# Classification of Systems and Maintenance Models
## 4 Classification of Maintenance Models
Prescriptive maintenance can show us how particular actions on an asset or system restoration affect the output, rather than only predicting when the equipment is likely to fail. Industrial businesses utilize prescriptive maintenance and analytics solutions to reduce unplanned downtimes, increase equipment reliability, and maximize profits. According to ISO 13372 [91], \"condition monitoring is detection and collection of information and data that indicate the state of a machine.\" Both intermittent and continuous condition monitoring is possible. Periodic monitoring in production systems is performed with the use of portable indicators such as hand-held measurement equipment, acoustic emission units, and vibration pens at specific intervals [38]. In real-time monitoring, a machine is continuously monitored, and at any time an error is found, a warning alarm is set off. Continuous condition monitoring is used to increase system availability or reduce maintenance costs for deteriorating systems. Sensors for condition monitoring may use vibration, ultrasonics, thermography, fiber optics, and other  
Figure 2: Hierarchical classification of maintenance models: (**a**) classification of models depending on the reliability structure; (**b**) classification of models depending on the type of inspection; (**c**) classification of models depending on the degree of the system’s unit restoration; (**d**) classification of models depending on the system’s external manifestations of failure. technologies. Maintenance based on continuous condition monitoring is a potential strategy to increase operational reliability and lower the operating costs of various deteriorating systems [33; 39; 42; 43; 50]. Therefore, depending on the type of inspection, the class of models \(M_{1}\) is divided into four subclasses \(M_{11}\), \(M_{12}\), \(M_{13}\), and \(M_{14}\) that is,  
\[M_{1}=\mathop{\cup}\limits_{b=1}^{4}M_{1b} \tag{8}\]  
One of the four types of inspection can be utilized for every system unit when performing maintenance on multi-unit systems. To simplify the classification, we selected from the classes of models \(M_{2},\ldots,M_{7}\) only those subclasses, \(M_{a1}\), \(M_{a2}\), \(M_{a3}\), and \(M_{a4}\) (\(a=2,\ldots,7\)), in which the same type of inspection was used for all units of the system. Figure 2b illustrates the classification of maintenance models by sign \(b\). _Third classification sign._ A sign of the degree of the system's unit restoration (\(c=1\): perfect repair: after repair, the unit becomes as-good-as-new; \(c=2\): minimal repair: after repair, the unit is in the pre-failure condition; \(c=3\): imperfect repair: after repair, the unit is in the condition between \"as-good-as-new\" and its pre-failure condition). Those units rejected due to inspection are shipped for restoration. These units can be restored with varying degrees of correspondence to the initial state. The degree of recovery is characterized by the reliability properties that the unit acquires after restoration. The degree of recovery can be full, minimal, or partial. The first degree of recovery corresponds to perfect repair. In this case, the repaired units acquire the same reliability properties as the new units with a zero-operating time. Such restoration is a model of a practical situation in which the reliability of a unit is primarily determined by a non-repairable element. In the second case, the repaired unit had the same reliability characteristics as the unit that had been in use for the same amount of time without experiencing failure. This recovery model applies to real-world scenarios, in which a unit includes many components with similar reliability indicator values. When one failed element was replaced, the reliability indicator readings of such a unit were comparable to those of a block with no failures. In the third case, we describe imperfect repair as one that leaves the system in a condition halfway between its pre-failure \"as-bad-as-old\" state and an \"as- good-as-new\" state.