Reliability Services in the Design Phase
Fault Tree Analysis (FTA)

DEFINITION
Fault Tree Analysis (FTA) is a top-down approach to failure mode analysis. An FTA identifies failures and strives to eliminate the cause of the failure.

SITUATION
While troubleshooting a failure or trying to identify possible causes to a specific failure effect, an FTA can be a very useful tool.

OBJECTIVE
FTA is a systematic, deductive method for defining a single specific undesirable event and determining all possible failures that could cause the event in question to occur.

VALUE TO YOUR ORGANIZATION
Although an FTA can be very useful in the initial product design phase as an evaluation tool, it is probably more powerful as a troubleshooting tool after an event (or proposed event) has taken place.

RELIABILITY INTEGRATION
An example of Reliability Integration during Fault Tree Analysis is as follows:

Using FTA's during HALT planning
When a FMECA identifies a critical effect, an FTA is often deployed to evaluate all possible failure modes that can also cause the same critical effect. This is especially helpful when planning a HALT so that the appropriate stresses can be applied and so that the failure can easily be troubleshot if the critical effect is exposed during HALT.

METHODOLOGY
When we perform an FTA, we start with an undesired event. The undesired event constitutes the top event in a fault tree diagram. We then brainstorm (just like the FMECA) as to the possible failure modes that can result in this undesired effect.

FTA's and FMECA's are very similar in this regard but the goal is much different. Whereas a FMECA is trying to identifying all possible failure modes in a system and the effects of these failure modes, an FTA starts with one specific failure effect and then identifies only those failure modes that can cause the particular effect.

How we decide whether to use an FTA or a FMECA

FTA is preferred over FMECA when:

• A small number of top events can be identified

• Product functionality is highly complex

• The product is not repairable once initiated

FMECA is preferred over FTA when:

• The events cannot be limited to a small number

• Multiple successful functional profiles are possible

• Identification of "all possible" failure modes is important

CASE STUDIES/OPTIONS
The following case studies and options provide example approaches. We shall tailor our approach to meet your specific situation.

1) Using FTA's to Identify Safety-Critical Failures
A Computer manufacturer had a safety-critical failure in their product and they wanted to identify if there were any other failure modes in the product that could result in the same failure effect. For this, we turned to an FTA. We made the safety-critical failure the top event. From there, we reviewed the FMECA and held brainstorming sessions to help identify other failure modes that could cause this top event. Next, we reviewed the reliability prediction to identify how often the event will occur.

2) Using FTA's During Failure Analysis
For a Medical Device company, we showed them how to use FTA as a powerful failure analysis tool after a failure occurred to help identify the cause of the failure. Troubleshooting isolated which component failed but an FTA was needed to determine what caused the failure.

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