## 1 Introduction
The disadvantage of this approach is the unpredictability of the duration, type of necessary repairs and the possibility of secondary damage or damage to other parts of the system caused by the initial fault. Periodic maintenance (time-based maintenance) represents a systematic approach to system maintenance where actions are planned and take place at certain time intervals. If a fault occurs within the period between two scheduled interventions, only minimal interventions are performed to return the system to a functional condition. The best example of this method is a thermal energy production and distribution system with a critical period for the operation of the heating system being the winter season. With this system, the best time for shutdown, maintenance, overhaul and preparation of the system for a new cycle is outside the heating season, i.e., summer and autumn. The advantage of periodic maintenance is that all interventions as well as the time required for regular maintenance are planned. The disadvantage of this approach lies in the fact that the cycle length is predicted based on statistical indicators, thus, this can lead to an excessively long cycle. This approach can ensure very high reliability and availability of the maintained system, which is naturally accompanied by high maintenance costs. Condition based maintenance is a combination of the two above-mentioned approaches, i.e. periodic maintenance and maintenance after a fault [6; 7; 8; 9; 10]. The idea is to use the advantages of the latter, that is, to use the reserves of components and parts of the system as much as possible and, at the same time, avoid the disadvantages of periodic maintenance. Maintenance according to the system condition uses all available methods to determine the technical level of the system and equipment condition to access maintenance only when the condition of system components falls below a certain critical level. The condition of the system is determined by tests, inspections, diagnosis, measurement and the analysis of measured data [11; 12; 13]. The advantages of this approach are high reliability and maximum utilization of system parts reserves. The disadvantages are that the system depends on the ability to perform adequate measurements and the reliability of system diagnosis. In addition, since the time for carrying out the overhaul is not planned but depends exclusively on the condition of the system, the execution of major procedures on the system may fall at an inappropriate time, i.e., time when the market demands high availability of the system. When making decisions regarding maintenance based on risk assessment, we distinguish between two types of risks. The first would be a risk in terms of the safety and reliability of production and risk in estimating the cost of system maintenance and production maintenance. The risk related to the assessment of the cost of maintenance and operation can be expressed as the product of the frequency of fault and the amount of its consequent damage. Equation (1) shows risk \(R\) for carrying out or not carrying out maintenance, where \(H\) is the frequency of fault occurrences or irregularities and \(S\) represents  
Figure 1: Maintenance strategy diagram. the level of potential damage. The frequency of faults \(H\) is determined with the help of statistical methods and reliability theory, while the level of damage \(S\) is determined as the cost of production downtime plus the cost of maintenance or the cost of repairing of fault. The total amount of risk for a system represents the sum of risks for each part of the system:  
\[R=\sum_{i=1}^{n}R_{i}=\sum_{i=1}^{n}S_{i}\cdot H_{i} \tag{1}\]  
where \(R_{i}\) represents risk for the \(i\)-th component, \(S_{i}\) the inherent cost and \(H_{i}\) the frequency of the fault on the \(i\)-th component.