1. Introduction The main function of a Distributed Control System (DCS) is to control, monitor, manage, and make decisions regarding the production process. Therefore, it must possess high reliability to ensure the safe and economical operation of the factory. With the continuous development of large-scale computer systems and computer communication networks, reliability has become a crucial issue, and its theories are constantly evolving and improving. The research content of reliability technology can be broadly divided into four aspects: reliability design, reliability analysis, reliability testing, and reliability management. 2. Reliability Measures for DCS Systems In DCS systems, many technical measures are employed to improve reliability. These measures are based on the following four fundamental principles: ① Making the system itself less prone to failure, i.e., fault prevention; ② Minimizing the impact of system failures, i.e., fault protection and fault mitigation; ③ Enabling the system to continue operating when a failure occurs, i.e., fault tolerance; ④ Allowing maintenance without stopping system operation when a failure occurs, i.e., online maintenance. Based on these four fundamental principles, various reliability measures are employed in DCS systems to reduce the occurrence of accidents and the resulting losses. 2.1 Strictly manage quality and improve system hardware level In order to achieve hardware reliability of DCS system, DCS system manufacturers have taken a series of measures to improve hardware quality, such as strict screening of components, de-rating of aged components, full consideration of the impact of parameter changes, use of low power components, and use of noise suppression technology. 2.2 Reliability measures for DCS system on-site design After a problem occurs in DCS, there will be situations such as inability to operate or monitor, resulting in equipment loss of control. At the very least, equipment shutdown will affect economic benefits, and at worst, it will endanger personal and equipment safety. Therefore, in the design, the following reliability points of the system must be carefully considered according to the specific situation on site: (1) Power supply system The power supply system of DCS system is the foundation of its operation. The power load should not exceed 60% to avoid high load operation of the power supply. At the same time, high-quality power supply should be considered. Generally, the author uses UPS power supply. It is best to use two UPS as backups for each other. If one fails, the backup machine can switch without disturbance, improving the reliability of the power supply. The UPS input power supply should not use the same busbar as the electrical equipment. It should be based on the electrical plant wiring and use different busbar sections with high reliability. This will ensure that if a problem occurs on one section of the electrical equipment busbar, it will not affect the power supply of the DCS system. When installing the DCS power system (UPS, power cabinet, etc.) on site, it should be installed in an electronic equipment room that meets the requirements of dust-free, vibration-free, and corrosion-resistant. At the same time, the cable holes should be sealed before powering on after installation to prevent leaked steam from entering the power system through the cable holes and causing a short circuit. After the DCS power supply is powered on, a sign saying "Equipment is powered on and operation is prohibited" should be hung on the UPS, power cabinet, etc. It is strictly forbidden to connect loads unrelated to the DCS system to the DCS power system, such as maintenance power supply, maintenance lighting, charging equipment, air conditioning, etc. (2) Redundancy system On site, it is necessary to make important I/O modules redundant and consider the N:1 redundancy structure of the operator station. This will prevent equipment shutdown due to hardware failure of the card or operator station. (3) Use backup measures ① Manual backup For important control loops, manual backup can be used to improve reliability. If automatic control fails, manual operation is used directly. In this case, the manual operation station directly outputs a 4-20mA or 1-5V analog signal to control the actuator or directly grounds the equipment for operation, allowing manual control of the production process. ② Automatic Backup: Automatic backup uses redundancy to set up one or more backup control devices. When the automatic control device in operation fails, the backup control device automatically activates to maintain the system's automatic control. Automatic backup is a form of redundant system, including redundancy of main control units, power systems, networks, I/O modules, operator stations, and servers. When operating equipment fails, the hot standby device will automatically operate without interruption, without affecting the maintenance of the faulty equipment. 2.3 Ensuring a Safe State During System Failures: The DCS system continuously performs online fault detection during operation. Once a fault is detected, the faulty equipment is isolated from the system to prevent it from affecting the normal operation of other equipment. An audible and visual alarm will be issued simultaneously after a fault occurs. Maintenance personnel will analyze the cause of the fault based on the alarm prompts and handle the fault as soon as possible to prevent the fault from escalating. During the handling process, proper on-site technical measures should be taken to avoid easily restarting the CPU, which could cause disturbance to the equipment. If a CPU restart is necessary, it is necessary to coordinate with the operators to take preventive measures. Generally, the DCS will reset the analog output signal when the CPU is reset. This allows for manual switching control of the actuators and frequency converters. The actuators can also be powered off or the analog output card can be removed (the actuator has a signal hold function). After the system is back to normal, the signal should be returned to its normal value before powering on the actuator and reinstalling the card. However, it is essential to consider the characteristics of the DCS itself. Some designs have switch safety outputs (on, off, hold, etc.). If the user selected "on" as the safety output during configuration, the switch output will issue an "on" command after the CPU is restarted. 