Chapter 6. Modelling and optimization of SIS including MooN voting architectures
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6.3. MODELLING OF STR FOR MooN ARCHITECTURES 6.3.1. Effects of bypass on the STR This section is dedicated to study the behaviour of MooN during normal operation and their reconfiguration during test from the perspective of STR modelling. The analysis follows the same structure as for the one made for PFD modelling in Section 6.2. The bypassing philosophy established in that section is that the bypass will be closed for 1ooN (parallel) architectures closed and open for MooN (M>1) architectures. It has been seen that the specific bypass for one component influences how the system fault tree is reconfigured during test. However, the effect that a bypass has on the spurious trips is different than the effect of a bypass on PFD. Actually, as with the PFD quantification, a bypass has an effect that resembles a failure, but in the opposite direction. Remember that spurious trips are safe failures. For PFD the bypasses behave in a similar way to a dangerous failure of the corresponding basic event in the fault tree. A closed bypass resembles a dangerous failure (and thus it takes a similar value in the PFD fault tree). However, regarding safe failure this would resemble the opposite, and thus a zero in the STR fault tree. This can be easily understood for the open bypass. This would resemble a nofailure for PFD (since the action to trip is an open switch). But for the STR this resembles a failure (because a false trip is always the result of an open state). Therefore, an open bypass corresponds to a value of one in the STR fault tree. In conclusion, the bypasses take the opposite value in the PFD and the STR fault trees. For STR a closed bypass is equivalent to a value of zero, and an open bypass a value of one. 6.3.2 Reconfiguration of STR fault trees with bypasses
Figure 6.12. Reconfiguration of STR fault trees of Moo2 architectures