I'm guessing this power supply only has a two wire line cord, right?
Switchers have X capacitors across the line and Y capacitors line to
chassis to attenutate conducted EMI. If the chassis is not grounded, the Y
capacitors form a nice voltage divider, putting half the line voltage on
the chassis. This is considered acceptable if the "leakage current" is low
enough (in the neighborhood of 1mA, less for patient contact medical
equipment). Your DVM, of course, drew considerably less than 1mA, so you
were able to measure the unloaded voltage out of the voltage divider.

Even linear supplies with 50/60 Hz magnetics will have capacity to ground
between the primary and the core. This will place a voltage on the
chassis. Again, leakage current is the concern.

Now for another story...  The first PC compatible computer I bought was a
big 286 machine (still have it!). It had a line cord for the computer and
another for the monitor. Plugged in the computer, then plugged in the
monitor. Loud humming noise and smoke starts coming out of the monitor.
Quickly pulled the plugs! Turns out the monitor line cord had line and
safety ground reversed, so line went to monitor ground, then through the
video cable to the computer, where it found ground. The smoke out of the
monitor was the ground lead from the power inlet to the chassis. Replaced
the line cord and all was well...

Harold


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