Prospective Fault Current

All electrical equipments are rated to operate with in permissible current limits depending on the power it can deliver.Any abnormal condition of operation( Single phasing, overloading etc), earth fault or short circuit will draw a fault current from the system which will cause heat and fire, if not switched of with in safe time limits. The fault current will depend on system voltage and type of load.The protective relays are selected depending on the permissible fault current that can be switched off with out damaging the system . This fault current is termed prospective fault current . Depending on the fault it can be some percentage higher than the rated current or even very near to infinity.

Wikipedia is a good resource, too.