Abstract: Current transformer (CT) is key primary equipment in open-type substations. Breakage of the secondary circuit may easily trigger severe accidents such as overvoltage and protection failure to operate. Taking the abnormal vibration and secondary circuit breakage fault of LCWB6—110W2 oil-immersed current transformer at the 110 kV bay of a 220 kV substation as a case study, combined with on-site tests, X-ray inspection and disassembly analysis, it is identified that the root cause of the fault lies in inherent cold solder joint defects of the soldered connection for secondary leads. The solder joints degrade continuously under long-term electromagnetic vibration and temperature cycling, eventually resulting in circuit breakage. According to the fault mechanism, a special inspection scheme for current transformers adopting the aging soldering process is proposed, which provides a reference for hidden danger treatment of similar equipment.