**6. Conclusions**

In this paper, we introduced Weibull-based ageing systems that undergo discrete maintenance optimization. The Weibull-based ageing process is considered imperfect CM, where each failure and follow-up repair degrade the system to some extent. After the occurrence of the *n*-th failure, where *n* can be determined for each component as an optimal value for the decision variable of the optimization process, FBM is launched and the component is replaced with a new one. The optimization is realized in a context with minimal system costs and a prescribed unavailability restriction. The corresponding reliability mathematics for the unavailability quantification of terminal nodes was derived in this article because this unavailability is used as the main input in system unavailability quantification using AGs as the system representation. Although the renewal process model that originates from the new renewal cycle was only developed in this article, in general, we can conclude that it is a limited case of the model developed in [31] where the length of the inspection period approaches zero. A cost model respecting the imperfect CM process with FBM was further introduced because it is indispensable for discrete maintenance optimization.

Numerical experiments showed that the discrete maintenance optimization method is a viable method to make an optimal decision for FBM—replacement of system components that undergo real ageing. Although the computing process may in some cases be heavy on computing time (see the 81 system configurations of the four component systems), none of the computations in this paper exceeded 4 min. All these computing experiments, including both the development of the algorithm for the unavailability function *u*(*t*) and the discrete maintenance optimization, were numerically realized with the high-performance language MATLAB on computing equipment with the following characteristics: Intel (R) Core ™ i7-3770 CPU @ 3.4 GHz and 3.9 GHz, 8.00 GB RAM.

Our future research work will be based on our original achievements resulting from our cooperation with power industry experts ([17]). It will involve maintenance optimization of power networks with a particular focus on circuit breakers.

**Author Contributions:** Conceptualization, R.B. and P.J.; methodology, R.B. and P.J.; software and validation, R.B.; formal analysis and investigation, R.B. and P.J.; writing—original draft preparation, R.B. and P.J.; supervision, R.B.; funding acquisition, R.B. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was partly supported from ERDF "A Research Platform Focused on Industry 4.0 and Robotics in Ostrava Agglomeration", No. CZ.02.1.01/0.0/0.0/17\_049/0008425 and partly by the VSB-Technical University of Ostrava project "Applied Statistics and Probability", No.SP2020/46.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** This work was partly supported from ERDF "A Research Platform Focused on Industry 4.0 and Robotics in Ostrava Agglomeration", No. CZ.02.1.01/0.0/0.0/17\_049/0008425 and partly by the VSB-Technical University of Ostrava project "Applied Statistics and Probability", No.SP2020/46.

**Conflicts of Interest:** The authors declare no conflict of interest.
