Abstract: This paper introduces the specific design method and characteristics of the relevant mechanisms of the WS1-100 (direct-heated) molded case circuit breaker. Keywords: circuit breaker; design 1 Introduction Molded case circuit breakers are widely used products in low-voltage electrical appliances. With the concentration of load density in low-voltage power distribution systems, the increase in distribution transformer capacity, the increase in system short-circuit current, and the increasing requirements for automation and reliability of control systems, molded case circuit breakers widely used in power distribution devices are required to develop towards higher ultimate breaking capacity, smaller size, lighter weight, more complete functions, and novel structure. At the same time, the usage of circuit breakers is also increasing, and they are increasingly becoming international commercial products, leading to fierce market competition. Due to serious shortcomings of molded case circuit breakers such as the DZ20-100: ① large size; ② long arc distance; ③ lack of 660V rated voltage level; ④ imperfect protection functions. Wenzhou Switchgear Factory introduced advanced technology from Terasaki Corporation of Japan to independently design the WS1-100 (direct-heated) molded case circuit breaker, developing a new generation of molded case circuit breakers designed independently in China. As a replacement for domestically produced DZ20-100 and TO-100BA molded case circuit breakers, it boasts advantages such as small size, high ultimate breaking capacity, multiple specifications, complete varieties, and short arc distance. 2. Characteristics of Direct-Heated Bimetallic Strips The overload long-delay characteristic of thermomagnetic molded case circuit breakers is achieved through bimetallic strips. There are three heating methods for bimetallic strips: indirect heating, direct heating, and combined heating. Direct heating, also known as direct-heating, is characterized by its simple structure, high thermal efficiency, and low power loss. Current passes directly through the bimetallic strip, utilizing its own resistance to generate heat. Molded case circuit breakers using direct-heating bimetallic strips are called direct-heated molded case circuit breakers (hereinafter referred to as circuit breakers). For example, the DZ10-100 trip unit with a rated current of 50A and below, and the DZ20-100 and HFB-150 trip units with a rated current of 40A and below (small current ratings), all adopt a direct-heated structure. The WS1-100 direct-heated circuit breaker with a rated current of 100A and below also adopts a direct-heated structure. Directly heated bimetallic strips have higher resistance to obtain sufficient heat, meeting the requirements of the circuit breaker's long overload delay characteristics. However, this often limits the circuit breaker's breaking capacity. Therefore, when calculating and selecting the dimensions and metal material of the bimetallic strip, in addition to considering meeting the circuit breaker's long overload delay protection characteristics, it is also necessary to consider its heat capacity to withstand large short-circuit currents. Since the short-circuit time is very short, the short-circuit current passes directly through the bimetallic strip, and the bimetallic strip does not have time to dissipate heat. Therefore, heat dissipation can be disregarded, and it can be assumed that all the heat loss of the short-circuit current is used to heat the conductor. The power loss of a directly heated bimetallic strip per unit time is: P=I2R (1) Where P is the power loss, W I is the effective value of the short-circuit current, A R is the conductor resistance, Ω According to the energy balance formula: Pt=mCθ (2) Where t is the heating time, s m is the mass of the conductor, kg C is the specific heat capacity of the conductor material, J/kg℃ θ is the temperature rise of the conductor, ℃ And R＝ρ/S, m＝Sγ, substituting into formula (1) and formula (2) and rearranging, we can get: θ＝ρ×I2t/γCS2 (3) Where ρ is the resistivity of the conductor, Ω.m γ is the density of the material, kg/m3 S is the cross-sectional area of ​​the conductor, m2 Bimetallic strips have certain requirements for the temperature range. When the temperature stress exceeds the elastic limit of the bimetallic strip material, the bimetallic strip will deform. If the temperature is close to or reaches the melting point of the material, it will break first at the narrowest part of the cross section and the part where heat is least likely to dissipate. Equation (3) shows that after the bimetallic strip material and size are determined, the short-circuit current value is strictly limited in order to keep its temperature within the upper limit of the allowable operating temperature. 3 Methods to reduce the limitation on the breaking capacity of the circuit breaker 3.1 Contact system The WS1-100 circuit breaker designs the moving and stationary contacts into a parallel conductor structure. The moving contact rotates upward under the repulsive force, while the stationary contact is a rigid body structure. According to calculation, the mass ratio of the moving and stationary contacts is 1:2.6. Under the same electric repulsive force, the moving contact moves faster, and the repulsive force on the moving contact accelerates the repulsion speed, introduces the arc earlier, and the arc voltage between the contacts rises rapidly, which suppresses the rise rate of the actual breaking current and plays a current limiting role. 3.2 Coil with strong arc initiation and current limiting effect Because the WS1-100 circuit breaker is small in size, in the circuit breaker with a current rating of 16A-31A, the stationary contact and the coil are designed as one piece, as shown in Figure 1. Once the moving contact opens and generates an arc, the arc is attracted towards the arc-extinguishing chamber by the magnetic field generated by the coil, and the coil acts as a magnetic blow-out mechanism. More importantly, the coil, made of a high-resistance