# Classification of Systems and Maintenance Models
## 3 Classification of Systems
For subclass \(I_{12}\), that is, for systems operating in two modes, the dominant factor in evaluating the consequences of a failure is the fact that a safety-critical function is not performed at an arbitrary start of the \"operation mode.\" As a rule, for this system subclass, an economic assessment of the consequences of the failure is not possible. Therefore, mission availability should be used as an indicator of maintenance effectiveness. We define mission availability as the probability that the system will be in an operable state at any time (except for scheduled maintenance intervals, during which the system is not used for its intended purpose), and from that moment, will operate failure-free within a specified interval of time. Therefore, in the third stage of the classification, depending on the value of sign \(k\), the subclasses of the systems are divided into the following nine groups:  
\[ \begin{array}{l}I_{11}=I_{111},\,\,I_{12}=I_{121},\,\,I_{13}=I_{131}\cup I_{ 132},\\ I_{14}=I_{141}\cup I_{142},\,\,I_{21}=I_{211},\,\,\,I_{25}=I_{251}\cup I_{252 }\end{array}  \tag{4}\]  
At this stage of classification, maintenance effectiveness indicators can be uniquely defined for the six groups of systems. For group \(I_{111}\), the effectiveness indicators were OPFFO and UC; for group \(I_{121}\), mission availability; for groups \(I_{132},I_{142}\), and \(I_{252}\), AOPUT or AMCUT; and for group \(I_{211}\), APFFO and UC. The performance indicators of groups \(I_{131}\), \(I_{141}\), and \(I_{251}\) can either be inherent or achieved availability. The final choice of indicators is made after these groups are divided into subgroups depending on the temporary mode of the system's use. Figure 1b,c illustrates the classification of systems by sign \(k\). Fourth classification signA sign of the temporary mode of system use \(l\) (\(l=1\): continuous mode; \(l=2\): intermittent mode). In the continuous system use mode, equipment downtime is caused by failures and the need for scheduled maintenance. When a system is used intermittently, planned maintenance can be performed if it is not in use. Each group of systems \(I_{111}\), \(I_{211}\), \(I_{251}\), and \(I_{252}\) on the fourth level of classification was divided into two subgroups:  
\[ \begin{array}{l}I_{111}=I_{1111}\cup I_{1112},\;\;I_{211}=I_{2111}\cup I_{2112 },\\ I_{251}=I_{2511}\cup I_{2512},\;\;I_{252}=I_{2521}\cup I_{2522}\end{array}  \tag{5}\]  
because the values of \(l\) do not contradict the previous signs in these groups. For group \(I_{121}\), sign \(l=2\) because the system operates in two modes and switches to operating mode only after an alarm signal. For group \(I_{131}\), sign \(l=1\) because the value of sign \(l=2\) contradicts the value of sign \(j=3\). Indeed, with the intermittent use of the system, scheduled maintenance can be carried out at intervals in which the use of the system is not planned. Therefore, there will be no downtime owing to the scheduled maintenance. For the same reason, for group \(I_{132}\), the sign \(l=1\). For groups \(I_{141}\) and \(I_{142}\), the sign \(l=2\) because \(l=1\) is incompatible with \(j=4\). It takes time to repair a system when a failure is detected in a continuous mode of use. Therefore, for groups \(I_{131}\), \(I_{132}\), \(I_{141}\), and \(I_{142}\) at the fourth stage of classification, we obtain  
\[ \begin{array}{l}I_{121}=I_{1212},\;\;I_{131}=I_{1311},\;\;I_{132}=I_{1321},I_ {141}=I_{1412},\;\;I_{142}=I_{1422}\end{array}  \tag{6}\]  
Figure 1b,c illustrates the classification of systems by sign \(l\). During continuous use, it is necessary to consider the downtime of the system owing to scheduled maintenance. Therefore, the achieved availability is an effective indicator for subgroups \(I_{1311}\) and \(I_{2511}\). As mentioned previously, scheduled maintenance does not result in downtime in intermittent usage. Therefore, the effectiveness indicator for subgroups \(I_{1412}\) and \(I_{2512}\) is inherent availability.