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Safety Aspects

Requirements for wireless systems in the railway domain can deeply depend on the criticality of the communication and also on the safety aspects of the system, that are defined in this part.


Systems whose failure can lead to damage to property, damage to the environment or loss of human life are considered as safety-critical systems [16]. Safety manages design of the system but also the operational environment in which the system is used. Indeed, safety of a system can be dramatically modified by changes in its operational environment. Programmable electronics, often controlled remotely via communication networks, progressively replace the mechanical and mechatronic devices to ensure safety of the devices. Safety-critical systems can no more be based only on the control of errors and failures, they must manage the security of the data used in their operation.

A range of standards relating to the use of electronics components and software in safety-critical systems are defined. These standards aim to design, procure and deploy safety-critical systems that provide some assurance on the safety features of the systems. The IEC 61508 presents a generic approach of all activities dedicated to functional safety of Electrical/Electronic/ProgrammableElectronic (E/E/PE) devices. The standard proposes a global approach of safety, that can be compared to the ISO 9000 system in the quality domain [17]. The IEC 61508 promotes a design methodology framework that aims to prevent the presence of dangerous failures, or control them when they arise by providing guidance on each phase of the safety life-cycle.

The safety requirements of a Safety-Related System (SRS) must be specified in terms of the functions to be performed by the SRS and the integrity required of each. Each safety-related function is then specified by a Safety Integrity Level (SIL). The SIL is defined by the necessary action to reduce the risk of a function, from an uncontrolled risk to a tolerable risk. The SIL can then quantify the level of security of a system. The level 1 (SIL 1) corresponds to the lower safety integrity level and the level 4 (SIL 4) represents the higher safety integrity level:

  • • SIL 4: catastrophic impact (higher level);
  • • SIL 3: impact on the community;
  • • SIL 2: major protection of installation and production or injury risk on persons;
  • • SIL 1: minor protection of installation and production (lower level).
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