The feeding-line protection equipment would not operate when the all-parallel AT traction network is interrupted with grounding .Aiming at the phenomena, the impedance of the fault is analysed and calculated by circuit knowledge and mathematics method . It is compared to the impedance of short fault . The fault is a high-impedance grounding one . The impedance value is too much bigger than the short fault’s . The equipment don’t operate because it is out of the fixed value of feeding-line protection . The result is simulated and validated by Matlab/Simulink .