What is safe failure fraction (IEC 61508)

Safe failure fraction is a term defined in the IEC 61508 standard, which is an internationally recognized functional safety standard for electrical, electronic and programmable electronic safety-related systems. This article aims to explain the concept of safe failure fraction and its importance in ensuring the safety and reliability of these systems.

The Definition of Safe Failure Fraction

The safe failure fraction refers to the proportion of failures that do not lead to a dangerous or hazardous condition in a safety-related system. In other words, it represents the fraction of failures that can be detected and controlled by the system itself, without causing harm to people or the environment.

According to IEC 61508, the safe failure fraction is a measure of the system's ability to maintain a safe state during its operation, even in the presence of failures. It is calculated as the ratio between the number of safe failures and the total number of failures that occur within a specified time period.

The Importance of Safe Failure Fraction

The concept of safe failure fraction plays a critical role in ensuring the overall safety performance of safety-related systems. By specifying a target safe failure fraction during the design and development phase, engineers can evaluate and improve the system's ability to handle failures in a safe manner.

A high safe failure fraction indicates that the system has a greater capability to detect and mitigate failures before they escalate into hazardous conditions. This helps to reduce the likelihood of accidents and minimize the potential consequences of failures. On the contrary, a low safe failure fraction implies that the system may have vulnerabilities that could lead to significant safety risks if not addressed properly.

Factors Affecting Safe Failure Fraction

Several factors can influence the safe failure fraction of a safety-related system. These include the design of the system, its architecture, the quality and reliability of its components, as well as the effectiveness of the implemented diagnostic and control mechanisms.

The choice of appropriate redundancy techniques, such as duplication or diversity, can significantly improve the safe failure fraction. Redundancy allows for the detection and compensation of failures, increasing the overall reliability and safety level of the system.

Furthermore, the maintenance and testing strategies also play a vital role in maintaining a high safe failure fraction throughout the lifecycle of the system. Regular inspections, functional tests, and preventive maintenance activities help to identify and rectify potential failures before they occur.

In conclusion, understanding and optimizing the safe failure fraction is crucial for achieving the desired level of safety in electrical, electronic and programmable electronic safety-related systems. By aiming for a higher safe failure fraction, engineers can enhance the system's resilience against failures and provide a safer operating environment.