Abstract: In the field of relay protection, the contact problem of the electrical secondary circuit of the measurement system is a major factor leading to equipment failure. In the face of such problems, most of the first-line maintenance workers of the power system infer this conclusion through appearance and experience, and then conduct circuit inspection until the problem is solved. Although this working mode can solve practical problems, it lacks specific phenomenon induction, cause analysis, test verification, and follow-up tracking of the process result of poor contact problems, which can only discuss one thing at a time. Combining the whole process of one contact fault repair of the measurement system on ultra-high voltage series compensation(SC) platform in China, deeply analyze and study the root cause of the atypical contact fault. Through the analysis and summary of the atypical contact fault, it helps to improve the operation and maintenance technology of SC. Moreover, the research results show that it can also be applied to the relay protection operation and maintenance work in the field of power system relay protection, which directly improve the operation and maintenance work efficiency and even the safe and stable operation level of the regional power grid.