In article , Andrew Gabriel
writes

One other thing - there's often a tiny capacitor between the mains
inlet (if unearthed) and the secondary. I'm not sure what it's for,
but my guess is that with the secondary floating, there's a good
chance the whole thing could be capacitively coupled to the primary
oscilating at 40kHz (and harminics) at ~160V (half the primary voltage
swing), and acting like a LW radio jammer. The capacitor would "short
out" the 40kHz (and even more so, the harmonics), which capacitively
leaks through from the primary, and prevent the output acting like a
jamming antenna.

Wot, never designed a mains switcher ;-)

Yes, it shorts out the common mode pump that would otherwise have the
load and any cables connected to it jumping up and down like nobody's
business. They'd never pass EMC testing without it.
--
fred
it's a ba-na-na . . . .