I have a Chinese scope with a two-pin power cord and a plastic body, and measure megaohms between its ground terminals and the actual earth, so I believe it has an internal isolation transformer (I suppose I could open it to confirm...) I've used it to measure small floating voltages before without problems (connecting the ground to +5V relative to earth, and the probe to a current sensing resistor also around +5 V), but now I want to measure power supply voltages where the ground of my scope will be connected to a rectified wave at maybe 200 VAC relative to Earth. Any problems? Does an isolation transformer make everything good?
I know that what you're supposed to do is use two separate probes and use the Math function to subtract them, but that doesn't work in practice because the common-mode voltages are so much higher than the differential voltage.
... Oh. Besides the, um, safety reasons. The whole scope will then be at 200 VAC relative to Earth ground, so if I touch one of the scope's BNC connectors and a real ground at the same time I'm in trouble. I guess that's a big problem, but is it the only problem? Floating the device under test with an isolation transformer would not have this problem? But that transformer would have to be a lot bigger to handle the high power. Does it have other problems due to parasitic capacitance, etc.?
Is there any trick to using the differential math function method by reducing the common-mode voltage with a capacitor or large resistor, but not connecting it directly enough to produce a safety hazard?