Abstract: In view of the extreme conditions of 6kV F-C in the ground short circuit fault at the moment of closing, there will be a long inherent delay phenomenon of opening, resulting in the safety hazard problem of protecting the occurrence of leapfrog tripping. In this paper, a 6kV F-C opening inherent delay test and its protection coordination tuning method are proposed. By analyzing the wiring and protection setting value of the control loop of the F-C, combined with the experimental design and the execution of safety measures, the action opening situation of the switch closing in the ground short circuit fault is truly simulated. The matching relationship between the inherent delay of the opening of the F-C and the setting value of the zero-sequence protection action time was further checked, avoid the protection trip due to the inherent delay problem of the switch opening when the switch is closed with a ground short circuit fault at the moment of closing, improved reliability of equipment operation, it provides a strong guiding basis for reasonably setting the zero sequence protection action time setting of the switch.