Abstract: This article introduces the potential failure mode analysis and optimized design of the magnetic circuit system, driving system and contact system of a low-height, high-dielectric-strength medium-power relay, and briefly explains the technical difficulties and material selection. Key words: electromagnetic relay; design; high dielectric strength 1 Introduction According to IEC 335 "Safety of household and similar electrical appliances" and IEC 730 "Safety of household and similar electrical automatic controllers", household appliances and automatic controllers are classified into Class 0, Class I, Class II and Class III according to the level of electric shock protection. In Europe, safety performance requirements are high. For products with rated voltages above the safety voltage, there is an increasing demand for Class II electrical appliance or controller protection against electric shock, meaning they must provide double or reinforced insulation. To meet this requirement and reduce costs, more and more customers are requesting electromagnetic relays that meet reinforced insulation requirements. Simultaneously, to achieve product miniaturization, the relay height must be less than 12.5mm. Research revealed that most medium-power relays on the market cannot meet this requirement. Therefore, our company decided to develop a low-height, high-voltage medium-power relay. 2. Product Performance Indicators The main technical indicators of the product are shown in Table 1. 3. Structural Design 3.1 Overall Structure (See Figure 1) 3.2 Magnetic Circuit System The magnetic circuit structure used in medium-power relays has the following forms (see Figure 2). The most commonly used is the scheme in Figure 2(a), which has fewer parts, a simpler structure, and a higher cost advantage. However, because the moving spring and yoke are riveted together, it is difficult to meet the reinforced insulation requirements between the magnetic circuit system and the contact system. In the scheme shown in Figure 2(b), the magnetic circuit system and the contact system are connected by a vertical push rod, which can meet the high isolation requirement. However, because the magnetic circuit system and the contact system are placed vertically, they cannot meet the customer's low height requirement. If the length of the push card is shortened and the height is reduced, it will be difficult to meet the enhanced insulation requirements between the magnetic circuit system and the contact system. This product design adopts the scheme shown in Figure 3. The horizontal magnetic circuit system is installed in the unidirectional open base cavity. The side wall of the base is used to isolate the magnetic circuit system and the contact system, which can well meet the enhanced insulation requirements of electrical clearance greater than 8mm, creepage distance greater than 8mm, and penetration distance greater than 2mm between the magnetic circuit system and the contact system. In order to avoid magnetic loss during the riveting of the yoke and iron core, effectively improve the magnetic circuit efficiency, and reduce the coil power consumption, the yoke and iron core adopt an integrated structure (Figure 4). The coil lead is inserted into the coil frame with round wire, which is less difficult and less expensive than using insert injection molding. By reasonably designing the flattening size of the round wire (Figure 5), the insertion and extraction force of the lead is guaranteed, and the generation of plastic debris is reduced. The armature uses a U-shaped notch to engage with the coil frame (Figure 6) and is limited in the front and rear directions by a compression spring. The armature rotates on the spring surface, avoiding rotation on the plastic surface, which would generate plastic debris and cause inflexible rotation. 3.3 Drive System To meet the requirement of a high insulation distance between the magnetic circuit system and the contact system, a pusher (Figure 7) is designed along the length direction. When the armature is attracted, it drives the pusher towards the contact system, making the moving contact contact the normally open stationary contact. The pusher moves along the side of the base, effectively reducing the product height to meet the requirement of less than 12.5mm. In existing designs, the armature (Figure 8) with a punched inclined plate is inserted into the notch of the pusher (Figure 7) to achieve positioning and connection between the armature and the pusher, which easily generates plastic debris or causes the pusher to crack. This product uses the boss of the armature to engage with the inclined surface of the pusher to achieve vertical positioning of the pusher (Figure 9), avoiding interference fitting that generates plastic debris or causes the pusher to crack. (1) As shown in Figure 10(a), first position the armature and the push card at an acute angle so that the armature boss passes through the push card. Since the gap of the push card is greater than the thickness of the armature boss, no plastic debris will be generated or the push card will crack. (2) As shown in Figure 10(b), rotate the armature so that the armature and the push card are positioned at a near right angle, and the armature boss contacts the inclined surface of the push card. Since the gap between the push card and the armature is less than the height of the armature boss at this time, the armature will not fall off. (3) As shown in Figure 1, combine the armature and the push card and install them into the magnetic circuit. Due to the limitation of the yoke and the base, the angle between the armature and the push card is always kept at about 90° when the product is working. 3.4 Contact System In general, the deformation of the relay spring is generated along the length direction, as shown in Figure 11(a), but this product is very low in height and only deforms in the height direction, which is not flexible enough. To overcome this shortcoming, a planar U-shaped spring can also be used, as shown in Figure 11(b). Although the situation has improved to some extent, the contact system cavity is still not fully utilized. This product uses a moving spring as shown in Figure 11(c). The moving spring can deform in all three dimensions, making full use of space to achieve good flexibility. The stress-strain measurement results of the moving spring structures in Figure 11(b) and Figure 11(c) are shown in Figure 12. As can be seen from Figure 12, the flexibility of the reference moving spring is worse than that of the moving spring in this product. At the same time, after the reference moving spring is subjected to force at the pushing point, it tilts forward, causing contact at the upper end of the contact and reducing electrical life performance. After the moving spring of this product is subjected to force at the pushing point, it deforms forward, and the moving contact connects with the stationary contact. At this time, if the moving spring is pushed forward again, the upper part of the moving spring will generate a torsion forward with the contact as the fulcrum, thereby generating contact tracking and using the relative rotation of the contacts to effectively pull off the possible adhesion between the moving and stationary contacts. Because the product is small in size, the plastic on the back of the upper stationary spring (Figure 13) fixed on the base (Figure 14) is thin. Under high temperature, the upper stationary spring will tilt backward, causing the contact overtravel to disappear and the product to fail. Therefore, this product adopts an L-shaped upper stationary spring that fits into the groove of the base. Compared with the method of fixing with a plane, it has better thermal stability. [b]4 Process difficulties[/b] This product uses an integrated iron core. In order to ensure the driving ampere-turn value of the product, the small cross section of the iron core is designed to be 2.2mm wide and 1.8mm thick. Because the width-to-thickness ratio is close to 1, it is easy to twist and deform during stamping. After many tests, the company's advanced precision mold manufacturing technology was used to manufacture qualified parts and achieve mass production. 5 Application of new materials After the successful development of this product, it is mainly sold to Europe, the United States and other regions. Therefore, the material selection must meet the requirements of European standard VDE0435/0110 and American standard UL508. VDE standard requirements: (1) Hot ball pressure test: temperature higher than 125℃. (2) Glow wire test: Temperature above 850℃. (3) Tracking resistance index: Above PTI 250. (4) Flammability test meets requirements. UL standard requirements: (1) Flammability test meets requirements. (2) Coil insulation level meets requirements. Based on the above requirements, UL certification focuses on the materials used in the coil frame and housing, while VDE focuses on the materials used in the base and push block insulation. 6 Conclusion The product performance meets the design requirements and reaches the level of similar international products, and has passed VDE and UL certifications. During the product design process, potential failure mode and consequences analysis was carried out well, ensuring the smooth progress of product trial production and mass production. The product is exported to Europe and the United States in large quantities, with stable quality and has received positive feedback from customers.