Fatigue Life Analysis of Main Tension Spring in Molded Case Circuit Breaker and Optimize Design
Li Kui1,2, Zhang Haotian1,2, Hu Bokai1,2, Niu Feng1,2, Guo Ze1,2
1. State Key Laboratory of Reliability and Intelligence of Electrical Equipment Hebei University of Technology Tianjin 300401 China; 2. Key Laboratory of Electromagnetic Field and Electrical Apparatus Reliability of Hebei Province Hebei University of Technology Tianjin 300401 China
Abstract：A molded case circuit breaker is a low-voltage switchgear used in the distribution end of power systems. It has the advantages of small size, broad applicability, ease of installation and operation, and is widely used in line protection. When overload or short-circuit faults occur in the circuit, the molded-case circuit breaker can promptly interrupt the faulty line, thereby ensuring equipment and personal safety. However, the operating mechanism of molded-case circuit breakers can withstand significant alternating stress during frequent opening and closing operations, leading to cracks or even sudden fractures in the mechanism and seriously affecting the safety of the power system. Therefore, accurate prediction of the mechanical life of molded-case circuit breakers can enable timely maintenance or replacement of critical components before fracture. Optimizing the structure of key components can improve the service life and reliability of molded-case circuit breakers. This paper focuses on predicting the fatigue life and optimizing the structural design of the main tension spring of an MCCB. The location of stress concentration and the failure mechanism of the main tension spring in the circuit breaker are analyzed. A rigid-flexible coupling dynamic model is established, and the stress-strain variation and stress-strain cloud map characteristics of the main tension spring of the fragile component are analyzed during opening and closing. The position of the stress concentration in the main tension spring is identified. The influencing factors of random fatigue fracture of the main tension spring are analyzed, and the distribution characteristics of its fatigue life are examined. Based on fatigue life data from standard specimens of primary tension spring materials under different strains, the probability distribution of material fatigue performance is derived using a Bayesian method. A Monte Carlo simulation was used to calculate the fatigue life of the main tension spring, which was found to follow a lognormal distribution. The simulation results are validated through mechanical life tests on molded-case circuit breakers. The effects of wire diameter, surface processing quality, and structural shape on fatigue damage in the main tension spring are investigated. The design of the main tension spring structure is optimized to reduce fatigue damage, thereby improving its fatigue life and enhancing the mechanical life and reliability of the circuit breaker. Finally, the optimization design effect of the main tension spring structure is verified through simulation.
李奎, 张昊天, 胡博凯, 牛峰, 郭泽. 塑壳断路器主拉簧疲劳寿命分析及优化设计[J]. 电工技术学报, 2026, 41(2): 689-700.
Li Kui, Zhang Haotian, Hu Bokai, Niu Feng, Guo Ze. Fatigue Life Analysis of Main Tension Spring in Molded Case Circuit Breaker and Optimize Design. Transactions of China Electrotechnical Society, 2026, 41(2): 689-700.
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2026, Vol. 41 
