# Methodology for Studying the Reliability of Interlocking Devices in Bulgarian Railways
## 3 Results
### Statistical Assessment of Failures of the Interlocking Devices and Signallinig Equipment
#### 3.1.3 Statistical Assessment of Failures in STD--Gorna Oryahovitsa  
Analysis of failures and their duration was carried out also for the region of STD--Gorna Oryahovitsa. Using the Equations (1)-(4), the total number of failures of all types of signalling devices and their duration, as well as the corresponding percentage ratios, were calculated, and the results obtained are shown graphically in Figure 13a-d and in Figure 14a-d (respectively, for years 2020, 2021, 2022 and for the entire period).  
The significantly predominant number of ESDs failures and their duration are evident. These are mainly failures of rail current circuits in the station areas (30.12% of all failures and 36.34% of all recovery time)--the state of the isolated joints depends a lot on weather conditions [51]. The bulbs of traffic lights also burned out very often (17.41% of all failures and 12.91% of all recovery time). It should be noted that the signalling equipment in this area is relatively old; many of the installations are depreciated and outdated. A large part of them are more than 40 years old. That is why the failures are significantly more than those for STD--Plovdiv.  
Modernization procedures have rarely been carried out. There are programs for the phased replacement of signalling equipment at stations, but they need to be implemented more quickly.  
There are also a relatively large number of failures of the interlocking devices--more than those for STD--Sofia and their share is approaching that of IsDs, and in 2021 it even surpassed it.  
It should be noted that at the time of the study there are no route computer interlocking devices in this area--the first RCID was put into operation at Sindel station in 2023.  
Figure 11: Proportion of failures of different types of IIDs for STD—Plovdiv.  
Figure 12: Proportion of duration of failures of different types of IIDs for STD—Plovdiv.  
The data on failures of different types of IIDs are presented graphically in Figure 15 and the data on their duration--in Figure 16. The obtained results after processing for the proportions are displayed graphically, respectively, in Figures 17 and 18--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). There is a visible increase in the total number of failures, while their duration is decreasing.  
The highest number of failures and, accordingly, the longest duration are observed at RRIDs--63.81% and 58.98%, respectively. These are 10.14% of failures of all signalling devices and 8.13% of the total recovery time. Due to the long-term operation of the relays and relay blocks, their contact systems have been corroded and burned. During the research period, there was still available spare relay equipment of various types, and in case of damage it has been replaced regularly.  
An increase in the proportion of failures of EMIDs over the last year can be observed, which proves the need for their gradual replacement with RCIDs.  
The largest number of failures and, accordingly, the longest recovery time are observed with RRID--72.1% of failures and 79.2% of duration, but these are only 6.06% and 6.17% of failures and duration of all signalling devices for the three-year period.