Calculating Probability Metric for Random Hardware Failures (PMHF) in the New Version of ISO 26262 Functional Safety - Methodology and Case Studies

The Automotive Functional Safety standard ISO 26262 introduced a PMHF (Probabilistic Metric for random Hardware Failures) in Part 5 and Part 10 by calculating the system failure rates and assessing the ASIL (Automotive Safety and Integrity Level) for functional safety. The new version of the standard expands the PMHF concept by further promoting a new metric “average probability of failure per hour over the operational lifetime of the item”, which has not been commonly used by the reliability engineering community. In order to clarify how PMHF is calculated within the content of ISO 26262, this paper will discuss how to calculate both the failure rate and the average probability of failure per hour in terms of definitions, sources of the data, applications, and advantages and disadvantages. It will also present examples of calculating PMHF including the average probability of failure per hour for a non-exponentially distributed failure population as well as an example of a system with redundancy.