Abstract: This paper elucidates that the selection of relays is a key issue in effectively improving the reliability of system operation. Combining the system's operating characteristics, working environment, and relay product characteristics, it discusses the selection principles of relays, the selection of quality levels in relay applications, and the principles of relay derating. It also analyzes relay installation and protection technologies and problems encountered in relay applications. 1. Overview Faced with a wide variety of relay products, how to rationally select and correctly use them is a practical problem that system developers and designers must pay close attention to and prioritize. To achieve rational selection and correct use, it is necessary to fully study and analyze the actual operating conditions and actual technical parameter requirements of the system, and, in accordance with the "value engineering principle," appropriately propose the technical performance requirements that the selected relay products must meet. In the reliability design of the entire system, the rational selection of components is required. The selection and control of components is a task that requires multidisciplinary knowledge and should generally be completed jointly by component engineers, reliability designers, overall and circuit designers, and failure analysts. First, the importance of the entire system, reliability requirements, environmental conditions, and cost requirements must be comprehensively considered and selected. Specifically, the following elements can be analyzed and studied one by one to confirm the required level and value range. The following aspects must be emphasized during selection. 2. Selection of Operating Environment Conditions Climatic stress factors mainly refer to temperature, humidity, atmospheric pressure (altitude), coastal atmosphere (salt spray corrosion), sand and dust pollution, chemical gases, and electromagnetic interference. Considering the system's universal applicability across various industries and natural environments nationwide, and taking into account the special requirement for reliable operation year after year, key components of the system must use fully sealed relays (metal-sealed or plastic-sealed, with metal-sealed products superior to plastic-sealed products). This is because only fully sealed relays possess excellent long-term resistance to harsh environments, good electrical contact stability, reliability, and stable load switching capability (unaffected by external climate). 2.1 The Influence of Temperature on Relays Relays are heat-sensitive components. High temperatures can accelerate the aging of internal plastic and insulating materials, cause contact oxidation and corrosion, make arc extinguishing difficult, and deteriorate electrical parameters, thus reducing reliability. Therefore, the design requires that relays not be placed near heat-generating components and that good ventilation and heat dissipation conditions be provided. Although relays are heat-sensitive components, the effects of excessively low temperatures (such as -55℃ in military aviation conditions) cannot be ignored. Low temperatures can exacerbate the cold-sticking effect of contacts, causing condensation on the contact surface and the formation of an ice film on the armature surface, preventing normal contact switching, especially for low-power relays. Tests have shown that some domestically produced low-power relays manufactured according to national standards, although specified for use at -55℃, actually cannot switch normally under these conditions. It is recommended to allow sufficient margin when selecting relays. For critical military electronic systems, it is recommended to choose products meeting national military standards. 2.2 The Effect of Low Atmospheric Pressure on Relays Under low atmospheric pressure, the relay's heat dissipation deteriorates, and the coil temperature rises, causing changes in the relay's set pull-in and release parameters, affecting normal operation. Low atmospheric pressure can also reduce the relay's insulation resistance, making it difficult to extinguish the arc at the contacts, easily causing contact melting and affecting the relay's reliability. For harsh operating environments, it is recommended to use a fully sealed system. 2.3 The Effect of Mechanical Stress on Relays This mainly refers to stress factors such as vibration, impact, and collision. For control systems, the primary considerations are resistance to seismic and mechanical stresses. Therefore, small intermediate relays with balanced armature mechanisms are recommended. Electromagnetic relays use cantilever structures with low natural frequencies, making them susceptible to resonance from vibration and shock. This can cause a drop in relay contact pressure, leading to momentary disconnection or contact chatter, and in severe cases, structural damage. The movable armature can also malfunction, affecting relay reliability. It is recommended to incorporate vibration-damping measures in the design to prevent resonance. Based on the impact of environmental conditions on relays, relay selection must first ensure that the relay meets all environmental requirements specified for the entire system. Failure to comprehensively consider all environmental conditions will result in the developed system failing to meet the contractual technical requirements. For example, when using JRC-5M small electromagnetic relays in military airborne electronic equipment, focusing solely on ambient temperature may meet the system's requirements while neglecting vibration and shock conditions. If these conditions are not met, corresponding preventative measures must be taken; otherwise, reliability cannot be guaranteed. 