> Hook the isolation transformer to your device under test, not
> your scope. If you hook the isolation transformer to your scope,
> the scope ground will become live when you hook it to your circuit.

That's quite true, better to use the IT on the circuit . The one I
have is only 150VA which rules out using it on anything that takes
more than that (eg if the load is a motor / heater / lighting), but it's
enough to run the scope if I have to do it the dodgy way around.
Nothing quite like having a "live" ground for focussing the mind
or tightening the sphincter

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