# Methodology for Studying the Reliability of Interlocking Devices in Bulgarian Railways
## 3 Results
### Reliability Analysis of Failures of the Interlocking Devices
#### 3.2.1 Reliability Analysis of Failures of the IIDs in STD--Sofia  
The values of the time for failure-free operation \(\tau(\Delta t)\) of each type of IID for one year (2020, 2021, 2022) and for the three-year period are presented in Figure 19.  
The obtained data for the statistical evaluation of the intensity of the failures flow \(\hat{\omega}_{i}(\Delta t)\) is shown in Figure 20, and for _MTBF_--in Figure 21.  
Figure 19: Time for failure-free operation \(\tau(\Delta t)\) of different types of IIDs for STD—Sofia.  
Figure 18: Proportion of duration of failures of different types of IIDs for STD—Gorna Oryahovitsa.  
It can be seen that despite the small number of failures of RCIDs and their short duration, the values of the intensity of the failure flow of this equipment are high and _MTBF_ is relatively short. This is due to the small number of route-computer interlocking stations in this division--only three, which were implemented at the beginning of the studied period at railway stations around Vidin. After passing the initial tests and adjustments, the _MTBF_ is greater.  
It should also be specifically noted that the _MTBF_ of RRIDs has higher values than that of RCIDs.  
These results lead to conclusions opposite to those drawn after statistical evaluation of the information. This proves the need for reliability analysis and determination of its indicators. In addition, it can be concluded that _MTBF_ reflects the reliability of the equipment better than the time for failure-free operation \(\tau(\Delta t)\), since it takes into account not only the recovery time, but also the number of the specific devices and the number or  
Figure 21: Mean time between failures flow _MTBF_ of different types of IIDs for STD—Sofia.  
Figure 20: Statistical evaluation of the intensity of the failures flow \(\hat{\omega}_{l}(\Delta t)\) of different types of IIDs for STD—Sofia.  
their failures. It can be seen that \(\tau(\Delta t)\) varies within narrow limits for all devices, while _MTBF_ varies significantly, decreasing for most of the older interlocking devices.  
The values determined for the coefficient of availability are shown in Figure 22 and those for the coefficient of unavailability--in Figure 23.  
It can be seen that the coefficient of availability of the RCIDs reaches high values regardless of the low _MTBF_ (over 0.99 for 2021 and 2022 and over 0.97 for three-year period). Respectively, the coefficient of unavailability of these IIDs is low. This can be explained by the shorter recovery time than other types of IID (see Figure 4).  
The coefficient of unavailability is highest for RRIDs, which are a large part of IIDs in this area and are also relatively old. It reaches very high values, especially in 2020. The duration of failures of these IIDs is also significantly higher than that of all the devices under study. Therefore, some of the RRIDs should be gradually replaced by route-computer interlocking devices. Another way to reduce the recovery time is to implement RRIDs into a SCADA system for remote monitoring and control.  
Figure 23: Coefficient of unavailability of different types of IIDs for STD—Sofia.  
Figure 22: Coefficient of availability of different types of IIDs for STD—Sofia.  
The average values of the availability and unavailability coefficients were also calculated using Equations (11) and (12), and the obtained values are systematised in Table 2 (compared with the values for the three-year period).  
It is clearly visible that the average values of the availability coefficients for several years are higher than the values for a three-year period for all ILDs and therefore should not be calculated, as this may lead to wrong decisions regarding maintenance, system upgrades and modernization of the devices.  
Coefficients of availability and unavailability for the entire research period should be determined only according to the described methodology.