# Methodology for Studying the Reliability of Interlocking Devices in Bulgarian Railways
## 3 Results
### Statistical Assessment of Failures of the Interlocking Devices and Signallinig Equipment
#### 3.1.2 Statistical Assessment of Failures in STD--Plovdiv  
Similar to the analysis for STD--Sofia, a study of failures and their duration was conducted for the STD--Plovdiv region.  
Using the Equations (1) and (2), the total number of failures of all types of signalling devices and the corresponding percentage ratio was calculated and the results obtained are shown graphically in Figure 7a-d (respectively, for years 2020, 2021, 2022, and for the entire period). The total duration of failures of all device types and the corresponding percentage ratio for the four periods was calculated using Equations (3) and (4) and the results obtained are shown graphically in Figure 8a-d (respectively, for years 2020, 2021, 2022, and for the entire period).  
Overall, the failures are significantly fewer than those for STD--Sofia. However, it can be seen that the failures of interlocking devices and their duration have significantly higher values compared to those for STD--Sofia. Accordingly, their proportions approach the share of failures and duration of the ESD and IsD. The reason is the modernization of some railway sections in this area: the oldest point machines have been replaced with new ones (in terms of type and functionality requirements); the incandescent lamps of traffic lights have been replaced with LED ones; the network of railway lines equipped with automatic interlocking with axle counters has been expanded; in the PAB, replacement of existing copper cables with optical ones has also been carried out; analogue ATCs have been replaced with digital ones, etc.  
All installation and assembly activities require time and are accompanied by a large number of failures until the correct adjustment, regulation, and fit of all elements is achieved--not only of the specific equipment, but also of all devices that work together in the signalling system.  
Figure 6: Proportion of duration of failures of different types of IIDs for STD—Sofia.  
Figure 8: Duration of failures of devices of signalling systems and their percentage distribution for STD—Plovdiv: (**a**) 2020; (**b**) 2021; (**c**) 2022; (**d**) period 2020–2022.  
Figure 7: Number and percentage distribution of failures of devices of signalling systems for STD—Plovdiv: (**a**) 2020; (**b**) 2021; (**c**) 2022; (**d**) period 2020–2022.  
The number of failures of different types of IIDs are presented graphically in Figure 9 and the data on their duration--in Figure 10. After calculating the proportions, results obtained are shown graphically, respectively, in Figures 11 and 12--for each type of IID by year and for the entire period (as 100% are the values accepted only for IIDs, not for all devices). In this STD, there are no EMIDs, they were replaced in stages with RCIDs and subsequently included in the SCADA system for remote monitoring and control. The large number of RCIDs failures and their duration, as well as the high values of corresponding proportions, are mainly due to the need to ensure the interoperability of the individual software modules (applications) of the computer interlocking device during its installation and especially during implementation into the SCADA system. After commissioning, a particular component may be incorrectly set or adjusted, manufacturing defects may appear in some electronic modules, etc.  
On the other hand, the failures of the RRIDs and their duration are significantly lower compared than those for the STD--Sofia (most of them are registered in the oldest RRIDs WSSB type). The recovery time, however, is relatively higher than that of RCIDs--need to replace some relay groups.  
The large number of failures and the correspondingly long recovery time of the RCIDs (58% of failures and 51.93% of their duration) represent only 15.17% of the failures of all signalling devices for the three-year period and 11.99% of their duration.