Sorry I blasted you earlier, mrmain!
It's just that mains AC voltage safety is not something to be taken lightly - either
know what you're doing there, or stay away from it & leave it to people who do (and what you described
seemed outright stupid/dangerous).
None of the conductors in question is accessible, except during installation &/or servicing.
So people doing installation/servicing are aware that something marked "+5V" might be read as "AC live voltage +5V" :?: And that awareness is guaranteed to be the case X years after equipment is installed, company that installed it has moved on & other folks are doing the servicing now? And physical construction is such that people other than those aware of "high voltage, dangerous" have no way of coming into contact with any of that circuitry, without
breaking into a box that says "high voltage, keep out" on it?
Not saying it does, but just supposing that
all of that holds true (and will keep doing so), what exactly would be the problem using mains AC live wire as 0V reference point? I mean if circuit as a whole is adequately isolated from outside world (a few kV at least), then the safe-to-touch ground (not the 'neutral' but the safety one, that connects to the planet we live on with very low resistance & high current capability) wouldn't come into play, never carry current & therefore never cause any fuse or whatever to trip, right?
If that doesn't hold true: please explain how current could
possibly flow from mains-connected circuit (
either live or neutral) to safety ground? Whichever route could be pointed out = what would make above condition(s) fails = what would make the circuit unsafe = what could make you responsible if someone gets electrocuted.
Just the plain fact that chosen reference point might have live voltage on it, shouldn't matter: if it happens to be the neutral wire, the other wire (live) will have 120VAC on it (with respect to chosen 0V point). If reference point happens to be the live wire, the other wire (neutral) will have 120VAC on it (with respect to chosen 0V point). Same thing in both cases, shouldn't affect circuit operation.