3. Site Conditions 3.1 Site Environment DCS systems are generally placed in a control room or computer room. The computer room should be heat-insulated, dust-proof, and away from strong electromagnetic interference and strong vibration and noise. The net height of the computer room should be no less than 3.2m, and the area should be no less than 20m2 (one computer and its peripherals). There should be electrical outlets in the room. The floor should be a smooth, dust-free, and static-free floor. Raised floors are preferred, with a minimum height of 150mm. The computer room should maintain an indoor temperature of 18-25℃ with a temperature change rate of less than 5℃/h and a relative humidity of 45%-70%. Condensation is not allowed under any circumstances. When the air conditioning equipment malfunctions, the room temperature should be maintained within 24 hours, not exceeding the manufacturer's allowable value. The above conditions need to be strictly controlled before the DCS system is powered on. The working environment is the foundation for ensuring the reliable operation of the equipment. If the environmental requirements are not met, the DCS system will be affected to varying degrees during operation. For example, high temperature, high humidity, and high vibration will all affect the normal operation of the DCS system. 3.2 Cable Laying (1) The connecting wires inside the control panel, console, relay cabinet, etc., should be copper core PVC insulated wires. The connecting wires that need to be plugged in (or the connecting wires of FK type changeover switches) should be copper core PVC insulated flexible wires. (2) The connection between the thermocouple and the cold junction compensator, the constant temperature chamber, or directly to the instrument should be a thermocouple wire extension or a compensating wire that matches the thermoelectric characteristics of the thermocouple wire. (3) Analog input/output signals and low-level switching signals should be connected using shielded cables, and the cross-sectional area of ​​the signal cable core should be greater than or equal to 1 mm2. High-level (or high-current) switching input/output signals can be connected using general twisted-pair cables (control cables), but should be separated from analog signals and low-level switching signals and run in a separate cable tray. (4) Weak signals and low-level signals, especially those requiring anti-interference (such as those in front of computers and phase-splitting instrument converters), should not share a cable with high-voltage circuits or be laid in the same protective pipe. (5) Cables with different signals or different voltage levels should not be laid in the same layer of brackets; when laid in the same layer of brackets, they should be arranged separately and separated by partitions if necessary. (6) Cables must not be laid parallel above oil pipelines or gas pipelines, or pass under oil pipeline interfaces. (7) Computer input and output signal cables should be laid in covered cable trays, and the cable trays and covers should be properly grounded. Single signal cables should be laid in steel cable conduits, and the cable conduits should be properly grounded. Copper tape shielding or aluminum foil shielding is recommended for cable shielding. 3.3 Common precautions for field grounding DCS systems have strict grounding requirements. When grounding in the field, it is essential to strictly follow the grounding specifications of the DCS manufacturer, otherwise the reliability of the DCS system will be affected. The following points should be noted when grounding the DCS system in the field: (1) The power ground of the I/O cabinet and the power ground of the UPS must be connected to the same ground to ensure equipotentiality. (2) The grounding cable must meet the requirements of the DCS manufacturer and be connected to the dedicated grounding screw in the cabinet. (3) System grounding and shielding grounding cannot be combined in the cabinet and must be connected to the grounding network separately. (4) The cabinet grounding cannot be replaced by the cabinet parallel bolts, and must be strictly connected with the grounding wire. (5) The grounding must be firm and reliable, and the busbar connection plate must be coated with anti-corrosion paint. (6) The grounding resistance must meet the requirements of the DCS system and must be strictly measured. (7) The grounding must be strictly in accordance with the grounding requirements of the DCS manufacturer. Shortcuts should not be taken to save trouble or grounding should not be carried out in accordance with the grounding wire diameter requirements. If the grounding of the DCS system does not meet the requirements, unpredictable faults will occur during operation, such as signal acquisition errors, communication failures, computer crashes, control failures, and protection malfunctions. 4 System Maintenance The following points are summarized for DCS system maintenance: (1) Regularly check the DCS system power supply. For redundant power supply systems, regular switching tests should be carried out. At the same time, the UPS power supply should be regularly switched and checked, and the battery should be regularly discharged and charged as required. (2) Regularly check whether the network connectors and all connecting wires are firm and reliable, and whether all wiring terminals in the control cabinet are firm and reliable. (3) Frequently check whether the control unit, I/O module, and other modules are working properly. (4) Regularly check whether the grounding is secure and test whether the grounding resistance meets the requirements. (5) Regularly check the workload of the controller, computer, etc., and pay attention to whether there is an increase. (6) Regularly check the MMI hard drive and delete fragmented files. Historical files should be frequently archived and backed up by peripherals. (7) During unit maintenance, reset the DPU (Distributed Processing Unit) and MMI (Human Machine Interface) of the DCS system one by one to eliminate the accumulated errors of the computer running for a long time. (8) For the interface between the DCS system and other systems (such as MIS (Computer Management System), SIS (Plant Monitoring Information System), etc.), it is recommended to install a virus firewall on the gateway station on the other system side and update the virus database in a timely manner. At the same time, update the operating system patches in a timely manner to improve the system security. (9) Regularly check whether the system fan is working properly and whether the air duct is blocked to ensure that the system can operate reliably for a long time. (10) When the unit is shut down for major and minor maintenance, pay attention to checking the power of the DPU host card CMOS battery. When CMOS data is lost due to CMOS battery failure, the CMOS batteries of the entire motherboard should be replaced. (11) Regular inspection habits, regular checks, and regular cleaning are necessary to ensure that the DCS system operating environment is clean and tidy.