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IIT Kharagpur
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A A BS B ST TR RA A CT C T
The thesis is concerned with the study and reliability analysis of signalling systems of Indian Railways (IR). The research work, reported in this thesis, has been carried out to address a few operation- and technology-related critical issues of railway signalling systems. In particular, the research work, with reference to railway signalling systems, has been specifically directed (i) to model its interacting variables, parameters, and physical components in different scenarios and conditions, (ii) to analyze the causes of failures of different components and their effects on signalling performance, and (iii) to find out an appropriate methodology for its reliability analysis.
The working principles of railway signalling systems are very complex in nature. A signalling system is represented in terms of several types of working components and variables, such as panel, cable, relay, ‘point-and-point machine’, ‘track circuit’, back indication, ‘signal-unit’, point-drive and signal-drive cards, power supply, and level crossing gate, in all there possible combinations. There exists 31 such combinations in IR signalling systems. In order to measure and evaluate reliability under different situations, it has to be generically represented in terms of types of interlocking, number of tracks, types of approaching and departure signals, and signalling routes in all possible combinations.
Failures of signalling systems may occur due to many types of causes. As enumerated from the cause-and-effect (CE) diagram, several types of failures, such as hardware failures, software errors, operator mistakes, and poorly-designed ‘human-machine’
interfaces, may occur in signalling systems. In order to identify the critical subsystems and components, responsible for signalling failures, failure mode and effect analysis (FMEA) is carried out to estimate the risk of the subsystems during its operating phase in terms of Risk Priority Number (RPN), measured as a product of occurrence, severity, and detection. It is observed that ‘point-and-point machine’, ‘signal-unit’, and ‘track circuit’ are the critical subsystems. Fault tree analysis (FTA) is carried out on the signalling systems to define and enumerate specifically the causes of the
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IIT Kharagpur
vi Abstract
system failure modes or ‘top events’ in terms of the subsystem or component failures and human errors, represented by ‘basic events’. Fault tree quantification also enables the probability of top events to be calculated in addition to cost components (cost of both repair and replacement) of each basic event. On the basis of the frequency of occurrence and cost implication, 8 out of the 39 components are considered as critical.
The findings of this study may be useful to the designers and operating personnel of IR.
Operations of three vital and critical subsystems, namely ‘point-and-point machine’
‘signal-unit’, and ‘track circuit’, are affected by a number of problems in relation to their intensity of use, repair and maintenance, and environmental stress. In this thesis, the reliability modelling of ‘point-and-point machine’, ‘signal-unit’, and ‘track circuit’, considered as repairable systems, is elaborately described taking into account the effect of the problems on their operational efficiency as well as the performance of signalling systems. It is found that, even by derating the system specifications (i.e., lowering the speed of the train), the reliability of a ‘point-and-point machine’, ‘signal- unit’, and ‘track circuit’ has been poor (0.44 after 1100 days of continuous operation, 0.50 after 250 days, and 0.50 after 300 days of operation, respectively) with existing preventive and corrective measures. Two graphical methods, namely, Nelson-Aalen and total-time-on-test (TTT) plots, are used to analyze the trends of the failure time data for a set of representative ‘point-and-point machines’ ‘signal-units’, and ‘track circuit’s at the Kharagpur Division of South Eastern Zone of IR. The parametric model, as recommended for estimating the reliability of subsystems, is capable of correctly predicting its failure rate probability. The analysis of data obtained from homogeneous Poisson process (HPP) for ‘point-and-point machine’, and nonhomogeneous Poisson process (NHPP) for both ‘signal-unit’ (power law model), and ‘track circuit’ (log-linear model) results in an improved and better assessment of reliability pattern with which the problems as mentioned may be adequately and properly addressed. The reliability estimation methodology, considering a number of superposed processes, of railway signalling systems is a new approach. In this regard, the major effort has been directed to specifically develop a generic reliability evaluation and measurement framework for a signalling system and its components and subsystems.
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IIT Kharagpur
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Abstract
Keywords: Railway Signalling Systems, Interlocking, ‘Point-And-Point Machine’,
‘Signal-Unit’, ‘Track Circuit’, Repairable System, ‘Human-Machine’ Interface, Failure Analysis, Failure Mode and Effect Analysis, Fault Tree Analysis, Reliability Analysis
