[julie_m]

There is certainly a case to be made that a mains-common chassis doesn't try to lull you into a false sense of security; you know it's not supposed to be safe to touch, and it is insulated accordingly. A set fed from a proper mains transformer by a 2-core lead with no connection to Earth is usually safe to touch anywhere with one hand, by virtue of this lack of a return path; but there will be no tripping of RCDs or blowing of fuses if a transformer-fed set develops an unwanted Earth connection, which would then render the set unsafe to touch.

Of course, there's also a case to be made that you shouldn't be treating anything as "safe to touch", at least not until you have tested it with something a bit more sophisticated than the old neon screwdriver.

I reckon an autotransformer is really the worst of all possible worlds. On first glance, it looks like a transformer, but does not actually provide isolation; while it also cannot be used on a DC supply. And an autotransformer can't really have offered much cost saving over a proper isolating transformer anyway .....