Analyzing the Physical Process

Analysis starts with the physical process, identifying the physical and operational controls and any potential physical failures, dysfunctional interactions and communication, or unhandled external disturbances that contributed to the events. Ibe goal is to determine why the physical controls in place were ineffective in preventing the hazard. Most accident analyses do a good job of identifying the physical contributors to the events. 

The most hkely reason for water getting into tanks 701 and 702 were inadequate controls provided to keep water out during a recent rainstorm (an unhandled external disturbance to the system in figure 4.8), but there is no way to determine that for sure. 

Accident investigations, when the events and physical causes are not obvious, often make use of a hazard analysis technique, such as fault trees, to create scenarios to consider. STPA can be used for this purpose. Using control diagrams of the physical system, scenarios can be generated that could lead to the lack of enforcement 

Prevent inadvertent release of toxic chemicals or explosion Convert released chemicals into a nonhazardous of less hazardous form Provide indicators (alarms) of the existence of hazardous conditions Provide protection against human or environmental exposure after a release Provide emergency equipment to treat exposed individuals 

Emergency and Safety Equipment (Controls) Partial list 

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