Analysis of relay failure modes and countermeasures

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A relay is an electronic control device, commonly used in automatic control circuits. In fact, it is a kind of “automatic switch” that uses a smaller current to control a larger current. Therefore, it serves functions such as automatic regulation, safety protection, and circuit conversion in electrical circuits. An electromagnetic relay is mainly composed of a coil, iron core, return spring, movable contacts, and fixed contacts. The structure of a common electromagnetic relay is shown in the figure below. http://yunrun.com.cn/upload/201908/05/201908050416089817.png Basic structure of electromagnetic relays; Working principle of electromagnetic relays. ◆ When the coil is energized, the iron core at its center becomes magnetized and generates a magnetic force that attracts the iron plate as shown in the diagram. The iron plate, through leverage action, pushes the connected movable contacts, causing the normally open contacts to close while the normally closed contacts open ； ◆When the current to the coil is cut off, the core immediately loses its magnetism. The compressed spring returns to its original position, exerting a force that pushes the iron plate back to its initial position; at this point, the normally open contacts open while the normally closed contacts close. Analysis of relay failure modes and countermeasures: yunrun.com.cn/tech/2662.html In some special application scenarios, relays that should function normally operate abnormally due to various factors, resulting in losses that can often be immense. Therefore, when using relays, we should be aware of the specific reasons that cause them to fail, and take appropriate corrective actions to minimize such failures, as this can be of great significance in protecting human life and property. As for relays, there are as many as twenty different failure reasons that can cause them to malfunction, and these reasons vary depending on the specific operating environment and conditions in which the relay is used. Changhui Instruments lists only the following common causes of relay failure along with corresponding solutions for reference. Common causes of relay failure and solutions 1. Abnormal contact operation ① Welding of relay contacts: The contact surfaces are subjected to high-load surge currents, and arc heat is generated due to frequent switching. Corresponding solutions: a. Avoid contact vibration caused by a drop in supply voltage or coil voltage ； b. Be careful with loads whose inrush current exceeds the rated current ； c. Be careful with the high-frequency switching of relays. http://yunrun.com.cn/upload/201908/05/201908050420217254.png Faulty contacts caused by welding at the relays’ contacts. ② Transfer of contact material: Arc heat is generated when the contacts switch between DC loads, leading to the melting of the contact material and its subsequent evaporation and transfer. Corresponding solution: a. Reduce the load inrush current by connecting a series current-limiting resistor ； b. Diodes connected in parallel with the contact part absorb the back electromotive force. http://yunrun.com.cn/upload/201908/05/201908050422007088.png Faulty contacts in relays caused by adhesive bonding of contacts and material transfer. ③ Carbides leading to relay contact failures: This issue often occurs when the contacts are connected to electromagnetic loads such as solenoids, contactors, and valves. After the relay contacts are opened and closed repeatedly, the surges resulting from these operations cause the decomposition of organic gases in the vicinity as well as the carbonization of dust, which then adheres to the contacts, thereby leading to poor contact quality. Corresponding solution: a. The back EMF of inductive loads is greater than that of resistive loads, so carbonization is more likely to occur. b. Surge absorption elements such as RC circuits or diodes are connected in parallel to the contact part to absorb back electromotive force. http://yunrun.com.cn/upload/201908/05/201908050426539522.png Faulty contacts in relays due to carbonization. The three types of contact failures mentioned above are among the common failure modes of relays that we encounter during their use. To ensure contact reliability, extend their service life, and reduce noise, Changhui Instruments recommends the use of surge-absorbing components in circuitry for contact protection. Furthermore, at this time, there will be a certain delay in the load being released; please verify using an actual load before use. Furthermore, please note that improper use will lead to the opposite effect. Typical example of a relay contact protection circuit: 1. Contact protection circuit ① CR method: http://yunrun.com.cn/upload/201908/05/201908050500017897.png It can be used in AC power circuits where the load impedance is less than the RC impedance. C:0.1~1μf ； R: The resistance value equal to the load. http://yunrun.com.cn/upload/201908/05/201908050500362203.png It can be used in both AC and DC power supply circuits. C:0.1-1μf ； R: The resistance value equal to the load. ②Diode mode: http://yunrun.com.cn/upload/201908/05/201908050507177352.png Designed for DC power supplies. Please use diodes with the following rated values. Reverse tolerance voltage: supply voltage of the load circuit * 