###### Abstract  
Maintenance policies are crucial for ensuring the reliability, safety, and longevity of a system, as well as reducing the risk of accidents. Preventive maintenance (PM) is an effective strategy to keep equipment and systems in good working order by fixing potential issues before they cause downtime or safety hazards. However, optimizing the time intervals between PM activities is essential for minimizing the overall maintenance cost. This paper proposes an innovative approach that considers the intervention level of maintenance activities as an independent variable of PM times. The approach provides greater flexibility in creating maintenance plans, as it considers practical aspects that may impact maintenance activities beyond the time interval between PMs. The proposed approach uses a reliability model that incorporates imperfect preventive maintenance and a variable improvement factor based on age reduction. The improvement factor of each preventive maintenance activity (PMA) is defined based on the intervention level of the activity itself, which is determined by the number of tasks performed, execution time, and the number of items replaced in the maintenance plan. The proposed maintenance strategy determines not only the optimal times for PMAs and the respective intervention level but also the optimal number of maintenance activities that minimize the total maintenance cost along a fixed and user-defined planning horizon. The effectiveness and precision of the approach have been demonstrated through a series of numerical examples and a comprehensive case study involving three heat exchangers situated within the hydroelectric power plant. m m  
preventive maintenance generally result in lower corresponding costs. However, longer intervals between PM activities also increase the expected number of failures, which may require more corrective actions that increase the overall cost of the maintenance process. In general, the costs of corrective maintenance tend to be higher than those of preventive maintenance [4]. Preventive maintenance actions are associated with an intervention level that considers the number of tasks performed, execution time, and the number of items replaced in the maintenance plan [5]. The intervention level is usually considered dependent on the times between successive preventive maintenance actions, meaning that the longer the time interval between PMs, the greater the intervention level of the maintenance action, and the higher its cost. However, in practice, the number of tasks performed, execution time, and the number of items replaced in the maintenance plan may not depend solely on the time interval between PMs. Economic issues and contractual rules may also affect decision-making, while equipment usage regimes may vary significantly from the standard established in the manufacturer's manuals. This paper proposes an innovative approach that considers the intervention level of preventive maintenance activities as an independent variable of PM times. This approach provides greater flexibility in creating maintenance plans, as it considers practical aspects that may impact maintenance activities beyond the time interval between PMs. In practice, companies often operate under budgetary and time constraints, so an intervention that restores equipment to a state that is \"good enough for safe and efficient operation\" may be deemed sufficient, especially if complete restoration is significantly more costly or time-consuming. Additionally, the proposed approach uses a reliability model that incorporates imperfect preventive maintenance and a variable improvement factor based on age reduction. The improvement factor of each PMA is defined based on the intervention level of the activity itself, which is determined by the number of tasks performed, execution time, and the number of items replaced in the maintenance plan, according to [6].