Application of FTA for Design of Safety Critical Systems

SUMMARY & CONCLUSIONS This paper provides a case study on application of the Fault Tree Analysis (FTA) methodology for design of Safety Critical Systems such as Power Control and Energy Management Systems. These systems are widely used in many industrial and commercial applications and failure of these devices can potentially lead to Safety or Regulatory issues. In many industries, FTA is used reactively to quantify the probability of the Top Unwanted Event once there is a safety incident in the field or to meet a compliance standard such as ISO26262, it is seldom used as a proactive tool to drive design improvements. In this paper, we have delineated a process that includes identification of safety risks and development on an FTA from a P-diagram (Parameter-Diagram) driven Design, Failure Modes & Effects Analysis (DFMEA). Next FTA is analyzed Qualitatively to derive Minimum Cut Sets (MCS) and appropriate Failure Tolerance/Fail Safe features are designed in to mitigate the risks due to Single Points of Failure (SPOF) and combination of events that are of concern. Lastly, the impact of the designed in fail safe features was quantified to show improved safety level as compared to original design. We have applied the aforementioned methodology on the design of Industrial Circuit Breaker, where risk of Arc Flash Hazard, which can potentially lead to human injury, was mitigated by systematically analyzing the design by diving deep into MCS generated by Qualitative FTA and incorporating Firmware fail-safe features (Application Specific Integrated Circuit (ASIC) control) that trips the circuit breaker in the event of a short-circuit condition, thereby preventing the Arc Flash. This risk mitigation was quantified using Quantitative FTA, where the probability of occurrence of Arc Flash hazard was reduced to a negligible level.