7. Notion of minimal architecture

7.1 IEC 61511 requirements

PFD calculations _avg are fraught with uncertainty, partly due to doubts about the reliability data used (λ, etc.). To make this PFD assessment process _avg more pragmatic, the IEC 61511 standard provides minimum requirements in terms of redundancy to justify a SIL level.

To justify a SIL level, in addition to the PFD _avg or PFH requirements, IEC 61511-1 defines minimum architecture requirements. According to the standard: "For safety instrumented functions, sensors, logic units and terminal elements shall have a minimum hardware fault tolerance (HFT)".

To say that a device...