If you haven't already watched the video above from Dave, watch it.
It's a good idea to have an isolation transformer for the device under test, especially when working with high voltages, but don't be fooled into thinking that one will make everything okay. You really need to be awake when working with scopes because its very easy to attach your ground clip to the wrong part and boom. If you are working with power supplies, specifically the primary side, then you need to look at the alternative methods for keeping yourself and the scope safe. Placing your ground clip on the primary side (hot) ground will fry your scope and maybe also you. The two options commonly used are differential probes and floating the scope, but many folks here with advise you not to float it for safety reasons. Differential probes are wickedly expensive however.
Two more options. Find a scope with isolated inputs (channels) these are usually expensive bench scopes or handheld scopes, or, find a bench scope with a battery like some of the Owon ones. Note: Owen specifically advise use of the earthed mains plug even when on battery, because you are effectively floating the scope otherwise.
Much reading you must do young skywalker.
Out of interest, me and my colleague have always floated scopes (I am not endorsing it mind) but once in our new workshop he forgot to remove the scopes earth but told me he had. Next time I probed the smps primary side there was a bang, the rcd tripped and I thought, shucks, that's the scope fried. Luckily, it was fine and I just needed to reset the rcd. Scopes been calibrated since with no problem, but it is a great tektronixs scope of ten years, doubt the Rigol or any modern DSO would stand the same.
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