First of all I'm pretty sure I know the theory behind the effect. I know about the inductive and capacitive coupling, about 3-phase balance problems and about ground bounce, so this question isn't about normality of existence of voltage between ground and neutral line.
What I want to know is in general are there established levels at which the voltage is normal, at which I should start investigating the voltage and at which I should kill power and start panicking?
!!!PLEASE READ THIS FIRST BEFORE ANSWERING!!!
Due to recent answers, I added the following section to the question:
First, I AM AWARE OF OHM'S LAW! I hoped that it would be obvious from the second sentence of the question, but it isn't.
Second, the type of answer I'm looking for is something like: "The difference in a properly installed should be below XX V because it is written so in regulation YY". Bonus points for information on how number XX has been derived.
Alternatively acceptable answer would be: "There's no limit in any regulatory documents because of ZZZ." if it's accompanied by a good description of ZZZ.
I will down-vote answers which basically say: Ohm's law.
I know that this sounds extremely rude, but unfortunately I don't see any other way to explain exactly what I'm asking. If you don't know the answer, that's fine by me.