alloy material, significantly increases the phase resistance of the circuit breaker itself, effectively preventing an increase in the actual breaking current. In the WS1-100 circuit breaker, the coil with a rated current of 16A for the trip unit is made of iron-chromium-aluminum alloy (Cr13A14), with a resistance of approximately 0.018Ω. This increases the phase resistance by 11.38 times compared to a coil without series connection, providing significant current limiting during breaking capacity tests and protecting the directly heated bimetallic strip. The schematic diagram is shown in Figure 2. In Figure 2, the coil current limit Ra is much larger than the bimetallic strip resistance Rb, resulting in a significant current limiting effect throughout the circuit. Another function of the coil is to increase the magnetic field strength between the moving and stationary contacts, thereby increasing the electromagnetic repulsion of the moving contact. Table 1 shows that the actual peak current and current coefficient of the 16A product of WS1-100 circuit breaker using coil current limiting are about 40% lower than those of the 100A product without coil current limiting. 3.3 Reduce static contact pressure spring to reduce repulsion current Another factor that generates contact repulsion force is the junction electric repulsion force caused by the contraction of the contact current line around the contact. This repulsion force is directly related to the final pressure of the contact. The smaller the pressure, the stronger the current contraction line contraction and the greater the electric repulsion force. Under the premise of meeting the circuit breaker temperature rise requirements, WS1-100 circuit breaker uses a small contact spring tension of 3N. This moving contact can be repelled earlier, and the arc is introduced earlier, which greatly reduces the I2t that the bimetallic strip bears in the ultimate breaking capacity test and effectively prevents the bimetallic strip from burning out. 3.4 Features (1) In the operating mechanism of circuit breakers such as DZ20 series, the contact between the trip and the latch is a surface contact, and the contact may produce dead contact phenomenon. To solve this problem, the WS1-100 circuit breaker uses two limiting components to limit the tripping mechanism and design it to contact the shaft, thus successfully achieving line contact between the tripping mechanism and the locking mechanism. This makes the circuit breaker operation more flexible and reliable, and speeds up the circuit breaker's breaking speed. (2) In order to make up for the shortcomings of the instantaneous trip current setting value of 10In for products such as DZ20-100 with In≤40A and below, the WS1-100 circuit breaker is designed with several turns of enameled wire coil on the instantaneous iron core, achieving the shortcomings of the instantaneous trip current setting value of 10In for 32A and above. This improves the protection function of the circuit breaker and also speeds up the breaking speed of the circuit breaker. 3.5 Adoption of new materials (1) The contact material is a key component in the circuit breaker. The physical properties and metallographic structure of the alloy contacts have a crucial influence on the breaking capacity level of the circuit breaker. Since the burnout of the short-circuit breaking current mainly occurs in the active contact, the WS1-100 circuit breaker uses CagW50, which has good arc resistance, as the active contact material and CagWC12C13, which has good anti-welding properties, as the static contact material. Two materials with different hardness are used in combination, and they have the characteristic of low contact temperature rise. (2) The contact support adopts an integral type, which solves the problem of loose contact support when the contact support is riveted in the past and the protection characteristic test such as temperature rise occurs. The support material is UP-100 molding compound with good insulation performance, high strength and strong arc resistance, which prevents the high temperature and strong arc generated during ultimate breaking from damaging it. The circuit breaker successfully passed the 660V rated ultimate short-circuit breaking capacity. It makes up for the disadvantage of DZ20-100 and other circuit breakers not having a 660V rated voltage level. (3) To improve the arc voltage, the arc extinguishing device is designed with a large capacity and many grid plates. The arc extinguishing chamber uses D324 melamine laminated glass cloth board to ensure the arc extinguishing performance of the arc extinguishing chamber. 4 Technical measures to solve the temperature rise of circuit breakers The test shows that the highest heat-generating point of the circuit breaker is usually not at the contact, but at the connecting plate a where the thermal element and the moving contact are connected. Since the original DZ10 series products and their upgraded products welded the moving contact and thermal element to their respective soft connections separately, as shown in Figures 3 and 4, and then connected these two parts at point a and fastened with screws. Therefore, in order to reduce the temperature rise at the highest point, the WS1-100 circuit breaker changed its structure and combined the two parts into one, that is, welded its 8 parts into one piece, as shown in Figure 5. Since this connection method reduces the contact resistance, it reduces the temperature rise of the circuit breaker. 5 Comparison of technical indicators of plastic case circuit breakers, as shown in Table 2. 6. Conclusion When designing directly heated bimetallic strips in circuit breakers, it is essential to consider not only the requirements for long overload delay characteristics but also the temperature rise during short circuits. Employing methods such as electrodynamic repulsion contact systems and series current-limiting resistors to reduce the limitations imposed by high breaking capacity on directly heated bimetallic strips is of significant practical value.