The neutral is a conductor with potential 0 or potential difference 0. Its function is precisely to create an unbalance, a potential difference that allows the existence of electric current through the phase conductor.

The neutral wire of an electrical installation serves as a return conductor for the current that circulates in single-phase circuits. If a light bulb for example is connected only to the phase, it cannot work as it does not have a neutral so that the electrons can circulate

The UPS will turn off only when the utility grid fails and there is no load connected to the UPS. This function serves to extend the life of the battery.