isolation transformer question

[lokeycmos]

when using an oscilloscope to make measurments across components to a mains connected device, is it better to put the isolation transformer on the scope or the device being tested?

 

[MikeMl]

If you isolate the scope (by breaking its ground prong connection), and then connect the scope probe ground clip to the "hot" side of the line, you have created a death trap. Touching the scope cabinet with one hand, while also touching something that is grounded with the other hand may kill you...

Putting a 1:1 Isolation Transformer between the Line and the Device under test allows you leave the scope cabinet grounded.