Safety Case Development

A safety case is the argument, supported by evidence, that an item achieves functional safety. ISO 26262-2 requires it to be developed progressively: it compiles the work products generated across the safety lifecycle and links them through an explicit argument to the claim that the safety goals are satisfied. A safety case therefore has three ingredients - claims about safety, arguments that explain the reasoning, and evidence drawn from work products such as the HARA, the safety concepts, safety analyses, and test results. It is a required work product for items developed to ISO 26262 and a central input to the functional safety assessment.

GSN is a graphical notation for safety arguments, popularized by Tim Kelly's work on safety case management. It expresses an argument as a tree of goals (claims), strategies (the reasoning that decomposes a goal), and solutions (the evidence that discharges a leaf goal), annotated with context, assumptions, and justifications. GSN makes the reasoning between a top claim like "the item is acceptably safe" and individual work products explicit and reviewable, and its modular extensions let large programs split the case into maintainable modules.

Evidence follows the lifecycle. From the concept phase: item definition, HARA, safety goals, and the functional safety concept. From system development: the technical safety concept and architecture. From hardware and software development: architectural metrics, safety analyses, and design and unit verification results. Then integration and test results, vehicle-level safety validation, production and operation evidence, and the management and supporting-process records - safety plan, confirmation measures, configuration management - that show the case was produced under a controlled process.

At the start of the concept phase, not at the end of development. ISO 26262 describes the safety case as a progressive compilation: the argument skeleton can exist as soon as safety goals do, and each phase then adds its evidence to already-defined claims. Starting early exposes evidence gaps while they are still cheap to fix, keeps the argument aligned with design changes through change impact analysis, and fits naturally with incremental development approaches where the case grows sprint by sprint.

The guide has 12 chapters: foundations and the six-step method, a full GSN chapter, seven evidence chapters spanning concept through production and supporting processes, management of functional safety, pitfalls and assessment tips, and a use-cases chapter with eight program archetypes. It maps 19 ISO 26262 work products to the claims they support and anchors the concept phase in a worked steer-by-wire safety goal set. A free account starts you off, and the Pro and Expert plans unlock more of the concept library.