## 2 Review of Current Maintenance Strategies  
According to EN 13306, maintenance is defined as the combination of technical, administrative and managerial actions during the life cycle of an item that is intended to maintain it in, or restore it to, a state in which it can perform the required function or combination of functions considered necessary to ensure that the service in question can be provided. The maintenance of a company's production equipment typically involves maintenance activities such as repairing, replacing components, checking, adjusting, testing, measuring and fault-finding. These actions are aimed at avoiding breakdowns that can lead to interruptions in the normal operation of the production line. If these actions are effective, they can reduce the frequency of failures, their effects and extend the service life of the production equipment. The implementation of maintenance implements the maintenance strategy, which is defined through action plans to ensure that direction is maintained and that guidelines are met to carry out subsequent maintenance activities required by the system. In the literature, the maintenance strategies can be divided in various ways, such as (Figure 1): corrective maintenance (CM), preventive maintenance (PM) and predictive maintenance (PdM), extended from the condition-based maintenance [19; 20; 21]. The first two of these strategies (CM and PM) have been applied since the early 1990s [22].Corrective maintenance (i.e., reactive maintenance or failure-driven maintenance) is intended to repair or restore a system after a failure has occurred. According to [23], this strategy often results in unpredictable consequences, including higher production costs, longer repair times, but also costs and penalties due to machine failures. Preventive maintenance is based on planning and performing maintenance at planned time intervals or based on the number of hours worked to keep the equipment in good condition [24]. The goal is to repair the system before a failure occurs. Decisions regarding maintenance activities are made by specialists based on their experience, previous failures and their features. However, it is difficult to ensure a flawless maintenance schedule well in advance. Due to increasing requirements for the reliability, availability and security of systems, preventive maintenance is becoming less and less effective. Additionally, the downside of this strategy is that the actual state of the system is not always taken into account when planning subsequent maintenance, which can lead to unnecessary maintenance and additional costs [25]. Compared to the aforementioned two types of maintenance strategies, the third of these strategies (PdM) is a trade-off between the two [21]. According to [26], the essence of the PdM is to predict the next error in a manner that preventive maintenance can be performed before the failure takes place. In practice, PdM reduces the frequency of maintenance to the lowest possible level to prevent unexpected failure, thus avoiding reactive maintenance and reducing the expense of conducting unnecessary preventive maintenance. The concept of PdM is a step further from Condition-Based Monitoring (CBM). PdM enables proactive maintenance and includes anomaly detection, diagnostics, prediction and maintenance decision-making. Its advantage lies in the implementation of a system that constantly monitors the state of the system to ensure data-driven maintenance [27; 28]. For many enterprises, a predictive maintenance strategy is an opportunity to reduce costs, increase the working life of assets, ensure the required quality of products, improve operational safety and reduce the negative environmental impact of machine failure. According to [29], implementation of predictive maintenance (PdM) systems saves up to 60% of maintenance costs. [PERSON] et al. [30], referring to [31], indicate that preventive maintenance can reduce maintenance costs by 25-35%, reduce failure rates by 70-75%, reduce downtime by 35-45% and increase production by 25-35%.