Thesafety levels(SIL) summarize the potential risks of systems. 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 recognized 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 range between 60% and 90%, the range 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.