I had a discussion with my colleges regarding Oscilloscope measurements and DUT (device under test) isolation. Specifically a device connected to a AC/DC wall-plug.
I think I am aware of the concepts and I have read How is using a transformer for isolation safer than directly connecting to the power grid?, Why do we need an isolation transformer to connect an oscilloscope? and Isolation transformer and scope when troubleshooting SMPS.
From my understanding, if you have the DUT behind an isolation transformer, one has to be potentially careful not to connected the scope GND to the DUT, because there may be a large potential difference between GND/PE (normally connected on the scope side) and the DUTs "GND reference".
A common way I often used to circumvent this problem is to use the “channel A minus B” pseudo-differential approach. Another would be to float the scope / disconnect PE which is obviously a safety risk but may be a acceptable approach when working with low voltages.
Back to the question:
A college asked why it seems that there is no need to consider the "isolation issue" when measuring on a DUT that is powered from a wall plug, since those use an isolation transformer as well.
My initial thought was that there may be a Y-cap between primary and GND that serves as kind of a potential reference. However thinking about that, one would want to connect that cap to neutral not to phase which can't be guaranteed.
If wall-plugs do not have some reference to the primary side, wouldn't it be even an issue to, say connect a USB cable to some wall-plug powered device, effectively connecting PE to "some" potential? Am I just missing something obvious here?