2.4 Insulation Withstand Voltage: The exposed insulators at the leads of unsealed or sealed relays are subject to long-term dust and moisture contamination, leading to a decrease in insulation strength. Under overvoltage conditions during inductive load switching, this can cause insulation breakdown failure. Considering the inherent insulation characteristics of relays, the following technical characteristics must be considered when selecting a relay: 2.4.1 Sufficient creepage distance: Generally >3 mm (operating AC 220V); 2.4.2 Sufficient insulation strength: >AC 2000V (operating AC 220V) between conductors without electrical connection, >AC 1000V between contacts in the same group; 2.4.3 Sufficient load capacity: DC 220V inductive; 5～40 ms, >50W; 2.4.4 Long-term resistance to climatic stress: Coil anti-mold breakage, long-term stable and reliable insulation dielectric level. 2.5 Sealed Relays vs. Unsealed Relays Some engineers believe that unsealed products offer intuitive operation and convenient failure analysis, while the operation of sealed products is invisible and difficult to understand. The advantages and disadvantages of each are as follows: Unsealed relays typically use snap-fit ​​armatures, resulting in simple structures, easy manufacturing processes, convenient installation and maintenance, intuitive operation, easy failure analysis, and lower prices. Their main disadvantages are: high sensitivity to changes in the operating environment (climatic stress, mechanical stress); susceptibility to long-term climatic conditions due to environmental contamination and damage over time; poor electrical contact stability and reliability; and susceptibility to moisture and impurities leading to electro-corrosion, mold growth, and failure. Sealed relays typically use balanced rotating armatures, with a fully sealed structure isolating them from external climatic stress, providing excellent resistance to harsh environments; stable and reliable contact performance; and coil resistance to corrosion and mold growth, resulting in excellent long-term reliability. Their disadvantages include complex structures, specialized manufacturing processes, difficult failure analysis, inability to be repaired and reused, and high cost. Therefore, considering long-term climatic stress resistance, resistance to harsh environments, and stable and reliable electrical contact, sealed relays are significantly superior to unsealed relays. For aerospace, aviation, and military applications with high reliability requirements, metal-shrouded relays are primarily selected. For critical production process automation control systems requiring long-term stable and reliable operation, sealed relays should be the primary choice. 3. Excitation Coil Input Parameters These mainly refer to factors such as over-excitation, under-excitation, isolation between low-voltage and high-voltage (220V) outputs, the influence of temperature changes, long-distance wired excitation, and electromagnetic interference. These are all factors that must be carefully considered to ensure reliable system operation. Excitation according to the relay's specified excitation amount is a necessary condition for its reliable and stable operation. Relay technical specifications generally provide the operating voltage, pull-in voltage, and release voltage for the coil. To ensure the relay's normal operation, the circuit connection must ensure that the three voltages meet the values ​​specified in the technical specifications under all circumstances. Otherwise, the relay will not switch normally. 3.1 Regarding Series Power Supply Excitation Methods: Many users use a series voltage divider power supply method to apply excitation to the relay coil to drive the relay. This excitation method is generally not recommended. This is because the relay's pull-in time mainly depends on the circuit's time constant T, and T = L/R. When the relay coil is powered by a series resistor R1, R = R1 + R2, then L/R2 > L/(R1 + R2). Clearly, connecting R1 in series reduces T, accelerating the relay's pull-in time. Especially when R1 ≥ R2 and the voltage is very high, the pull-in time will be significantly reduced. Excessive speed of moving parts will increase the impact, collision, and rebound during engagement, thereby increasing contact bounce, accelerating mechanical wear, and reducing the contact's load capacity and mechanical life. Therefore, the series power supply excitation method alters the original design's normal operating state of the relay and is generally not advisable. Only when contact bounce and mechanical wear do not pose a significant threat to actual use, and a faster operating speed is particularly needed, can the excitation voltage be increased or the series resistor power supply excitation method be used. 3.2 Use of Series Relay Coils Using multiple relay coils connected in series and then excited with a DC 220V power supply is an excitation method that must be used with caution. 3.2.1 For relays of the same type and specifications, since the impedances (including DC resistance and transient reactance) of each coil are roughly the same and the difference is small, the series voltage divider excitation method is generally feasible. Practice has proven this to be effective. 3.2.2 For relays of different types or specifications, since the impedances of different relay coils are inconsistent, and the difference varies greatly depending on the transient reactance, the difference in excitation voltage (determined by the instantaneous voltage division ratio) across each relay coil at the moment of series excitation will inevitably be large. This will inevitably result in some relays being over-excited while others are under-excited, causing fundamental changes in the switching sequence and speed of the relay contacts. This will inevitably lead to situations such as reversed action order, fast/slow switching, and unreliable switching. Therefore, the series voltage divider excitation method is not suitable for relay coils of different types and specifications. 