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Introduction The Safety Lifecycle (SLC) of a Safety Instrumented System (SIS) is a structured process that encompasses all the steps required to meet high levels of functional safety requirements throughout the lifecycle of such a system, from its conception and design to operation and maintenance. SLC can be divided into three phases in sequence: the analysis phase, the implementation phase, and the operation phase. Corresponding activities are carried out in each phase; they do not exist in isolation but follow a logical order, as shown in Figure 1-1. It can be said that SLC begins with conceptual process design and ends only after the SIS units are retired. Furthermore, security issues must be considered from the very beginning of the conceptual process design and integrated throughout the entire SLC. Figure 1-1 SLC flowchart. The necessity of creating an SLC: Creating an SLC is not only intended to assist SIS designers in building safer systems, but it also helps companies develop more cost-effective systems. Through hazard risk analysis, a system can be designed to reduce specific risks to a tolerable level, rather than over-designing or under-designing it. Some of the safety assessment activities that are currently being vigorously promoted are carried out in strict accordance with SLC. For example, during the analysis phase, the HAZOP and LOPA methods are used to conduct process hazard analysis and SIL classification, thereby enabling a comprehensive identification of process risks and determining the necessary SIFs along with their SIL levels ； During the implementation phase, SIL verification is carried out to confirm whether the SIF resulting from the current instrumentation meets the desired SIL requirements. The SLCIEC 61508 standard among the relevant standards defines overall SLC, SLC for E/E/PE systems, and software SLC; the overall SLC is shown in Figure 1-2, and it can be said that both the SLC for E/E/PE systems and software SLC are included within it. Compared to Figure 1-1, the three stages of SLC in Figure 1-2 are still clearly visible, but provide more details. For example, the overall planning and comprehensive installation and commissioning of steps 6 to 8 need to be completed in the second phase, namely the implementation phase ； If a comprehensive modification and redesign are carried out in Step 15, it is necessary to return to the corresponding SLC stage and go through the process again. It should be noted that, for clarity, the activities related to functional safety are not shown in the diagram, but they are addressed in the IEC61511 standard later on. Figure 1-2: The overall SLC for IEC61508. Similarly, the SLC defined by the IEC61511 standard also has distinct phases of analysis, implementation, and operation, as shown in Figure 1-3. This standard is specifically developed for the process industry; therefore, many of the requirements of this SLC are tailored for process safety, outlining relevant functional safety activities such as emphasizing good management of functional safety, including developing thorough plans for each step and carrying out verification activities. Figures 1-3: Conclusion on SLC in IEC61511. Regardless of which standard’s SLC is referenced, it essentially constitutes a feedback process that involves two key validations: the first takes place after conceptual design to determine whether the requirements for risk control have been met, and the second involves regular testing and review during operation. Only by enhancing the participants’ understanding of the SLC concept and ensuring proper documentation of each step can all aspects related to SIS be effectively managed, thereby fundamentally improving the overall level of functional safety. This article is reprinted from the WeChat official account: Haopeng Technology