Differences between the interlock functions of SIS and those of DCS

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The SIS system and the DCS system operate at different safety levels within the production facility. The interlocks in the DCS system are of the type commonly used in production processes, such as switch interlocks or interlocks for starting and stopping equipment, and they are part of normal operational procedures； Most of the interlocks in SIS are related to safety interlocks for personnel and equipment (especially shutdown interlocks), and they represent fault and accident interlocks for which the SIL level is strictly defined. The nature and configuration of the two are different. The Safety Instrumented System (SIS) and the Distributed Control System (DCS) play different roles in industrial production processes, as shown in the figure: Safety levels of production units. From a safety perspective, production units can be divided into three levels; the first level is the production process level ； The second layer is the process control layer ； The third layer is the safety instrumented system shutdown protection layer. During the initial engineering design, equipment selection, and installation phases of the production facility, safety regarding processes and equipment was taken into account; thus, the facility itself constitutes the first line of defense for safety. By using a control system to perform continuous dynamic control of the process, the device operates stably at the set value, not only producing various qualified products but also reducing the risks associated with the device by one level, thus serving as a second line of defense for safety. A safety instrument system must be installed over the process to monitor and protect it, minimizing the likelihood of serious accidents and ensuring the maximum protection of the production facilities and human safety by preventing such accidents from occurring; this system constitutes the most reliable and crucial final line of defense for the production facilities. Therefore, the roles of SIS and DCS in the production process are completely different. The DCS system and SIS are two systems with different functions. The differences between SIS and DCS are as follows: 1. DCS system: ① DCS is used for continuous process measurement, conventional control (continuous, sequential, intermittent, etc.), and operational control management, in order to ensure the stable operation of production facilities. ②DCS is a “dynamic” system that continuously monitors, processes, and controls process variables, thereby enabling dynamic control of the production process and ensuring product quality and output. ③DCS can automatically display faults ； DCS is not demanding in terms of the length of maintenance time or the requirements involved ； DCS can switch between automatic and manual mode. ④The DCS system is used only for general interlocking, starting and stopping of motors, sequencing, etc., and its safety requirements are not as high as those of a SIS. ⑤A DCS system generally consists of a human-machine interface operation station, a communication bus, and field control stations; it does not include detection and actuation components. ⑥To automate the production process, operators often change some of the output actions of the DCS system. 2. SIS system: ① The SIS is used to monitor the operating status of production facilities, to promptly address any abnormal conditions, thereby minimizing the likelihood of failures and ensuring the safety of both people and equipment. ②SIS is a static system; under normal operating conditions, it continuously monitors the operation of the equipment, with no change in system output and thus no impact on the production process. In abnormal conditions, it performs logical operations according to a pre-designed strategy to ensure the safe shutdown of the production equipment. ③SIS must test for potential failures ； The SIS maintenance time is extremely critical; severe cases can lead to the shutdown of the entire installation ； The SIS system must never operate offline, otherwise the production facility will lose its safety protection barrier. ④Compared to DCS, SIS requires stricter standards in terms of reliability and availability; IEC 61508-11 and ISA S84.01 strongly recommend that SIS and DCS hardware be installed separately. ⑤The SIS system consists of sensors, logic processors, and final units. ⑥The SIS system remains silent in normal operation and does not generate any actions; it only activates when interlocks are triggered.