Principles of Risk Assessment and Acceptance in RAMS studies


Some of the risk tolerance principles or criteria that should be taken into account when designing and identifying safety or SIL levels of a railway system are little known. These criteria are identified in CENELEC EN 50126, EN 50128 and EN 50129 standards, and they must be considered when developing a new equipment, system or installation or, when a change or a modification of the previous ones will be carried out:

ALARP - As Low as Reasonably Practicable

Every rail transport system must demonstrate that its level of risk has been mitigated as much as possible.

That is, the system must be as safe as reasonably possible, due to the state of the art or for a cost objective.

GAMAB - Globalement Au Moins Aussi Bon

Every rail transport system, as this evolves, will be at least as safe as an existing system.

Therefore, in other words, the evolution of the state of the art can never lead to worsen safety. This criterion is mandatory in France and its positive point is the obligation to study knowledge and past experiences for the improvement and evolution of new equipment and facilities.

MEM - Minimum Endogenous Mortality

This criterion is based on the fact that there are different mortality rates in society, depending on age and sex. These deaths are caused in part by technical systems.

In this manner, MEM criterion compares risks due to a new system with existing risks caused by natural mortality. MEM requires that the new system will not contribute significantly to the existing mortality caused by technical problems.

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