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safe failure fraction (SFF)

The safety levels (SIL) summarize the potential risks ofsystems. These safety levels depend on the Safe Failure Fraction (SFF) and the Hardware Fault Tolerance (HFT). The Safe Failure Fraction of a system is in turn determined by the ratio of two types of faults: safe faults

and faults detected as dangerous. Safe faults have no effect on safety-critical functions, whereas faults detected as dangerous can trigger safety-relevant malfunctions. Both errors determine the failure rate of the system.

In embedded systems, the SFF value range knows the probability of below 60 %, the between 60 % and 90 %, the between 90 % and 99 % and the value above 99 %. The SIL value resulting from the SFF probability depends on the hardware fault tolerance (HFT) and is SIL 1 for low probability and low hardware fault tolerance, SIL 3 and SIL 4 for high SFF probability and high hardware fault tolerance, i.e. high availability.

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Englisch: safe failure fraction - SFF
Updated at: 23.02.2009
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