It follows that the series/parallel equations derived in Sections 11.2 and 11.3 can be applied directly to a minimal cut set diagram. The procedure is to apply the equations for parallel system to each cut set in order to evaluate the equivalent indices for each cut set and then to combine these equivalent indices using the equations for series system to give the overall system reliability indices.
In order to illustrate the application of the equations to the minimal cut set method, reconsider Example 11.3
A visual inspection of figure 4.7 identifies two second-order and one third-order cut sets. These are (1 and 5), (2 and 5), (3 and 4 and 5).Equation 11.15 to 11.18 can be applied to the first two cut sets and Equations 11.20 to 11.22 can be applied to the third one. This gives the results shown in Table 11.1
After evaluating the reliability indices for each cut set, equations 11.11 can be used to evaluate the system indices. This is most conveniently accomplished by summating the values of λ to give λs, by summating the values of U to give Us and then evaluating Rs by dividing the value of Us by λs. These system indices are also shown in table 11.1 and can be compared with those obtained previously in Section 11.4 using network reduction.
Table 11.1 Minimal cut set analysis of example 11.3
The following comments can be mead in the light of these results and the analysis used to achieve them :
(a) The system indices are generally dominated by the low order cut sets which, in the case of the above example, are the two second order cuts. Sufficient precision is therefore generally achieved by ignoring cut sets that are more than one or two orders greater than the lowest order cut sets that exist. It must be stressed that this assumption may not be as valid if the components forming the lower order cut are very reliable and the components forming the higher order cuts are very unreliable.