Since the 2oo2 system has twice as much hardware as a simplex system, it has twice as many dangerous failures, therefore the 0.02 doubles to 0.04. For this system to have a nuisance trip, both channels would have to suffer safe failures. As before, the probability of two simultaneous failures is the probability of one, squared. This does not imply 2oo2 systems should never be designed. Most people are surprised to learn that a 2oo3 system has a higher nuisance trip rate than a 2oo2 system, and a greater probability of incurring a dangerous failure than a 1oo2 system.
Triplicated systems are actually a tradeoff.
Overall, they re pretty good, but not as good as 1oo2 and 2oo2 systems. A closer examination of the numbers included in the Simplex system diagram indicates the 1oo2 system is safer than 2oo3, and the 2oo2 system offers a better nuisance trip rate than 2oo3. It has been known for years that if a dual system could be designed to provide the best performance of both of the dual systems, such a system would outperform a triplicated system. But, it has been within the last 10 years that technology has reached the point where such a design can be properly implemented. These systems are commonly referred to as 1oo2D. Dual is not always better than single, and triple is not always better than dual.