The neutral is used as the return path for the current used by the AC device. The ground is used as a safety wire. If a device should become 'energized' because of an internal failure or some other means, then this energy will be diverted to ground and the device will no longer be energized. This is why you see three connectors in a wall outlet, hot, neutral, and ground. Now if your talking 3 phase power then thats a different story. Hope this helps