[Updated to now include download of relevant whitepaper]
Functional safety engineers are immersed in performance calculations involving failure rates, diagnostic coverage, proof test intervals, common cause and much more. It’s easy for engineers to focus on math. Yet such modeling only accounts for random hardware failures. But what percentage errors shown in the well-known pie chart from the United Kingdom Health and Safety Executive describing the causes of accidents involving control and safety system failures were due to random hardware failures? Few, if any.
The chart showed: incomplete and incorrect specifications – 44%, changes after commissioning – 20%, design and implementation – 15%, operation and maintenance – 15%, installation and commissioning – 6%
The more predominant factor in accidents is overlooked in the math: systematic failures. Systematic failures come in many forms, yet there is one common element: people. Human errors include oversights, lapses, mistakes, and decisions of not only people, but of organizations. Standards do address a significant portion of systematic failures, but not upon the basis of math. Preventing systematic failures is done by following the lessons learned and documented in the various lifecycle activities of relevant standards, in this case ISA/IEC 61511. The standard covers the specification, design, fabrication, installation, maintenance, operations, and management of change activities of safety instrumented systems that are potential sources of systematic errors. Simply put, reducing systematic failures reduces risk.
To read examples of systematic failures throughout the lifecycle, and to learn how to reduce them, read the full paper “Methodologies in Reducing Systematic Failures of Wired IPLs” by Rick Hanner of aeSolutions and Tab Vestal of Eastman.
Dive deeper into the United Kingdom Health and Safety Executive's stats and why systematic failures really occur:
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keywords: Systematic failure, random hardware failure, Independent Protective Layer, IPL, SIF, SIS, BPCS, common cause, Human Factor Analysis, SIL Verification, Systematic analysis of failures