© Handermann 2012 FunctionalSafety.net In the failure models are different failures: Safe failures with safe detected (Lambda SD) and safe undetected (Lambda SU) and Dangerous failures with dangerous detected (Lambda DD) and the most critically failures the dangerous undetected (Lambda DU). The architectural constraints in the standards IEC 61508 or IEC 61511 well-defined the SIL with the information of the HFT, type A or B and the SFF. Reliability analysis is required because we need to determine the SFF and the PFD. We need to document the failure behavior of the SF and the end user want to know the spurios trip rate of the SF. Reliability techniques are:  -Failure mode and effects   analysis (FMEA); -Reliability block diagrams; -Fault tree analysis (FTA); -Markov analysis;