Regional Basic Professional Training Course in Korea

several pumps due to an inappropriate lubrication.
❙ 583 ❙
9. Probabilistic Safety Analysis
CCFs have to be introduced in a PSA, since their impact on system reliability is
significant, especially in case of highly redundant systems. The steps for this introduction
are the following:
Choice of a method for modelling CCFs;
Identification of the groups of components which can be affected by CCFs (CCF
components groups);
Data collection.
Several methods can be used in the PSAs for CCF modelling: Multiple Greek Letters
(MGL), Binomial Failure rate (BFR), or Beta Factor [9.13]. For example, the Beta Factor
can be used if it is defined as the ratio (for a particular component), of the number of
common cause failures to the total number of failures. This method can be generalized by
using factors Beta (k, n), representing the proportion of common modes affecting k
components with a redundancy of a factor n. It is possible to calculate al the Beta (k, n)
using the three basic values which are Beta (2, 2), Beta (3, 3), Beta (4, 4).
It does not seem that the choice of a model is the most important problem for the
treatment of CCFs, since with the same basic data all the models give similar results. Two
aspects seem more important and can be the source of large uncertainties:
The list of components (and of failure modes) which can be submitted to a CCF
(CCF component groups);
The source of data, which are generally sparse and difficult to interpret. For
example, in experience feedback there is very often the occurrence of partial or
potential CCFs.