Tolerable Hazard Rate

The Norwegian National Rail Administration, Jernbaneverket, is responsible for the management of the national railway network, on behalf of the Ministry of Transport and Communication. In this context, Jernbaneverket has a need for setting distinct and verifiable safety requirements to railway command and signalling systems. Currently, safety requirements are imposed on equipment used to realise command and signalling systems (CSS). An alternative to this is to impose requirements on safety critical functions of these systems.

SINTEF presents in the report a proposal to a tolerable rate of faults for a defined set of safety critical functions. The proposal is based on risk analyses of accident scenarios related to the command and signalling systems. These types of accidents are included as part of the Jernbaneverket’s paramount, national safety goal. The scenarios presented are used to estimate the probability for, and consequence of, errors in the safety critical functions caused by errors in the command and signalling systems. This gives a model for the command and signalling system’s influence on the total risk from railway activity, expressed as PLL (Potential Loss of Life, expected number of casualties per year).

In addition to accident modelling scenarios in the form of risk analyses, a model showing how to derive requirements as tolerable hazard rates from the railway’s paramount, national safety requirements is presented.