A safety instrumented system for rolling stocks: Methodology, design process and safety analysis

Abstract

Modern equipment for rail transportation has to be compliant with the reliability, availability, maintainability and safety (RAMS) requirements of both national regulations and international standards such as EN 50126-1:1999 and EN 50126-2:2007. Two critical hazards for passengers and personnel of a rolling stock may arise from accidental external doors opening and from unmanned train travelling due to the sudden incapacitation of the driver. In order to reduce the risk of such hazards to tolerable or, preferably, to negligible levels, ad hoc smart monitoring systems, typically referred to as dead-man’s vigilance devices (DMVDs), are generally installed on trains. In this paper, the design process of a novel DMVD is thoroughly described with a special emphasis on safety issues. This process can be of interest for designers, engineers and practitioners developing safety and diagnostic systems for railway applications. The proposed DMVD is not only modular, flexible and able to meet the wanted safety specifications, but it is also characterized by lower development costs than other solutions available on the market, as it does not include micro-controllers (MCUs) or other programmable devices running software routines. In particular, if just hardware components and Register Transfer Level (RTL) modules synthesized in Field Programmable Gate Arrays (FPGAs) are used, the correct operation of both safety and diagnostic functions can be verified through techniques normally used for hardware-only systems. In this way, the long and expensive validation and verification strategies described in specific standards for software-based safety systems (e.g. EN 50128:2011) are no longer strictly required.