IEC 61508 functional safety standard indicates Safety Integrity Levels (SIL) needs to be evaluated by three design barriers:

• The Systematic Capability Rating
• The Architectural Constraints for the Element
• The Probability of Failure for the Product

Systematic Capability is achieved when the equipment used to implement any safety function is designed using procedures intended to prevent systematic design errors. This is evaluated through an assessment of the quality management system for suppliers of process control and instrumentation for safety. 

The Architectural Constraints for the product is achieved when Failure Mode Effects and Diagnostic Analysis (FMEDA) evaluates the product through the rules of Route 1H or Route 2H.

The Probability of Failure for the product is achieved when the random probability of a failure is calculated by PFDavg for low demand mode of operation or PFH for high or continuous demand modes. 

All three of these design barriers must achieve or exceed the target SIL level to achieve compliance with the standards. This is not a pick-and-choose deal. Think of the Three Design Barriers as track hurdles that each must be conquered to get to the finish line.   

The worst-case (the lowest) SIL determines the SIL level for the entire SIF. This is a case of weakest links— if your Architectural Constraints and Probability of Failure are SIL 3 rated, but your SIL capabilities are limited at a SIL 1 rating, your SIF is stuck at that SIL 1 level rating.