> Are you suggesting that ALL measurements on a system > which is not galvanically isolated from the mains should be > done through a 1:1 transformer ? No problem using a multimeter for most signals, and you don't *need* to isolate a circuit for that. But often you'd want to know the quality of those signals, and a scope is going to be the common tool for that. You can't just hook up scope ground to the Vss of such a circuit, so either the circuit (or the scope, not so likely, depends on the scale of what needs isolating) has to be powered through an isolating transformer. Vss becomes E when you earth it with the scope. You still don't want to be poking your fingers where they aren't welcome though, plenty of smoke and screams itching to be let loose -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist