Final answer:
The return conductor is at ground potential due to grounding, providing safety by ensuring the conductor remains at zero volts relative to the Earth in the event of a fault.
Step-by-step explanation:
The return conductor is at ground potential because it is grounded to the electric panel. Ground potential is a point where your potential is zero i.e. it may be earthed even. This will be taken as reference in most of the cases, whereas negative potential is potential which is less than ground potential or less than zero. These are just relative terms used for comparison.
Earth is considered as an infinite reservoir of charge and any addition or removal of charge is negligible on the total charge of earth. Hence, we may conveniently assume the charge on earth's surface as zero. Grounding ensures that any conductor connected in this way is at zero volts relative to the Earth.
This is an important safety feature that helps prevent electric shocks by ensuring that if a fault occurs in the system, the excess charge can be safely directed into the Earth, maintaining the conductor at zero volts. Grounding the metal case of an electrical appliance is a common example, ensuring it is at zero volts relative to the Earth for the safety of users.