Evaluation of the Useful and Residual Life of a Class of Relay-Based Centralised Traffic Control
Authors: Dimitrov R. 
Published: July 2022

Currently in Bulgaria, the main railway lines (corridors) are being modernised to increase train speed up to 160/200 km/h, which necessitates adoption of modern European traffic control systems (ERTMS/ETCS, classified as so-called class A systems). The remaining sections continue to operate mainly relay-based systems for ensuring safety of train traffic (the so-called class B systems). These systems include, first, station centralised traffic control systems, most of which are currently route relay-based ones. On the railways of Bulgaria, two classes of route relay-based centralised traffic control systems (RRCTCS) are operated, using respectively, the relays of so called «first class of reliability», and the relays of the so called «not first class of reliability». The inevitable increase in the age of RRCTCSs makes it necessary to assess their technical condition and technical suitability to clarify strategies for their further operation. Such an assessment is difficult due to the lack, first, of statistical data on the failures and reliability parameters during operation as per types of systems and their constituent elements and, second, of manufacturers’ recommendations on the service life of RRCTCS. The service life and residual service life, as well as the useful and residual life of RRCTCSs could, in the author’s opinion, be assessed according to the following criteria: • Electrical and mechanical useful life/wear of the components. • Reliability parameters of the relay components. • Electrical and mechanical useful life or the condition of the external and internal cabling. • Operating costs required to maintain RRCTCS within the range of predefined technical parameters. The article analyses and evaluates the average useful life and the average residual life of RRCTCS based on the first two criteria. It is concluded that neither electrical nor mechanical wear resistance of components is among leading characteristics for assessing useful and residual life. Two approaches are used to consider some of reliability parameters of the relay components: deterministic and probabilistic one, which are based on the «intensity of dangerous failures» characteristic of the relay of the first reliability class. Based on that characteristic, the probability of a dangerous failure of the first-class relays in operation was calculated for 58 Bulgarian RRCTCSs of N-68 type with non-routed manoeuvres, as well as the probability of their safe operation; the latter probability is proposed to be used to calculate the average residual life of the relay, and based on that, to calculate the average residual life of RRCTCSs of that type. By early 2021, the estimated maximum of the latter is 11 years. The consequences of implementing two strategies in subsequent years are considered: a) «nothing is done» and b) «RRCTCS useful life/age management» (based on the program of gradual decommissioning of N-68 type RRCTCSs and of re-equipment of relevant stations). This is also a prerequisite for safety management of train traffic. It is argued that an average useful life of RRCTCS could be estimated as no more than 60 years in operation. Considering additional factors, this period should be understood as a predicted value of the expected time after which the corresponding RRCTCS will reach the limit state and will be decommissioned. The average useful life and the average residual life should be considered the best guideline for the expected age limit for this type of RRCTCS.