How do sub panels work, how are sub panels wired, why are neutral and ground separated, what happens during a ground fault, what happens when we lose a neutral wire, what happens if the sub panels is incorrectly wired. All this and so much more covered in this video.
source.image: The Engineering Mindset
At the main panel, Neutral and Ground must be bonded together in order to provide a short circuit path in the event of a short in an appliance.Watch the video from The Engineering Mindset for more info:
The neutral and ground are only allowed to be connected at one point in the system. This connection is called the bonding point. Before the bonding point there is only one wire (a seperate wire goes from this point to the grounding electrode/ ground rod).
The wire before the bonding point, to the source, is designed to carry current, and the ground wire after the bonding point is not designed to carry current. After the bonding point, the ungrounded conductor or “neutral” wire carries currents from each “phase” conductor back to the bonding point and on to the center tap of the transformer.