Firouzabad Institute of Higher EducationAnnals of Optimization Theory and Practice2588-36663120201001On planned time replacement of series-parallel system11311791910.22121/aotp.2020.240152.1033ENTijjani AWaziriSchool of Continuing Education, Bayero University Kano, Nigeriahttps://orcid.org/00IbrahimYusufDepartment of Mathematical Sciences, Bayero University, Kano, Nigeria0000-0002-4849-0163AbdullahiSanusiSchool of Continuing Education, Bayero University, Kano, NigeriaJournal Article20200719This paper investigate some characteristics of the age replacement model with minimal repair, of series-parallel system with non-uniform failure rates and are subjected to two different types of failures, which are Type I and Type II failures. The six units of the system formed three subsystems, which are subsystems A, B and C. Subsystem A is having three parallel units, subsystem B is having a single unit and subsystem C is having two parallel units. We constructed age replacement model with minimal repair that will determine the optimal replacement time of the series-parallel system. Furthermore, we also considered some modifications of the age replacement model with minimal repair constructed. Finally, some numerical examples are given to illustrate the characteristics of the age replacement models with minimal repair constructed. From the results obtained, it was observed that the optimal replacement time of the system when the three units of A are in parallel is higher than when the three units of A are in series. It was also observed that, the optimal replacement time obtained from the standard age replacement model is higher than the optimal replacement time obtained from standard age replacement model with minimal repair.http://aotp.fabad-ihe.ac.ir/article_117919_ebaaee48f773332dfaecb6cf6e27facb.pdf