Hello everyone!
I know the question has been somehow answered, but I will do it anyway.
“tbtkdz” asked:
“I was taught that in alternating current, this latter change its direction in the circuit 50 time per second (50 hertz), or 60 in some countries. Does this not mean that there is actually no fixed phase and no fixed neutral, since the live wire is sometimes the current provider and the neutral is the return current and vice versa”
And my short answer is: NO; it does not mean that!
A better one could be something like this (putting together some pieces already answered by some other members and with a little addition):
The “Neutral” is neutral because it is grounded. The name Neutral does not refer to an absence of electrical currents, but to an absence of electrical tension between it and the ground electrode. It does not depend on phase configuration (monophase or polyphase) or load distribution (balanced or unbalanced).
In a typical commercial building’s three phase wye 120/208, the Neutral is the transformer’s secondary central point (!); but in a residential 120/240 the Neutral is a middle point between two phases. I both cases the Neutral is explicitly bonded to ground.
I know, someone could ask what happen if suddenly the Neutral-to-ground bounding gets broken? Well, we have a safety issue on hands, the system still works (electrically [at least most of it]); but the Neutral is not neutral anymore (it is not acting as a Neutral; is now a floating phase measured against ground [looking at it from the supply-side]).
Hope this helps,
Joe (another Joe).-
