P3: Considerations on the Safe Failure Fraction in High and Low Demand

Last edit: 26/02/2024

In a previous article we presented an important parameter, used in both high and Low Demand Mode Safety Systems: the Safe Failure Fraction.

In this article we discuss some critical aspects linked to the use of the SFF. We also provide some requirements for failures to be defined as Detectable.



From the previous article we remind that the Safe Failure Fraction is represented by the following equation:

SFF = (λS+λDD) / (λS+λD )


It is used together with the Hardware Fault Tolerance (HFT) of the safety subsystem to determine its maximum reachable SIL level.

In Low demand mode of Operation the SFF is used when Route 1H is selected and, depending upon the type of component, one of the following two tables has to be used.

In High demand mode of Operation, IEC 62061 has the following reference table, valid for any type of component. We remind that the classification Type A and Type B is not applicable in High Demand Mode.