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 61511-1. 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 61511-1, 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 61511-1.
- 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 61511-1:2016 – the entire standard is applicable to a SIS
- Wikipedia SIS article