Design, Analysis, and Testing of a New Microcircuit Breaker Thermal Trip Unit Concept Based on Compliant Mechanisms

Miniature circuit breakers (MCBs) are protective devices used in low-voltage applications. To perform their protection function, MCBs contain two different trip systems: a thermal unit and an electromagnetic unit. Current thermal trip unit designs were introduced decades ago and are based on bimetal technology. This paper introduces a new technology based on compliant mechanisms that replace bimetals in MCB. The proposed architecture uses a thermosensitive compliant mechanism whose design is calibrated to perform as traditional bimetal designs do, but with the advantage of reduced heat losses. An additional advantage of the new design is its monolithic construction, which reduces the number of parts in the MCB. The design of an MCB equipped with this technology is presented. Basic sizing equations for design purposes are introduced. The performance of the thermal trip unit is analyzed using finite-element-analysis models, with special attention paid to the protective function and thermal losses of the new design. Laboratory experiments with prototypes are also presented. Results indicate that the performance of the proposed design is similar to that of the bimetal technology that it replaces, with reduced power losses.