Why do we need a dedicated neutral power line? Can’t we just use ground as neutral?
I’m an AC system, the hot wire and neutral swing back and forth, from positive to negative and back. Power flows from hot to neutral, stops briefly in the middle, and then reverses and goes negative for a while. Importantly, the neutral wire is part of the active electrical circuit. It carries power more or less all the time. Ground … Isn’t really the same thing. It’s a local connection to ground, usually a rod pounded into the lot on which your building sets. Now, you can make a circuit that way, but your ground system will become hot, so to speak, which is not the best idea, and certainly not the intent.
Because if something comes loose, current flowing through that line could cause it to sit at something other than neutral. The ground is a backup for safety in case the neutral isn’t really neutral, and it is safer. Because there is no power flowing through it, even if it is *weakly* connected to ground, it should still be at roughly neutral/ground voltage. If your neutral is weakly connected to ground it is likely to carry a moderate amount of voltage.
Neutral and hot lines are also commonly mixed up both in older building wiring and in poorly wired appliances.
Because ground is a separate pin that does not look like the first 2, and it is usually carried by a casing or conduit not by a wire, it is much harder to mistake ground and one of the hit or neutral lines. This helps to keep it safer even if there is some level of mistake in how things are wired.