10; forward current: greater than the load current. ③Varistor method: http://yunrun.com.cn/upload/201908/05/201908050511488692.png It can be used in both AC and DC power supply circuits. For optimal results, when using a 24V-48V power supply voltage, connect a varistor at the load side ； At a supply voltage of 100V–200V, a varistor is connected between the contacts. Note: Do not use the following contact protection circuit: http://yunrun.com.cn/upload/201908/05/201908050516477124.png. a) This protection circuit is very effective at extinguishing arcs when the contacts are disconnected. However, since the capacitor (C) charges when the contacts are disconnected, a short-circuit current flows from the capacitor when the contacts are connected, which makes the contacts prone to welding. http://yunrun.com.cn/upload/201908/05/201908050517564742.png b. This protection circuit is very effective at suppressing arcing when the contacts are disconnected. However, when the contacts close and the accumulated current flows into the capacitor, the contacts are prone to welding. In practical applications, almost all relay failures are caused by the frequent switching of the relay, which leads to the end of its service life, or by driving high-power inductive loads, resulting in contact welding or carbonization. Even though some devices have installed surge protection elements such as diodes, it is still not possible to completely prevent failures from occurring. Therefore, for control requirements where it is necessary for the contacts to actually open and close, such as using a relay to cut off the power supply via an emergency stop switch, etc., safety relays with redundancy and self-check functions must be employed to achieve this. So what exactly is a safety relay with redundancy and self-check functions? Next, the following information on safety relays will be provided for your reference. Definition and characteristics of safety relays 1. What is a safety relay? A safety relay is a module composed of multiple relays and logical circuits; it is a circuit component whose purpose is to compensate for each other’s shortcomings in fault conditions, thereby ensuring accurate operation with few false triggers, reducing errors and failures, and enhancing safety. To be more precise, it should be called a safety relay module. Different mechanical equipment and different process stages pose varying safety risks as well as different levels of hazard. Therefore, during product design, various safety relays were developed to protect mechanical equipment of different risk levels, with the main purpose of safeguarding machine operators exposed to different levels of danger. 2. Characteristics of solid-state relays: a. Safety relays are circuit modules rather than electronic components ； b. Safety relays are identified by a specific color, usually yellow or red ； c. Safety relays have a high diagnostic coverage rate, thanks to the different requirements imposed on them as well as the auxiliary design of other logic circuits; in some cases this coverage can even reach 99.9%. Therefore, they can be used in applications with higher safety requirements, and it is stipulated that certified safety relays must be used to meet such safety standards! d. Safety relays feature a structure with forced-guided contacts (or other protection mechanisms), which ensures safety even in the event of contact welding. Thus, a “safety relay” is not one that is free from failures, but rather one that takes predetermined actions in the event of a failure. Under the action of the internal forcing rod, the forced-guidance safety relay ensures that even if the contacts weld together, both pairs of contacts (NO/NC) will not turn ON simultaneously. A forced guidance structure is a simple and effective method for detecting contact welding faults. In accordance with EN 50205 requirements, in the independent combinations of NO and NC contacts, when the NO contact fuses, even if the power supply to the relay coil is disconnected (no excitation), the NC contact does not close, ensuring a contact gap of over 0.5 mm. Furthermore, when the NC contacts weld together, even if the power to the relay coil is applied (excited), the NO contacts do not close, ensuring a contact gap of over 0.5 mm. http://yunrun.com.cn/upload/201908/05/201908050436368188.png Melting of the NO contacts of the forced-guidance relay. http://yunrun.com.cn/upload/201908/05/201908050438086313.png Melting of the NC contacts of the forced-guidance relay. If the contacts of a regular relay melt, it is possible for both pairs of contacts (NO/NC) to be in the ON state at the same time. In short: safety relays are safer and more reliable, making them suitable for industrial safety applications. Application scenarios of safety relays: ① Emergency stop buttons, to stop the machine operation in emergency situations ； ②Appropriate safety light curtains are used to prevent workers from entering dangerous areas ； ③Applicable safety threshold switch for detecting the door’s position, used for open or closed modes ； ④Suitable for use with electromagnetic door lock switches to ensure the safety of the work area ； ⑤The appropriate key is used in conjunction with the safety door switch to detect whether the door is open or closed ； ⑥Use a safety pad switch to ensure that workers can enter the hazardous work area safely.