two nodes that are galvanically isolated, what voltage should one expect to measure?"  The answer is, "It could be just about anything."








simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1.&nbsp;&nbsp;&nbsp;What voltage does VM1 measure between Earth ground (neutral) on the primary side and either transformer wire on the secondary side, when the transformer's primary and secondary are galvanically isolated from each other?








simulate this circuit

Figure 2.&nbsp;&nbsp;&nbsp;What voltage does VM1 measure between Earth ground (neutral) on the primary side and signal ground on the secondary side? (NB: Earth ground and signal ground are galvanically isolated; they are not at the same potential.)


Imagine measuring the potential difference (voltage) between the AC neutral line and the negative "-" terminal on a battery, where the AC power source and battery are galvanically isolated. What voltage would one measure in this case? Very likely it'll be an AC voltage: but what does that measurement mean when the meter is connected to two nodes that are galvanically isolated?







simulate this circuit

Figure 3.



And for what it's worth, isolation transformers have parasitic capacitance between the primary and secondary windings, and between the windings and the transformer's metal enclosure/chassis. This capacitance can and does contribute to the potential difference (the voltage) between Earth ground on the primary and signal ground on the secondary. See Andy aka's answer to this post on EE.SE, Isolation transformer and AC potential between grounds.