Over the years, we’ve published a few blogs on dependent failures and the techniques used to quantify the dependent failures analysis, such as „Quantifying ISO 26262 Dependent Failures Analysis“ from 2020, the area not as thoroughly supported by ISO 26262, though other functional safety standards, like ISO 13849-1 and IEC 61508, offer more prescriptive guidance on the topic.
At its core, dependent failures analysis is basically analysis of common elements that could fail and cause two or more components to fail for the same reason, ultimately leading to a violation of a safety requirement(s). Over the years we have seen many examples e.g. systematic failures in microcontrollers from the same family, which could have resulted in catastrophic results if not caught in the development phase.
Dependent failure initiators
There are many sources that list the potential initiation of dependent failures. ISO 26262 handles the topic well but focusses more on the technical aspects. Other standards such as ISO 13849-1 and IEC 61508, additionally bring in more human factors including competence, training, and experience.
For technical considerations when analysing dependent failures, the key elements are:
- similar and dissimilar redundant elements
- different functions implemented with identical software or hardware elements
- functions and their respective safety mechanisms
- partitions of functions or software elements
- physical distance between hardware elements, with or without a barrier
- common external resources
Examples are wide and varied, including common software tools or libraries, power supplies, or clock signals, to name just a few.
Failures can also be subdivided into two basic groups: common cause failures and cascading failures.
Cascading failures involve interference between elements or components, while common cause failures focus on similar or identical elements or components.
Alastair Walker, Owner & Consultant
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