The short - circuit might be occur in 2 or 3 phases of electric power system . The amount of fault current is always high enough . It actually depends upon the voltage which has been short - circuited and upon the impedance of the circuit up to the fault point . The copper loss of the fault providing transformer is abruptly increased . This increasing copper loss causes internal heating in the transformer . Large fault current also generates severe mechanical stresses in the transformer . The maximum mechanical stress occurs during first cycle of symmetrical fault current .