To ensure the safety and reliability of a system, reliability agents such as engineers and maintenance personnel need to understand a fault and its impact on the system thoroughly. This understanding requires detailed analysis of faults and other undesired operational states of the machinery. There are many different types of approach and methodology when it comes to analyzing faults of a system. In this piece the main focus will be the failure analysis method known widely as Fault Tree Analysis.

Fault Tree Analysis is a failure analysis method created by Bell Laboratories in 1962 to be used in the aerospace field. This method focuses on the faults a system might encounter, relationships among these failures and interactions between other subsystems and elements.  In this analysis format the undesired state , a failure , is placed on the top of the tree as the main focus point. By working in a backward manner related subsystems components, machine elements and operational actions are examined. All this analysis is later compiled under a tree-like structure with a mapping created by symbols of events and logic gates such as AND, OR etc.

Fig. 1 Fan System Failure Diagram

What are the steps of FTA?

Defining the fault to analyze:

Defining a critical fault is essential to conduct a FTA. Undesired event, a fault, must be chosen by taking criticality, complexity and impact on the system into consideration. This is a vital stage since the top element in the FTA is unique and analysis conducted specifically for one failure. 

Understanding the system completely:

Once the focus point, failure, is chosen any related element should be studied thoroughly. Actions, subsystems, components, environmental elements etc. must be noted to obtain an understanding of the failure alongside any related input. Occurrence possibilities of the events are calculated and indicated for every event related to the undesired event.

Create and evaluate the fault tree:

After studying the system, construction of the tree representation begins. Every event and state leading to undesired state of fault is listed and existing relations between the conditions are represented by using AND or OR gates. Fault tree is evaluated for any improvements and all the possible hazards resulting in the undesired event is obtained. 

Taking action according to the fault tree analysis:

After the Fault Tree representation is complete and all the necessary studies are conducted actions must be taken to increase the reliability of the system and to decrease the probability of potential hazardous states leading to the undesired event.

An Example of FTA

In this example, failure of a fan system is determined as the undesired state. Main possible causes of the malfunction are Fan Element Malfunction, Component Failure and Motor Failure. All of these sub level malfunctions like Bearing Failure, Motor Failure, Broken and Stuck Impellar are taken into evaluation and mapped on the Fault Tree Diagram. 

Fig. 2 Failure Mode Diagram