What was Said, What was not Said

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.