Your confused on the EARTH and a actual ground, yes I know the NEC does not make it easy, but you have to remember this, that little hole in your receptacles does NOT GO TO THE EARTH. I cannot stress this enough.
An accident waiting to happen.
Here is another picture to study, this one actually involves a service without a neutral, but it may help you understand that the earth does nothing in relation to circuit breakers. Picture the motor shown, a well casing with a well pump that has a ground fault.