# Classification of Systems and Maintenance Models
## 8 Discussion  
Section 1 highlighted the categorization of maintenance performance indicators, which include equipment-related, maintenance task-related, and cost-related indicators. Nevertheless, it is imperative to have a guide for selecting suitable indicators for systems with varying purposes, objectives, and limitations. The article presents significant findings on the classification of systems that aim to select maintenance effectiveness indicators and classify maintenance models for the indicator calculation. Through a four-sign system classification with 11 different values, the study identified 13 subgroups with unique maintenance effectiveness indicators. Additionally, the classification of maintenance models resulted in the identification of 168 subgroups that varied in 16 values of 4 signs. The classifications presented in the article address two problems at once. Notably, we developed a new mathematical model of condition-based maintenance with inspections at discrete times for 1 subgroup out of 168, which enables the calculation of maintenance effectiveness indicators for all 13 system subgroups. It should be noted that there is a fundamental difference between the developed classifications and previously published ones. The literature review [14] classified the key performance indicators into maintenance process and maintenance result indicators. The maintenance results group includes three categories of indicators: equipment effectiveness, maintenance cost-effectiveness, and safety and environmental indicators. The classification of systems developed in this article outlines 13 subgroups, each with its own set of maintenance effectiveness indicators that can also be categorized into the same three categories as in [14]. However, unlike [14], the classification developed in the article makes  
Figure 4: The dependence of the achieved availability on the number of inspections at \(RT^{opt}=23.5\) kV (curve 1, red squares) and at \(RT=FT=25\) kV (curve 2, blue circles). it possible to clearly define for which systems indicators are intended. Thus, our article presents a unique classification of technical systems for selecting maintenance effectiveness indicators during the operational phase. While the well-known literature review [15] touched on various aspects of maintenance tasks, resources, metrics, and measurements in manufacturing companies, it did not include our categorization of systems that aim to choose maintenance effectiveness indicators during the operational phase. The contribution of this article fills this gap in the literature and provides valuable insights for maintenance professionals. Reference [16] conducted a survey in the Belgian industry to investigate the utilization of maintenance performance indicators. The study's objective was to determine how frequently different indicators are used. Interestingly, the results showed no correlation between the indicators used and the maintenance goals pursued. In contrast to [16], we propose an approach to selecting maintenance effectiveness indicators based on system classification according to specific signs. The study in [18] examined a multi-criteria model for measuring maintenance performance, which included three levels of performance indicators for industrial equipment: plant level, system level, and item level. While the study provides a list of indicators for each level, ranging from 16 to 26, it fails to specify the criteria for selecting indicators within each group. It is crucial to note that the model proposed in [18] is not a classification of systems aimed at the selection of maintenance indicators. It is worth mentioning that the CBM model we developed is distinct from previous studies. Many mathematical models for CBM have a flawed assumption of perfect inspections, which results in an error-free determination of the system's condition [30; 31; 32].