# Methodology for Studying the Reliability of Interlocking Devices in Bulgarian Railways
## 4 Discussion  
Any failure in the signalling systems requires time to restore the functions of the damaged device, which leads to increased operating costs and possible train delays. In order to ensure transport safety, all important components are redundant, and the system as a whole very rarely stops functioning--if one element fails, the backup element is immediately switched on. That is why in most cases the train delays are very small. The purpose of the study is to determine the components that have relatively low values of  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Type of IID** & \(\mathbf{A^{2020-2022}}\) & \(\mathbf{A^{Av}}\) & \(\mathbf{U^{2020-2022}}\) & \(\mathbf{U^{Av}}\) \\ \hline RRID & 0.9205264 & 0.973652878 & 0.0794736 & 0.026347122 \\ EMID & 0.9622748 & 0.987344869 & 0.0377252 & 0.012655131 \\ RSKD & 0.9868424 & 0.995368753 & 0.0131576 & 0.004631247 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Comparison between availability and unavailability coefficients obtained in different ways for STD—Gorna Oryahovitsa.  
Figure 33: Coefficient of unavailability of different types of IIDs for STD—Gorna Oryahovitsa.  
availability coefficients and to plan the technical maintenance correctly and to foresee a gradual modernization of the devices.  
The obtained results for all STDs clearly demonstrate the need for reliability analysis and determination of its indicators. It also can be concluded that it is necessary to calculate the complex reliability indicators according to the presented methodology. They give the clearest assessment of the state of the devices. If only a statistical assessment of failures and their duration is made or if only simple reliability indicators are calculated, erroneous conclusions can be drawn regarding maintenance and the need for modernization. On the other hand, the availability coefficients of all devices for the three-year period are lower compared to the values for one year--therefore it is better to calculate the indicators for a longer period and obtain a more accurate reliability assessment.  
The highest coefficients of availability are for RCIDs--complete failures of these interlocking devices are very rare. The values are also high for RSKD (over 0.98 for all STDs). However, these are devices located in small stations, where switching related to route ordering is very rarely required. That is why the elements wear out much more slowly and the failures are relatively few, but their restoration takes a relatively long time (usually over 4 h).  
The most common are RRIDs, and their availability coefficients are relatively low--for the three-year period, below 0.98 for STD--Plovdiv, around 0.92--for STD--Gorna Oryahovitsa, and even under 0.63--for STD--Sofia.  
The availability coefficients of EMIDs are also not particularly high--below 0.98 for the three-year period for all STDs.  
This proves the need to gradually replace these interlocking devices with route-computer ones, especially in large railway stations. The implementation of RRIDs in a SCADA system for remote monitoring and control is also an option for reducing the unavailability coefficient of these devices.