ANSI/ISA-84.91.03:2025

ANSI/ISA-84.91.03-2025, Functional Safety: Low Integrity Protection Layers, specifies the requirements for achieving functional safety using instrumented protection layers, commonly referred to as low-integrity protection layers (LI-PLs). These layers are not designed to meet the requirements of ANSI/ISA-61511-1 for safety instrumented systems. This standard applies across various industries in the process sector, including chemicals, oil and gas, pulp and paper, pharmaceuticals, food and beverage and non-nuclear power generation. It requires organizations to follow a formal lifecycle for LI-PLs.This standard emphasizes safety planning that assignsresponsibilities, maintains staff competence and requires performing functional safety assessments (FSAs) before commissioning and after any modifications to verify that the protection layer continues to meet its objectives. A formal identification and classification process must be used to distinguish which protections are LI-PLs and their specific functions. Each LI-PL requires a documented functional requirements specification (FRS) that details sensors, logic solvers, final elements, setpoints, response times, bypass capabilities, compensating measures, testing requirements and HMI needs. During detailed design, the LI-PL must be evaluated for independence from other systems, vulnerability to common-cause failures and the suitability of the chosen devices and infrastructure. Implementation must follow documented commissioning and functional testing procedures aligned with the FRS. Operational controls include documented fault response protocols, a comprehensive bypass management policy and operator training with regular refreshers. LI-PL devices should be included in an inspection, testing and preventive maintenance (ITPM) program with proper records, failure investigations and approved deferral procedures. Performance monitoring should track activations, spurious trips, bypass frequency and duration, systematic failures and obsolescence. Additionally, lifecycle documentation, such as classification results, FRS, device specifications, commissioning and ITPM records, FSA findings and modification histories, must be maintained.