The hazard analysis is a key document for safety critical systems (see Term Definition: Safe) as it combines the
description of the hazards, the level of risk, system faults that can lead to the hazard, and the control measures all
together in a single view. The Fault is a non-compliance of the system that can lead to the identified hazard. The
severity is a measure of how bad the hazard is (an arbitrary scale of 1-10 is used often used). The Risk is a value
computed by the likelihood multiplied by the severity of the occurrence.
All safety critical systems are real-time systems as well, since each fault has a period of time that it can be
tolerated (called its Fault Tolerance Time). To be considered safe against a specific fault, the sum of the fault
detection time and the control time (the time required for the control measure to handle the fault) must be less than
or equal to the fault tolerance time.
The hazard analysis is usually coupled with one or more Fault Tree Analysis (FTA) to show how faults and other
conditions can combine to lead to hazards.
The hazard analysis is created during the requirements analysis phase but not all aspects (such as the control
measures) may be identified early on. The hazard analysis should be maintained throughout the development cycle - as
new hazards or faults are identified, they must be added to the table. As control measures are specified, the analysis
must be updated as well.
|