Application of Fault Tree Analysis for Signaling Systems

Abstract

Fault Tree Analysis (FTA) is one of the key safety evaluation techniques used by New York City Transit (NYCT). First developed over 50 years ago, this technique continues to provide valuable insight for failure analysis of systems. Its use is widespread in safety-critical systems analysis across industry boundaries, including defense, nuclear, aerospace, chemical [1], and transportation industries. FTAs provide a systematic, top-down methodology to safety analysis. As such, it complements other safety analysis techniques, such as Failure Modes Effect Analysis (FMEA), which is a bottom-up failure analysis [2]. Formal Methods analyses, including Theorem Proving and Model Checking, are powerful development and analysis methodologies, both used by NYCT, that provide assurance of product’s correctness and safety. With these other safety analysis techniques, the FTA continues to play a key role in the NYCT Safety Program. This paper will examine how NYCT uses FTAs for the safety analysis of microprocessor-based signaling systems. FTAs are used by NYCT throughout the system lifecycle. Initially, during the system development phase, NYCT requires system suppliers to develop Fault Tree Analyses of their systems, as a requirement for NYCT safety certification and deployment. For the system maintenance phase, NYCT uses the outputs of suppliers’ analyses to develop and enforce maintenance and operational procedures. In this manner, NYCT’s use of FTA provides full lifecycle value by providing design, maintenance, and operational insight into the causes of hazardous events. Through the examination of example fault trees and an overview of the FTA process, this paper will present the NYCT’s implementation of this powerful analysis tool, and will describe the benefits gained from using this methodology.