Abstract: Aiming at the maloperation fault of a newly installed magnetic-actuated pole-mounted circuit breaker on a specific line, this paper conducts a disassembly inspection. The inspection reveals cracks at the bottom of the phase B and C pole columns, which cause water ingress and freezing inside the mechanism box. Simultaneously, the manual closing limit switch inside the operating mechanism box is corroded, and the copper contacts of the control circuit are ablated and melted. Through analysis, it is concluded that the causes of the pole column cracking include: stress cracking caused by epoxy resin curing shrinkage or excessive installation tightening torque; interfacial deformation under thermal cycling due to differences in the coefficients of thermal expansion of the materials; and microcracks generated by uneven mixing or insufficient purity of the casting material, which eventually propagate into penetrating cracks and ultimately trigger the moisture-induced fault in the mechanism. Consequently, this paper proposes strengthening the control of the epoxy resin casting process, strictly managing tightening torque, conducting condition monitoring on equipment of the same batch, and implementing anti-maloperation measures during power outages to guarantee the safe operation of the equipment.