Views: 0 Author: Site Editor Publish Time: 2026-08-11 Origin: Site
Translated from: Electrical & Energy Management Technology (2021 No. 4) Author: HU Jinli
In circuit breaker design, simulation methods facilitate the selection of materials for the bimetallic sheet and allow a preliminary evaluation of whether the tripping time of the circuit breaker satisfies the design requirements, thereby enhancing production efficiency and reducing development costs.
To obtain the accurate transient temperature rise and deformation of the bimetallic sheet, this paper utilizes Ansys finite element analysis software to perform thermal analysis on the bimetallic element. During the simulation procedure:
3D Modeling: First， Creo software is employed to construct the bimetallic sheet model, which encompasses four distinct components: the active layer, passive layer, intermediate layer, and welding point.
Import & Material Assignment: Second, the constructed model is imported into the Ansys APDL module, where element types are defined, material properties are assigned to each respective part, and finite element meshing is executed.
Boundary Conditions & Execution: Finally, the initial temperature of the bimetallic sheet is defined. Based on the ambient temperature and the applied current, the heat dissipation coefficients for each surface are calculated and assigned. Fixed constraints and current boundary conditions are applied to conduct transient thermal-mechanical coupled simulation. This yields the transient temperature field and deformation distribution, from which the exact time required for the bimetallic sheet to reach the tripping position is determined.
The miniature circuit breaker (MCB) examined in this study is illustrated in Figure 4. The bimetallic sheet is situated on the right side of the circuit breaker interior. Its left side is connected to the moving contact via a flexible braid, while its right side connects to the right external terminal wire. The rated current of this miniature circuit breaker is In = 20 A.
Figure 4: Structure of the Tested Miniature Circuit Breaker (MCB)
Internal layout showing the 20 A MCB mechanism, arc chute, moving contact, and the bimetallic thermal release assembly situated on the right side.
Focusing on the transient temperature profile and dynamic deformation, the bimetallic model was constructed according to the exact dimensions of the prototype, as shown in Figure 5.
Figure 5: Bimetallic Sheet 3D Simulation Model
The structural model consists of four distinct sub-domains: Active Layer, Passive Layer, Intermediate Layer, and Welding Point.
As modeled in Figure 5, the bimetallic assembly comprises four parts: active layer, intermediate layer, passive layer, and welding point. When energized, the lower end of the active layer serves as the current inflow surface, while the welding point acts as the current outflow surface, as detailed in Figure 6.
Figure 6: Current Inflow and Outflow Surfaces of Bimetallic Sheet
Shows the designated electrical contact boundaries where current enters at the bottom of the active layer and exits through the upper welding pad.
To guarantee high simulation fidelity, material parameters corresponding to the physical prototype were assigned to each region. The four materials involved correspond to the active layer, passive layer, intermediate layer, and welding point of the 20 A MCB. The physical and mechanical properties are summarized in Table 1.
Through finite element simulation, transient thermal-mechanical fields were calculated using Ansys. To benchmark against empirical tests, the ambient environment was set at 25℃ with a test current of 30 A （1.5 times rated current In). According to prototype mechanics, the operating mechanism trips when the tip displacement reaches 1.35mm. The simulation predicted a required trip duration of 13s.The temperature field and deformation field at the moment of reaching the trip position are illustrated in Figure 7 and Figure 8.
Figure 7 & 8: Simulation Results at Tripping Moment (t = 13 s, I = 30 A, Tamb = 25 °C)
• Figure 7 (Temperature): Maximum temperature occurs at the root/bottom (73.54 °C), decreasing gradually toward the top tip (minimum 70.51 °C).
Figure 8 (Deformation): Tip displacement reaches 1.330 mm, satisfying the mechanical release threshold (~1.35 mm).
As seen in Figure 7, at the point of tripping, the highest temperature occurs at the bottom root of the bimetal, reaching 73.54 °C. Overall, the temperature distribution exhibits a gradient that is highest at the base and decreases towards the upper tip, with a minimum temperature of 70.51 °C. Under these conditions (Figure 8), the top tip displacement reaches 1.330 mm, successfully actuating the release mechanism. Thus, under 25 °C ambient temperature and 1.5 In overload, the MCB trips at 13 seconds.
