Abstract: In the 110 kV and 220 kV power grids, some transformer neutral points operate ungrounded based on operational requirements. Under this operational mode, line faults can potentially cause breakdowns on the transformer neutral point gaps. To ensure the selectivity of protection, it is necessary to coordinate protection timing. This case involves a permanent single-phase fault on a 220 kV line where the single-phase recloser was unsuccessful, and improper timing setting of transformer gap protection led to misoperation of neutral point gap protection.