Building Fault Trees for BESS Failures, Fires, and Explosions

This paper presents a practical method for constructing and quantifying Fault Tree Analyses (FTA) for Battery Energy Storage System (BESS) failures, fires, and explosions. It defines precise top events—thermal runaway propagation, container fire, and container deflagration—along with boundaries, operating phases, and a reusable basic-event taxonomy. Standard gate patterns (OR/AND/k-of-n, PAND, inhibit) are applied, and protection layers (BMS trips, isolation, HVAC/venting, suppression) are modeled with PFDavg and common-cause factors. A worked example estimates order-of-magnitude frequency for container deflagration and demonstrates minimal cut sets and importance measures to prioritize engineering controls. Emphasis is placed on ventilation reliability, ignition control, isolation integrity, and commissioning/maintenance states. Guidance is provided for data collection, QA, and integration with event trees, LOPA, and consequence models to support design decisions, O&M proof testing, and insurance evaluations.