Issue
When designing a Safety Instrumented Function (SIF), it is mandatory to define the Process Safety Time and ensure that the function will be executed within this time. According to IEC 61508, part 2, 7.4.3.2.5: The Process Safety Time (PST) is defined as the period between a failure occurring in the EUC (Equipment Under Control) or the EUC control system (with the potential to give rise to a hazardous event) and the occurrence of the hazardous event if the safety function is not performed.
Resolution
Engineers must carefully balance the scan cycle time and CPU bandwidth to ensure that all safety and non-safety tasks are executed efficiently. This often involves optimizing the program code and possibly upgrading hardware to meet the demands of stringent Process Safety Time (PST) requirements.
Disclaimer
The views and opinions expressed in this article are those of the author(s) and do not necessarily reflect the official policy or position of Schneider Electric.
