Permissives are widely used in instrumented protective systems to prevent hazardous actions, verify readiness, enforce operating sequences, and establish operating modes. However, industry practice often defines performance verification boundaries based on the individual devices implementing the permissive rather than the protective function being performed. This can result in inconsistent performance analyses, disagreement between hazard analysis and verification activities, and omission of equipment whose failure can prevent successful completion of the credited protective function.
This presentation examines four permissive categories: command blocking, readiness/proof-of-state, sequence control, and operating mode selection. Representative examples illustrate how each category establishes different verification boundaries despite similar implementation techniques. The recommended analysis method provides a consistent, traceable engineering basis for determining which equipment to include in performance verification.