Fault Tree Analysis for the BLEDP Student meeting Vegard Joa Moseng

About the Fault Tree Analysis Fault tree analysis (FTA) is a top-down approach to failure analysis, starting with a potentially undesirable event called a TOP-event, and then you determine all the ways it can happen. The analysis proceeds by determining how the TOP event can be caused by individual or combined lower level failures or events. Causes are connected through logic gates, typically AND & OR gates. FTA is the most commonly used technique for a casual analysis in risk and reliability studies.

Main steps in a FTA Define the TOP event (potential accident) and boundary conditions. Construction of the fault tree. Identifications of the minimal cut sets. Qualitative analysis of the fault tree. Quantitative analysis of the fault tree. Report results.

Preparation for FTA Starting point is often an existing Failure Mode, Effects and Criticality Analysis (FMECA) and a system block diagram. The FMECA is an essential first step in understanding the system. The design, operation and environment of the system must be evaluated. Cause and effects of the events leading up to the TOP event must be identified and understood.

Components in FTA building

Boundary conditions Normal boundary conditions are: 1.The physical boundaries of the system (what is included?) 2.The initial conditions (what is the operational state of the system when the TOP event is occurring?) 3.What types of external stresses should be included? 4.The level of resolution.

Boundaries for the BLEDP Physical boundaries are limited to the card itself and the immediate connections (fiber optic cables) with causation being random failures (including failure due to human error in maintenance) and normal degradations over time. Operational state of the system is normal beam permit. There are three TOP events that have been discovered: 1.Maintenance – Non critical failures that can be fixed later 2.False dumps – Failures who are incompatible with safe operation. Dumps the beam to protect the system. 3.Blind failures – Failures which should cause false dumps but are undetectable. No external stresses are included. Resolution is set to minimal cut set.

A cut set is a set of basic events whose occurrence ensures that the TOP event occurs (single failure with no redundancy and several simultaneous with redundancy). It is considered a minimal cut set if the set can’t be reduced more without losing its status as a cut set. Qualitative assessments are done on the cut sets. Qualitative assessments

Quantitative assessments