For decades, the process industries have relied on layers of protection to prevent hazardous events. When risk reduction requirements were high, safety instrumented systems (SIS), governed by standards such as IEC/ISA 61511, provided a clear framework for design, operation, and life-cycle management.

However, below that threshold sat a large class of instrumented protection layers that reduced risk, were credited in process hazard analyses, and were vital in day-to-day operations. But they weren’t designed or managed as safety instrumented systems and weren’t governed by a dedicated consensus standard. These functions were often treated as part of normal control or operations, with expectations that varied widely from one organization to another.

That inconsistency created practical problems. Functions that were relied on for risk reduction weren’t always subject to formal management of change, bypass control, testing, or documentation requirements. In many organizations, engineers and operations personnel understood what should be done but had no standard requiring that those practices be applied consistently.

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