While in practical operation the breaker opens immediately once the trip threshold is reached, evaluating the steady-state temperature and ultimate deflection provides vital insight for bimetallic material and geometry optimization. The steady-state temperature and deformation distributions are presented in Figure 9 and Figure 10.
Figure 9 & 10: Steady-State Temperature and Deformation Distribution (I = 30 A)
Figure 9 (Steady Temperature): Maximum temperature reaches 86.06 °C at the base, and minimum temperature is 83.07 °C at the tip.
Figure 10 (Steady Displacement): Ultimate maximum tip displacement stabilizes at $1.558\text{ mm}$.
As indicated by Figures 9 and 10, when subjected continuously to 30 A current at 25 °C ambient until thermal equilibrium is attained, the peak temperature reaches 86.06 °C and the lowest temperature is 83.07 °C. The thermal pattern remains consistent with the transient profile (hotter at base, cooler at tip), and the maximum tip deflection reaches 1.558 mm.
The temporal dynamic responses are plotted in Figure 11 (short-term: 1 – 13 s) and Figure 12 (long-term: 0 – 200 s). These curves depict the time-dependent deflection behavior. As time progresses, deformation accumulates rapidly during initial heating and asymptotically approaches thermal stability at approximately 160 s, achieving the ultimate plateau of 1.558 mm.
Figure 11
Figure 12
To enhance the thermal trip design reliability and factory calibration efficiency, empirical tripping tests were performed on 20 sample groups of circuit breakers under conditions identical to the numerical simulation (25 °C ambient temperature, 30 A test current). The experimental setup is shown in Figure 13.
Figure 13: Experimental Test Rig for MCB Thermal Tripping
Precision test bench configured with constant current power supplies, automated timing equipment, and ambient thermal control.
Each breaker sample underwent 6 consecutive tripping test cycles under 30 A current load. The recorded trip times across all test runs are plotted in Figure 14, with maximum and minimum trip times for each sample summarized in Table 2.
Figure 14
As shown in Table 2, due to subtle manufacturing tolerances and individual component variations, the tripping times vary slightly among different breakers. Across all 20 sample groups:
The average maximum trip time is 14.9 s.
The average minimum trip time is 13.5 s.
These experimental values correlate exceptionally well with the FEA simulation result of 13.0 s, confirming the accuracy and practical utility of the finite element simulation method.
The measured maximum tip deformations of the bimetallic sheets across the 20 test breaker samples are recorded in Table 3.
As detailed in Table 3, the measured tip deflections range between 0.9 mm and 2.0 mm, reflecting noticeable dispersion due to physical assembly tolerances in mass production.
Simulating and analyzing the transient deformation of bimetallic elements in miniature circuit breakers plays a crucial role in predicting trip times and improving the stability of thermal protection mechanisms.
Integrated Numerical Modeling: By combining 3D CAD modeling (Creo) with finite element analysis (Ansys APDL), a refined thermal-mechanical coupling simulation model was established, capturing the exact structural layers (active, intermediate, passive, welding point) and electrical/thermal boundary conditions.
Transient & Steady-State Predictions: For a 20 A MCB subjected to 30 A current at 25 °C ambient, simulation demonstrated that the bimetal reaches the 1.35 mm mechanical trip threshold at 13 seconds (temperature: 70.51 – 73.54 °C). Under continuous load, thermal equilibrium is reached near 160 s with a maximum temperature of 86.06 °C and displacement of 1.558 mm.
Experimental Validation: Empirical tests across 20 group samples (6 trip cycles each) yielded an average trip time of 13.5 – 14.9 s, showing excellent agreement with the 13.0 s simulation calculation.
The proposed simulation workflow provides a reliable, cost-effective, and efficient methodology for optimizing thermal trip units, reducing development cycles, and guiding automated factory calibration for miniature circuit breakers.
