Popular methods of Risk Assessment (Part-II)

Fault tree analysis

The fault tree analysis (FTA) was first introduced by Bell Laboratories and is one of the most widely used methods in system reliability, maintainability and safety analysis. It is a deductive procedure used to determine the various combinations of hardware and software failures and human errors that could cause undesired events (referred to as top events) at the system level.

The deductive analysis begins with a general conclusion, then attempts to determine the specific causes of the conclusion by constructing a logic diagram called a fault tree. This is also known as taking a top-down approach.

The main purpose of the fault tree analysis is to help identify potential causes of system failures before the failures actually occur. It can also be used to evaluate the probability of the top event using analytical or statistical methods. These calculations involve system quantitative reliability and maintainability information, such as failure probability, failure rate and repair rate. After completing an FTA, one can focus the efforts on improving system safety and reliability.

FTA includes review of the following:

•  Establish the pathway to the root cause of the failure

•  While investigating complaints or deviations to fully understand their root cause

•  Ensure that intended improvements will fully resolve the issue and not lead to other issues

•  Evaluating how multiple factors affect a given issue.

The basic symbols used in an FTA logic diagram are called logic gates and are similar to the symbols used by electronic circuit designers. Two kinds of gates, "and" and "or," are described.

The partial FTA logic diagram in uses the "and" and "or" gates' symbols to analyze hazard to the patient. Inputs to the "or" gate at the top identify the four reasons this failure can occur. The analysis continues on, using the same technique, until the lowest levels such as operator error or open ground pin are identified.

When you perform an FTA, you systematically determine what happens to the system when the status of a part or another factor changes. In some applications, the minimum criterion for success is that no single failure can cause injury or an undetected loss of control over the process. In others, where extreme hazards exist or when high value product is being processed, the criteria may be increased to require toleration of multiple failures. 

Fault tree construction

1. Define the fault condition, and write down the top level failure.

2. Using technical information and professional judgments, determine the possible reasons for the failure to occur. Remember, these are level two elements because they fall just below the top level failure in the tree.

3. Continue to break down each element with additional gates to lower levels. Consider the relationships between the elements to help you decide whether to use an "and" or an "or" logic gate.

4. Finalize and review the complete diagram. The chain can only be terminated in a basic fault: human, hardware or software.

5. If possible, evaluate the probability of occurrence for each of the lowest level elements and calculate the statistical probabilities from the bottom up. 

Following is an example of FTA construction

Limitations

• Only as good as input
• Time and resource consuming (needs FMEA as a complement )
• Need skilled leader to focus on what is really important
• Need significant amount of information
• Human errors may be difficult to predict
• Many potential fault trees for a system.
• Some more useful than others
• Need to evaluate contribution

For more details on Risk Management approach, Please follow below link:

Fault tree analysis
http://www.uscgmp.com/2015/11/popular-methods-of-risk-assessment-part_17.html

Failure Mode Effects Analysis (FMEA)
http://www.uscgmp.com/2015/11/popular-methods-of-risk-assessment-part.html

A to Z Risk Management with statistical approach
http://www.uscgmp.com/2013/12/a-to-z-risk-management-with-statistical.html