AC: Why differentiate between Ground and Neutral?
If wires were 100% reliable and had zero resistance, there would be no difference between the neutral (groundED conductor) and the safety ground (groundING conductor).  Neither condition applies, however.

Even if the neutral-grounded and safety-grounding conductors are connected at the breaker panel, a current-drawing appliance some distance from the box may cause significant voltage drop in the neutral-grounded conductor.  Having any exposed parts of the device connect to ground using a separate safety-grounding conductor will avoid the voltage on its end of the neutral wire from appearing on its exposed parts.

Additionally, using separate conductors ensures that a variety of single failures may occur without creating an immediately dangerous situation (though a second failure which occurs without the first having been corrected could be immediately dangerous).


If exposed parts of a device are connected to the safety-grounding conductor, and a hot wire within the device accidentally touches those parts, short-circuit currents should trip the breaker.
If the hot wire fails between the breaker panel and device, the device would get no power, but there would be no dangerous voltages anywhere near the device.
If the neutral-grounded wire fails, the neutral wire in the device may be only a few ohms separated from direct hot potential, but no current would flow through it, and no path would exist from it to anything the operator might touch.  Exposed parts would still be safely connected to the safety-grounding conductor.
If the safety-grounding wire fails, the device would no longer be protected against the possibility of a hot wire touching the case, but no immediate danger would be created.


If the case were not connected to anything, failure #1 would create an immediate potentially-lethal situation; if it were connected to neutral, failure #3 would create an immediate potentially-lethal situation.  With both wires present, however, a single failure will not create immediate danger.