3.3 Regarding the parallel use of relay coils in complex control circuits, it is common to use two (or more) relay coils of different types (such as contactor K1, small sensitive relay K2) in parallel. In this case, practical problems may arise, such as delayed release of K1, reduced contact arc-breaking capability, repeated reverse excitation of K2, and contact malfunction. In DC control circuits, the magnetic energy stored in the K1 and K2 coils may differ significantly. When the coil power supply is lost, the energy stored in K1 (with greater magnetic energy) will be discharged through the K2 (with less magnetic energy) coil, generating a reverse current. This results in a prolonged release time for K1, a slow contact arc-breaking speed, and a prolonged arcing time between contacts; K2 has a short release time and is subsequently excited by the reverse discharge current, or even re-engages momentarily after release, causing malfunctions. In practical applications, care should be taken to avoid the unreliable phenomena caused by insufficient research. 4. Contact Output (Switching Circuit) Parameters These mainly refer to the nature of the contact load, such as lamp load, capacitive load, motor load, inductor, contactor coil, choke coil load, resistive load, etc.; and the contact load values ​​(open-circuit voltage value, closed-circuit current value), such as low-level load, dry circuit load, small-current load, large-current load, etc. Selecting a suitable relay based on the load nature and capacity of the relay-driven equipment is a fundamental condition for reliable relay operation. Relay failure or reliability primarily depends on whether the contacts can complete the specified switching circuit function. If the actual switching load capacity exceeds the specified switching load capacity of the selected relay, the relay cannot operate reliably. 4.1 Regarding the load relay contacts: Contact failure is the core cause of relay failure. When the actual load voltage switched by the contacts is less than the arcing voltage and the current is less than 1A, especially under medium current (test standard is DC 28V, 0.1A), low level (10-30mV, 10-50A) or dry circuit (meaning the relay contacts close first, then connect the millivolt/microampere level load), the failure mechanism and failure mode of the contacts during actual operation are completely different from those of the actual rated power load being switched. Precisely to meet the different requirements of different loads, different products have different design, manufacturing processes, testing, and experimental requirements. Therefore, when actually selecting relay products, one must not mistakenly assume that the relay contacts are suitable for all loads from zero to the specified rated value, nor should one assume that the smaller the actual load passing through the contacts is compared to the rated load specified in the product standard, the more reliable it is. For example, contacts that can reliably switch a 220V, 10A load may not be able to reliably switch a 10mA actual load. Furthermore, it should not be used to switch low-level or dry circuit loads. Therefore, for medium current, low-level, dry circuit loads, it is recommended to choose fully sealed products with excellent contact reliability and metal covers. According to field usage statistics, in relay use, failures due to improper contact load account for approximately 70% of the total relay failure rate. Properly designing the contact load stress is crucial to ensuring relay reliability. Generally, in reliability design, derating is the most effective measure to improve reliability. For other components, if other factors such as cost and size are not considered, the greater the derating, the higher the reliability. However, relays differ from other components; lower contact load stress does not necessarily lead to higher reliability. This is mainly determined by the contact failure mechanism. When the contact current reaches 100 mA, the arcing effect of the contacts is significantly weakened. Carbonaceous substances precipitated at high temperatures cannot be burned off by the arc and deposit on the contact surface, increasing contact resistance and affecting contact reliability. When the contact load is below 10 mA or below 50 mV, contact reliability decreases significantly because the voltage cannot break down the film resistance on the contact surface, resulting in low-level failure. Especially under high-temperature conditions, contact oxidation is accelerated, making low-level failures more severe. Therefore, loads below 10 mA and 50 mV are considered low-level loads. If the relay is required to operate under low-level conditions, a specific technical agreement needs to be signed with the manufacturer. The manufacturer must produce and screen relays according to low-level requirements; otherwise, low-level failures will occur, seriously affecting reliability. Although the load stress of a relay cannot be too low, the load stress given in the technical specifications is the maximum rated value of the contacts, a parameter that should never be exceeded under any circumstances. Exceeding this value during use can, at best, shorten the lifespan and reduce reliability, and at worst, burn out the contacts, causing failure. This is mainly because the arcing generated when the relay contacts operate under heavy loads causes the contacts to melt, forming unevenness on the contact surface, resulting in mechanical seizing that prevents separation. The greater the contact load, the larger the arcing, and the greater the possibility of contact burnout. From the above analysis, it is clear that appropriate derating remains an effective measure to improve relay reliability. For proper use of contact loads, under normal circumstances, the load stress should be designed to be above 100 mA and below 80% of the rated load value given in the technical specifications for reliability. It is worth noting that the rated load value of the relay contacts is given under resistive load conditions. When the load used is inductive, capacitive, or lamp-loaded, it can