Optimal staggered testing strategies for heterogeneously redundant safety systems

Staggered testings are effective in improving the availability of redundant safety instrumented systems, and the optimal staggered time of testings for a system of two homogeneous components with the same testing interval has been proved as half of the testing interval. In this study, the impact of staggered time on the effectiveness of staggered testings for heterogeneous systems is examined, and the optimal staggered time is found still as half of the testing interval for systems with components different in failure rates but same in testing interval. In terms of systems with two components different in testing intervals, the optimal testing time is revealed as half of the shorter interval. Case studies of safety instrument systems present the same results. And then, Monte Carlo simulation based on Petri net models for these systems also confirms the conclusions obtained by numerical formulas. Such findings are helpful to effectively apply staggered testing strategies in more redundant systems.