Reliability is a critical aspect of ensuring the safety and functionality of complex systems in various industries, including automotive and industrial control. Two prominent standards that address hardware reliability in these domains are ISO 26262 and IEC 61508. While both standards aim to enhance safety and dependability, they differ in their approach to calculating hardware reliability. In this article, we will explore the key differences in hardware reliability calculation between ISO 26262 and IEC 61508.

Industry and Application Focus

ISO 26262 primarily targets the automotive industry, focusing on the functional safety of road vehicles. In contrast, IEC 61508 is a more general standard applicable to a wide range of industries, including process control, power generation, and manufacturing. Consequently, the scope and context of the two standards differ significantly, influencing how they approach hardware reliability calculations.

Safety Goals and Hazard Analysis

Both standards emphasize the importance of hazard analysis to determine safety goals and requirements. ISO 26262 utilizes an Automotive Safety Integrity Level (ASIL) framework, which assigns levels (A to D) based on the severity of potential hazards. These ASIL levels then drive hardware reliability requirements. In contrast, IEC 61508 uses a similar approach but with Safety Integrity Levels (SIL) ranging from SIL 1 to SIL 4. SIL levels are determined based on a broader range of industries and hazards.

Failure Rates and Probability of Failure on Demand (PFD)

One of the fundamental differences between the two standards lies in how they define and calculate hardware reliability metrics. ISO 26262 primarily relies on the concept of “Probabilistic Metric for Hardware” (PMHF) or “Evaluation of each Cause” (EEC) to deal with the overall hardware failure rate, combined with the “Single Point Fault Metric” (SPFM) and “Latent Fault Metric” (LFM) to deal with the question whether the hardware design leads to more safe or more unsafe failures and how likely latent faults will be detected. ISO 26262 also introduces the concept of “ASIL decomposition,” which allows for the distribution of hardware reliability requirements among system components.

IEC 61508, on the other hand, uses the Probability of Failure on Demand (PFD) and the Average Frequency of Dangerous Failure (PFH) of  to assess the reliability of a the analyzed system. The PFD represents the probability that a system will fail to perform its safety function when required. SIL levels are associated with specific PFD/PFH values the quality of the hardware design is, other than in ISO 26262, measured with the “Safe Fault Fraction” (SFF) and the “Hardware Fault Tolerance” (HFT), which are specified for each safety level.

Lifecycle Considerations

Both standards consider the entire lifecycle of a system, including design, development, production, operation, and maintenance. However, ISO 26262 places more emphasis on the specific phases of the automotive development process, such as concept, product development, and production. In contrast, IEC 61508 provides a more generic framework adaptable to various industries and lifecycles.


ISO 26262 and IEC 61508 are vital standards that play a crucial role in ensuring the safety and reliability of hardware in their respective industries. While they share common principles, such as hazard analysis and safety goals, they differ significantly in their hardware reliability calculations due to their distinct industry focus, safety goal frameworks, and fault tolerance requirements. Professionals working in automotive or industrial control systems should be aware of these differences to ensure compliance with the appropriate standard and, more importantly, to enhance safety and reliability in their systems.