What is the safe failure fraction (IEC 61508)?

The safe failure fraction (SFF) is an important concept in the field of functional safety, specifically defined by the International Electrotechnical Commission (IEC) standard 61508. This article aims to provide a thorough understanding of what SFF is all about.

SFF Calculation

In IEC 61508, the safe failure fraction is calculated as the ratio of the number of safe failures to the sum of safe failures and dangerous failures in a given system. It represents the number of failures that do not lead to a hazardous condition, compared to the total number of failures.

To calculate the SFF, the first step is to identify all potential failure modes within the system. This can be done through hazard analysis, fault tree analysis, or other risk assessment techniques. Each identified failure mode is then classified as either a safe failure or a dangerous failure based on its impact on overall system safety.

Once the safe and dangerous failure modes are determined, the next step is to quantify the occurrence of these failure modes. This is typically done using statistical data, expert judgment, or historical records. By assigning probabilities or rates to each failure mode, it becomes possible to calculate the SFF for the system.

Importance of SFF

The safe failure fraction plays a crucial role in determining the overall reliability and safety integrity level (SIL) of a system, especially in industries where even minor failures can have catastrophic consequences. It helps engineers and designers assess the effectiveness of safety measures implemented in the system.

A high SFF indicates that the system has a lower probability of dangerous failures, making it more reliable and safer. On the other hand, a low SFF suggests a higher likelihood of dangerous failures, requiring additional safety precautions or design modifications to improve system performance.

Application of SFF

The concept of safe failure fraction is commonly applied in various industries, including but not limited to automotive, aerospace, energy, and healthcare. It is particularly relevant in the development of safety-critical systems such as airbag deployment systems, nuclear power plants, and medical devices.

By using the SFF as a quantitative measure of safety, engineers can compare different design options, select appropriate components, and assess the overall reliability of a system. This helps them ensure that the systems they develop adhere to the necessary safety standards and regulations, reducing the risk of accidents, injuries, and financial losses.