Functional hazard analysis Preliminary System Safety Assessment

Preliminary system safety assessment Functional hazard analysis, FTA. 

Model Based Safety Assessment aims at supporting the Preliminary System Safety Assessment (PSSA) [8]. Before the PSSA is performed, the Functional Hazard Analysis identifies the Failure Conditions (e.g. safety critical situations of the system) and assesses their severity on a scale going from No Safety Effect (NSE) to Catastrophic (CAT). Then, during the Preliminary System Safety Assessment, safety models (or alternatively fault-trees) axe built and analysed. A safety model describes formally in which node a fault occurs and how this fault propagates inside the system architecture in order to cause a Failure Condition. 

The DAL is an index number ranking the safety-criticality of the system functions. This ranking implies that in order to make the system safe, greater development rigor must be applied to each successively critical level. Table 2.3 correlates the hardware DALs to the five classes of failure conditions and provides definitions of hardware failure conditions and their respective DALs. Initially, the hardware DAL for each hardware function is determined by the SSA process using a functional hazard analysis (FHA) to identify potential hazards and then the preliminary system safety assessment (PSSA) process allocates the safety requirements and associated failure conditions to the function implemented in the hardware. 

The preliminary hazard list and the customer requirements form the basis for the Functional Hazard Assessment (FHA). While considering the required functionality and the respective system enviromnent the FHA shall answer the question How safe does the system need to be The analysis performed during the FHA provides the safety objectives and a first set of system safety requirements which are necessary in order to fulfil the safety goals and to prevent the identified hazards from occurring. The system requirements are amended by the system safety requirements. Doing this in the early project phase assures that the safety requirements are appropriately considered in the system design. 

---