  A set of general formulas is proposed for the probability of failure on
demand (PFD) assessment of MooN architecture (i.e. k-out-of-n) systems subject
to partial and full tests. Partial tests (e.g. visual inspections, imperfect
testing) may detect only some failures, whereas owing to a full test, the
system is restored to an as good as new condition. Following the proposed
approach and according to an example, performance estimations of the system and
test policies are presented, by using the feedback from partial and full tests.
An optimization of the partial test distribution is also proposed, which allows
reducing the average probability of system failure on demand (PFDavg).
