To answer the OP:
Some people say, “The neutral and earth ground wires are going back to same spot in the breaker box! That’s stupid… why not just run one wire? If you want to ground the chassis, fine. Just connect the neutral to the chassis.”
This might sound like a reasonable thing to do at first. But upon further reflection we are left to conclude that connecting the neutral to the chassis is a Bad Idea[sup]TM[/sup].
Let’s say you have a motor with a metal frame. The frame is supposed to be connected to earth ground via the “third prong,” but some genius installed a two conductor power cord and connected the neutral to the chassis. That way, if a hot wire shorts to the chassis, it will blow the breaker.
So you walk over to an outlet with the idea of plugging the motor in. You are grounded (as is often the case). The power switch is in the “on” position. (Or perhaps there is no power switch.) Your right hand is touching the chassis. You left hand is holding the plug.
So far so good, right?
Now when you insert a two-prong plug in an outlet, do you think the hot and neutral prongs make contact at the exact same time? Of course not; that would be impossible. So which makes contact first, the hot or the neutral?
If the neutral prong makes contact first, you won’t feel a thing. If the hot prong makes contact first, you will feel a mighty jolt. This will start when the hot prong first makes contact and end when the neutral prong first makes contact. If this interval is 83 milliseconds (for example), your body would be hit with five complete cycles of 60 Hz AC.
Another scenario is when there is a break anywhere in the neutral line. Still another scenario is when you accidentally pull on the cord while operating the equipment and the plugs comes halfway out of the outlet, with the neutral disconnected while the hot remains connected. In any case, the lesson here is that you can’t connect the chassis to the neutral. And as my previous post stated, it is usually a good thing to ground a chassis. Therefore, you must run two conductors in order to have a grounded chassis.