Probability assessment common mode failures

The PMS is designed to prevent common mode failures between itself and the PLS. However, in the low probability case where a common mode failure does occur, the DAS provides diverse protection. The specific functions performed by the DAS are presented in Table 6.7-2. The DAS functional requirements are based on an assessment of the protection system instrumentation common mode failure probabilities combined with the event probability (see subsection 7.7.1.11 of Reference 6.1). 

For each hardware assessment of PFDavg, there should be some consideration of dependent failure (ie common cause or common mode types of dependent failure) with other layers. For each of the human error probability assessments there should again be some consideration of dependent failure. Further guidance on this may be found in Handbook of Human Reliability Analysis with Emphasis on Nuclear Power Plant Applications NUREG/CR-1278. ... 

Safety. It is becoming increasingly common to conduct quantitative assessments of process risks by failure modes and effects, fault tree, or other analytical alternatives. Thus, the probability of an accident times the corresponding potential loss is a cost factor which, although probabilistic. 

Failure modes and effects analysis is a systematic look at hardware, piece by piece, to determine how each piece could fail. The effects of each type of failure on the surrounding pieces and on the system or subsystem as a whole, and an assessment of the risk associated with each failure, commonly in terms of severity and probability, are expressed as a risk assessment code (RAC). 

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