# Methodology for Studying the Reliability of Interlocking Devices in Bulgarian Railways
## 3 Results
### Reliability Analysis of Failures of the Interlocking Devices
#### 3.2.2 Reliability Analysis of Failures of the ILDs in STD--Plovdiv  
The values obtained for the time for failure-free operation \(\tau(\Delta t)\), for the statistical evaluation of the intensity of the failures flow \(\hat{\omega_{i}}(\Delta t)\), and for \(MTBF\) of each type of IID for the different periods are shown in Figure 24, Figure 25, and Figure 26, respectively. It is noteworthy that the \(MTBF\) values for RCIDs are low. This can be explained as follows: the structure of the RCIDs includes a large number of electronic boards--processors (Local Control Unit LCU, Remote Control Unit RCU), modules for providing connection to the computer of the dispatcher on duty and man-machine interface MMI, central, element, communication, and interface controllers, etc. All of them are duplicated (permanent redundancy is implemented)--one component works as the main one (primary), and the second (the backup element) works together with the main one and in case of a failure of the primary element, it immediately begins to perform its functions. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Type of IID** & \(A^{2020-2022}\) & \(A^{Av}\) & \(\mathbf{U^{2020-2022}}\) & \(\mathbf{U^{Av}}\) \\ \hline RCID & 0.9723948 & 0.9890655 & 0.0276052 & 0.0109345 \\ RRID & 0.6281414 & 0.8223516 & 0.3718586 & 0.1776484 \\ EMID & 0.9768748 & 0.9905424 & 0.0231252 & 0.0094576 \\ RSKD & 0.9877950 & 0.9949017 & 0.0122050 & 0.0050983 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Comparison between availability and unavailability coefficients obtained in different ways for STD—Sofia. Figure 24: Time for failure-free operation \(\tau(\Delta t)\) of different types of ILDs for STD—Plovdiv. The interface modules provide connection to external station devices--point machines, traffic lights, train presence detection systems. During modernization activities, when replacing old point machines with new ones and old incandescent lamps with LED ones, overvoltages sometimes occur during commissioning, which lead to damage to the interface modules of RCID connected to the control modules of the corresponding external devices. Each of the electronic boards also includes diagnostic modules--they record all error and fault messages, including false alarms (for example, for cutting points from the control modules of the point machines). All these error messages are sent to the system server and are recorded as failures in the event logs and are subsequently included in the Annual consolidated activity reports issued by NRIC. In reality, most of them do not lead to a complete failure of the relevant RCID, but only to a short-term suspension of the operation of an individual main module until standby is activated (usually of a few seconds). In the worst case, it may be necessary to restart only the relevant module. However, the duration  
Figure 26: Mean time between failures flow _MTBF_ of different types of IIDs for STD—Plovdiv. Figure 25: Statistical evaluation of the intensity of the failures flow \(\hat{\omega}_{i}(\Delta t)\) of different types of IIDs for STD—Plovdiv. of the failure is recorded as the time for replacing the defective board and turning it on as a hot spare--only then is the module considered to be working correctly. The values calculated for the coefficient of availability are shown in Figure 27 and these for the coefficient of unavailability--in Figure 28.  
It can be seen that the values of the coefficient of availability of the RCIDs are relatively low and, accordingly, those of the coefficient of unavailability are relatively high. As mentioned above, these values are determined from the information in NRIC Annual consolidated activity reports. However, in them, there is no specific data only on the failures that led to the complete failure of a RCID--this happens if a large number of safety violations has been accumulated. In reality, for the three-year period there were only four such failures, and for the entire period of operation of the RCIDs there were about one per year.