ETCS Safety Requirements for Technical Interoperability (Subset-091)

Scope

Specifies the quantitative and qualitative safety requirements for the technical interoperability of ETCS in Levels 1 and 2, including the tolerable hazard rates apportioned between on-board and trackside subsystems. Mandatory specification under the CCS TSI (Appendix A, index 027), used by suppliers, safety assessors and notified bodies when building and evaluating ETCS safety cases.

SUBSET-091 specifies the quantitative and qualitative safety requirements for the technical interoperability of ETCS operating at Levels 1 and 2, defining the tolerable hazard rates that must be apportioned between the onboard and trackside portions of the ETCS system so that the overall system-level safety target is met regardless of how individual suppliers implement their respective parts. Because ETCS onboard and trackside equipment from different suppliers must interoperate safely without either supplier having full visibility of the other's internal design, this specification's apportioned safety targets give each supplier a concrete, independently verifiable safety budget to design and demonstrate against. It is a mandatory specification under the CCS TSI, listed as Appendix A index 027, and is used by suppliers, safety assessors and notified bodies when building and evaluating the safety case for an ETCS onboard or trackside subsystem. Its quantitative hazard-rate targets are typically referenced directly within a structured signalling safety case, connecting the ETCS-specific safety apportionment to the general CENELEC signalling safety framework. The current edition is version 4.0.0, dated 5 July 2023, developed by UNISIG and ERA.