What is Safe Failure Fraction (SFF) in IEC61508?

In the field of functional safety, the concept of Safe Failure Fraction (SFF) plays a crucial role. SFF is defined as the ratio of safe failures to the total number of failures in a system. It provides insights into the reliability and safety performance of safety-related systems according to the international standard IEC 61508.

The Importance of Safe Failure Fraction

SFF quantifies the proportion of failures that do not lead to loss of the required safety function in a system. It is a fundamental measure used to assess the level of risk reduction achieved by safety measures implemented in accordance with IEC 61508. A higher SFF indicates a higher degree of safety integrity in the system and vice versa.

Calculation of Safe Failure Fraction

The calculation of SFF involves evaluating the failure rates of different components or subsystems within a safety-related system. Each component's failure rate must be considered along with its ability to prevent dangerous failures. These failure rates are then used to determine the SFF for the entire system.

Enhancing Safe Failure Fraction in Practice

To increase SFF, various techniques can be employed during system design and implementation. Redundancy, diversity, and diagnostic coverage are widely used approaches to minimize the probability of dangerous failures and improve the overall safety integrity of the system. They contribute significantly to achieving higher SFF values.

In conclusion, Safe Failure Fraction (SFF) is an essential metric used to evaluate the reliability and safety performance of safety-related systems as per the IEC 61508 standard. Understanding and calculating SFF allows engineers and practitioners to enhance system safety, minimize risks, and ensure the proper functioning of safety functions in critical applications.