To enable a breaker to trip, fault current has to flow back to the source (the power company transformer) via the incoming neutral, not the earth. The impedance of the earth by itself is too high. The main purpose of the earth ground is for lightning protection and to stabilize the supply voltage. IMO the use of the term "ground" when referring to the protection against fault current is a misnomer, because in reality, it is the return of this fault current via the service neutral that affords the protection.
http://img.villagephotos.com/p/2003-5/191557/circuit%20%20load.JPG
http://img.villagephotos.com/p/2003-5/191557/ground_fault.JPG
http://img.villagephotos.com/p/2003-5/191557/circuit%20%20load.JPG
http://img.villagephotos.com/p/2003-5/191557/ground_fault.JPG