Abstract: A 47-minute voltage anomaly occurred in the 35 kV switchgear of a 110 kV substation in a metro system. The fault was diagnosed as high-impedance grounding caused by a loose C-phase copper busbar bolt. The fault current remained below the protection setting, and no trip was triggered, exposing a protection dead zone. A simulation test was conducted jointly with the switchgear manufacturer. The test results showed that the contact resistance under hand-tightened condition was several to ten times that under standard torque. Under a 630 A current for 10 hours, the temperature difference at the poor-contact point was only 2–4 K, far below the temperature anomaly threshold. The damaged bushing had a partial discharge value of only 0.06 pC in an gas compartment and still met the acceptance criteria. These results confirm that such contact defects are extremely difficult to detect by conventional means during the incubation period. It is recommended that loop resistance test data after switchgear installation be reviewed during the construction of new metro lines and that a voltage trend alarm mechanism be established.