SIL Calculations

The combined consideration of hardware, fault tolerance, category, DC, PFHD and SFF provides the following SIL assignment. All conditions must always be met. If one single condition is not met, the SIL has not been achieved.

Compare the required SIL from the risk assessment with the achieved SIL. If the achieved SIL is greater than or equal to the required SIL, the requirement for the implementation is considered to have been met.

SIL Assessment

In EN 62061, the maximum achievable SIL is determined via the dependency between the hardware fault tolerance and the safe failure fraction (SFF). The SFF is calculated by assessing all possible types of component failures and establishing whether each of these failures results in a safe or unsafe condition. The result provides the system's SFF.

The structural analysis also indicates whether there is any fault tolerance. If the fault tolerance is N, the occurrence of N+1 faults can lead to the loss of the safety function. The following table shows the maximum potential SIL, based on the fault tolerance and SFF. 

Calculated Safety: SIL Calculation in accordance with IEC 61508/61511 in process-based applications

http://www.pilzamerica.com/articles/Calculated-Safety.pdf