Diversity and Multi-Version Systems - Nuclear Power Plant Instrumentation and Control Systems for Safety and Security - page 207

Figure 24. Dependence of P 2M on time and β

Figure 25. Dependence of ΔP 2M-1M and ΔP 2M-2D on time

For two-version system (Figure 26, b, c) CCF
metric β = N CCF / N , N CCF = CardF 1 ∩ F 2 ; the
value of N may be calculated as an arithmetic
mean N = (N 1 + N 2 ) / 2, N i = Card F i ; SF metric
α i = 1 - β ; DCCF metric β d = N D CCF / N ; UDCCF
metric β d = N UD CCF / N ; β = β d + β d . Besides, it
is possible to use metrics of a relative number of
DCCFs and UDCCFs: β d * = β d / β , β d

determining a part of CCFs of any two versions,
γ = 2 N P CCF / N ). Metrics of distinguishable and
undistinguishable PSSFs are calculated by anal-
ogy β d and β d . If γ = 0 (Figure 1,e), α =1 - β . This
approach is based on the results described in
(Gorbenko, A.; Kharchenko V. & Romanovsky
A. 2009) and may be extended to systems, in
which a set of faults is added a set of vulnerabil-
ities attacked by an external system.

* = β d / β .
For three-version system (Figure 26, d) α = 1
- β - 2γ, where γ is PCCF metric (metric