What is the 0V reference for MOV's input protection on power supply?
What I don't understand is what is the 0V &quot;ground&quot; reference these
application notes are referring to?

Well, despite you thinking this: -

It is both regulatory and practically impossible to use the PE (=FG)
since even the smallest MOV current could trigger the earth protection
circuit breaker.

The answer is still actually the &quot;protective earth&quot; (PE). If a surge causes a live-neutral imbalance current, then it may trip an RCD (UK) or GFCI (US). That cannot be avoided other than by designing the power supply AC interface to not require a protective earth point and hence it's impossible to utilize surge protectors connected to PE.