4. The organization of barriers in a defense-in-depth strategy

The latter is characterized by :

• prevention of system malfunctions :

  • in design: it is partly taken into account in Table B, in the search for design barriers and in the application of FMEA, in Module B,

  • by quality: taken into account in the qualification of barriers with table C (including additional barriers from table D),

  • by defining the authorized operating range. For example, based on major accident scenarios: setting the filling level of the propane sphere with warning and alarm margins. This leads to the definition of excluded failures: propane overflow both in the sphere and in the wagon. Untimely disconnection of the wagon. The Safety Critical Elements (SCE) then appear, which can also be Technology Barriers (TB) or Use Barriers (BU)....