In this ultimate guide to IEC 61511, also known as ISA 61511 in the US, we explain Safety Integrity Level (SIL) and outline the safety life-cycle.

In the early years of functional safety and attempts at SIL conformance, there was a definite over-emphasis on purchasing “SIL certified” equipment.

The technical aspect of calculating “probability of failure” is an attractive one for engineers to get embroiled with, but sadly it hides far more important but less technically appealing issues.

Although achieved failure probabilities are certainly part of the requirement for demonstrating integrity, the achievement of specific numerical failure targets is only a very small piece of the functional safety picture.


Background to IEC 61511

At the turn of this century, the International Electrotechnical Commission published the very first version of functional safety standard IEC 61508. For the first time, IEC 61508 provided an internationally accepted standard for employing electrical, electronic, and especially programmable electronic systems in safety-related applications.

A few years after the first publication of IEC 61508, sector standard IEC 61511 was issued; specifically designed for functional safety of Safety Instrumented Systems (SIS) in the process industry sector.

The functional safety principles are broadly identical in both standards, but IEC 61511 distils the four normative parts of IEC 61508 into a single normative standard of around 80 pages, aimed specifically at the application of SIS in chemical, oil & gas, non-nuclear power generation, pharmaceuticals, pulp and paper and food & beverage plants.

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