generate a surge current of 10 times. Therefore, if it is not a resistive load, it should generally be converted according to Table 1. [IMG=Table 1]/uploadpic/THESIS/2007/12/2007122811334879538L.jpg[/IMG] 4.2 Regarding capacitive loads, relay contacts, as self-holding contacts for switching capacitive load circuits, are prone to contact sticking and failure to release. This is because the charging and discharging process of the capacitor is similar to the capacitor energy storage spot welding process. Further analysis and experiments show that after applying a sufficient DC 220 V voltage to a 22μF capacitor and then energizing the relay to directly short-circuit and discharge its contacts, the pure silver contacts can develop a soldering failure phenomenon within 10 cycles. Theoretically, the discharge current of a capacitor is: [IMG=Centric discharge current]/uploadpic/THESIS/2007/12/2007122811335549738X.jpg[/IMG] Where: U - is the voltage across the capacitor; R - is the resistance of the discharge circuit; T - is the time constant; Since R is approximately equal to the contact resistance of the contact point, approaching zero, at the instant of discharge i = U/R; i is very large, that is to say: all the energy stored in the capacitor is discharged through the contact point in a very short time, thus directly causing the spot welding to fail. Therefore, long transmission lines, filters for eliminating electromagnetic interference, power supplies, etc. are all highly capacitive. Relays used for such loads should be selected in conjunction with the characteristics of the equipment. 4.3 Regarding the parallel use of relay contacts 4.3.1 Power cannot be increased by connecting contacts in parallel; sometimes, when one set of contacts cannot meet the power requirements of the circuit, two or more sets of contacts are connected in parallel to ensure the power requirements of the circuit. However, due to the slight time difference between the action of relay contacts (typically 0.1 to 0.2 milliseconds), the first set of contacts to activate will bear the full power and switch under overstress conditions. This makes them susceptible to burnout and failure due to the arcing caused by the high current. Therefore, it is required that parallel connection of relay contacts not be used to increase power when using relays. 4.3.2 Parallel connection of contacts is generally not used to improve reliability; in reliability design, redundancy can improve reliability. Some designers, utilizing the principle of redundancy, subjectively intend to improve the reliability of control circuits by using parallel connection of relay contacts. However, the function of a typical control circuit is to control the circuit through the switching action of contacts. If parallel connection of contacts is used, although the reliability of activation is improved, the reliability of deactivation is reduced. Therefore, for switching circuits generally controlled by relays, using parallel connection to improve reliability is not advisable. Only for special requirements, such as circuits that can complete a specified function with a single connection or disconnection (e.g., launching a satellite, where only the relay contacts need to connect the rocket's ignition system to complete the mission), can parallel connection of contacts improve reliability. 4.4 Correct Connection of Relay Contacts 4.4.1 Use normally open contacts as much as possible and normally closed contacts as little as possible; when connecting relay contacts, use normally open contacts as much as possible and normally closed contacts as little as possible. This is because normally open contacts bounce less during operation than normally closed contacts. It is well known that contact bounce has a negative impact on the circuit and shortens the contact life. 4.4.2 Correct connection of changing contact polarity; the connection of changing contact polarity has a significant impact on contact life. The correct connection should be that the movable contact is connected to the power supply cathode and the fixed contact is connected to the power supply anode. This is because tests on two different connections show that, under the same load conditions, the arcing time of the contacts is reduced by half when connected with the correct polarity as described above compared to the opposite polarity connection, thus improving contact life. Conclusion In the fierce market competition, market share is primarily determined by quality and performance, while market size is predicated on technological development and innovation. Therefore, comprehensive development and innovation to improve product quality and cost-effectiveness have become the focus of competition. China's relay industry, situated within the global market environment, should proactively participate in the global relay market's division of labor and cooperation, seeking its place in the new round of restructuring and reorganization of the global relay industry, securing a favorable position in the new landscape, and conforming to the general trend of global economic integration and product and industrial development. We must fully recognize the new trend that the center of relay production and development is shifting to developed countries with a certain industrial base, and that China, and even Asia, has become the center of the global relay industry. We must carefully analyze the new changes in relay technology and the market, formulate more practical and effective development strategies, focus on digital products, and upgrade technological levels. With structural adjustment and upgrading as the focus, and increasing market share as the goal, we must emphasize the market, reform, strategic structure, adjustment and reorganization, technological innovation, and sustainable development, pursuing higher, faster, and stronger achievements, seizing opportunities to promote development, and creating a new era for China's relay industry in the new century.