What was Said, What was not Said
Hamid Jahanian
TL;DR
The paper tackles challenges in creating an effective Safety Requirements Specification (SRS) for Safety Instrumented Systems (SIS) under IEC 61511. It advocates a pragmatic, staged SRS development approach with explicit ownership division between Plant Owner and System Integrator, coupled with a classification and traceability framework. It also argues for including untold negative requirements and introduces exploratory inspection methods—Failure Mode Reasoning (FMR) and automated SIS simulation testing—to uncover gaps not captured by the SRS. While not a universal remedy, these practices aim to reduce systematic faults, improve SRS clarity, and enhance SIS safety in the process industry and beyond.
Abstract
In the process industry, the configuration of Safety Instrumented Systems (SIS) must comply with a defined set of safety requirements, typically documented in the Safety Requirements Specification (SRS). The functional safety standard IEC 61511 outlines the necessary content and quality criteria for the SRS. However, developing an effective SRS can be challenging. This article examines some of these challenges and proposes good practices to address them. It discusses SRS ownership, "staged" development of SRS, and the classification and traceability of requirements. Additionally, it explores the issue of untold "negative" requirements and suggests exploratory "inspection" of SIS Application Programs (APs) as a potential remedy.