 The major problem in the construction of safety barriers is how to make them as independent of each other as possible. If two or more barriers are sensitive to the same type of impact, then one and the same destructive force can get rid of all of them in one swoop. For instance, if three safety valves are installed in one and the same factory hall, each with the probability 1/1,000 of failure, it does not follow that the probability of all three failing is \(1 \times 10^{-9}\). The three valves may all be destroyed in the same fire, or damaged by the same mistake in maintenance operations. This is a common situation for many types of equipment. 