SPRY347 June 2022
For industrial machinery safety, ISO 13849 is the successor to the older machinery European Standard (EN) 954-1 functional safety standard. ISO 13849 covers safety requirements (including software) through the life cycle of safety-related machinery and their components in control systems.
The process identifies the parts in the system that perform safety functions and uses statistical analysis to determine the probability of failure over time to determine a performance level (PL) and a safety integrity level (SIL) for the system.
Performance levels range from PLa to PLe. PLa indicates the least reliable and PLe indicates the most reliable. After the performance level has been established, the architecture required to reach that level is classified into safety categories. The category specifies resistance to faults, similar to a hardware fault tolerance. Categories range from CatB through Cat4. CatB indicates the least safe and Cat4 indicates the most safe. The specific categories are:
Typical industrial machinery systems are Cat3 or Cat4, PLd and in some instances PLe. For specific applications, such as with IEC 61508, additional standards refers to ISO 13849 and provide further clarification and guidance (such as for correlating categories and PLs).
PL and SIL both look at probability of failure per hour (PFH) and dictate requirements concerning structure, diagnostics, and confidence of fault detection. Their key difference is rooted in the degree of how certain parameters are dictated. For example, the performance level (PL) is not based on reliability calculations alone and has a MTTF (mean time to failure) metric as well. Despite some slight differences in formula, SILs can be mapped to PLs and vice versa. PLs are only referenced in ISO13849 whereas SIL is referenced in IEC 61508 and all derived standards. Using one metric over the other typically market-specific; PL is used more often in machine industries, while SIL is more commonly seen in process industries.