The Preliminary Hazard Analysis (PHA) consists in identifying, at the system level, which redundant events can lead to dangerous situations for people. These events are then classified by assigning a safety level.
This analysis is performed using a set of known hazards (or identified in an analysis of operational risks, for example) in different life situations of the system. The next step is to identify the causes (technical or operational) at the system and subsystem level that would lead to such dangerous events
Once these causes have been identified, requirements are provisioned to ensure that the system can respond effectively. These requirements and their treatment are followed throughout the study cycle system in the Hazard Log (HL).
The preliminary Hazard Analysis is an iterative process that should be initiated early in the development process because it will guide the choices in terms of design, operation, installation by the safety level required. In fact, stting up a PHA consists in drawing a table by matching a danger with one or more dreaded event(s) and potential cause(s) at the system and subsystem level. The last column will list the requirements in respect of each of the potential causes identified.
The last step is to assign a safety level or SIL to apply to each of the dangerous events identified