Definition:
A Safety Instrumented System (SIS) is a complete arrangement of sensors, logic solvers, and final elements that implement one or multiple SIFs. It is the entire system which is dictated per IEC 61511. Note IEC 61508 is more applicable at the component level.
There are three main portions
- instrument – the items that detect the process.
- logic solver – the brain that monitors the logic. Usually a safety PLC but can be relay logic.
- final element – the item removing energy and putting the process in a safe condition. Usually a valve but can be a contactor or STO.
As a SIS is the main focus of IEC 61511, entire books can and have been written about it, but we will leave it here for now.
Key Points:
- SIS must be designed, installed, and maintained according to safety lifecycle requirements per IEC 61511.
- SIS reliability is critical to overall plant safety.
- A SIS is distinct from the BPCS.
Example:
A plant Emergency Shutdown (ESD) can be a SIS consisting of multiple SIFs.
See Also: SIF, instrument, logic solver, final element
Cited Source:
- IEC 61511:2016 – the entire standard is applicable to a SIS
- Wikipedia SIS article