Abstract: To coordinate with the time grading and sensitivity of self-healing lines in distribution networks, the protection settings of transformers often require corresponding adjustments. However, due to the relatively low operation and maintenance management level at the county-level staff, the principle of protection grading is often abandoned during the setting calculation process, with excessive emphasis placed solely on the reliability of self-healing lines. This results in poor coordination of protection settings among different devices. In some cases, after a fault occurs, power is restored blindly without identifying the root cause, which expands the outage scope and increases the difficulty of fault diagnosis. This paper presents a fault case where a fault on a self-healing line led to an out-of-zone trip of the upstream switch at the incoming line of the substation. By analyzing the protection operation waveforms and combining with on-site investigation, it is concluded that the setting value of the self-healing line was excessively high, the setting miscalculation occurred, and the sensitivity of the protection starting element was insufficient. Based on the issues revealed in this case, corresponding rectification suggestions are proposed.