Fault Tree Analysis Builder

What is Fault Tree Analysis?

Fault Tree Analysis is a top-down, deductive analysis method. It begins with a single undesired Top Event and decomposes it into the events that can contribute to it, connected by logical gates. The result is a structured causal model rather than a list of ideas.

FTA is used in reliability engineering, functional safety, systems engineering and manufacturing quality, particularly where several failure paths can lead to the same outcome.

How does Fault Tree Analysis work?

1. Define the Top Event as an observed undesired outcome.
2. Identify the immediate contributing events one level below it.
3. Choose AND or OR logic for that decomposition.
4. Decompose intermediate events further where the mechanism is still unclear.
5. Stop where Basic Events are specific enough to act on or investigate.
6. Validate the logic and verify important events with evidence.

AND vs OR gates in FTA

An OR gate means any one child event alone is enough to produce the parent event. An AND gate means every child event must occur together. Choosing the wrong gate changes the meaning of the whole branch, so gate selection should reflect the physical or logical reality of the system.

Example: a leak occurs if the seal is damaged OR the joint is loose. A protected circuit fails only if the primary fuse fails AND the redundant protection fails.

Fault Tree Analysis example

Top Event: Assembly fails final functional test. The events below are candidate logic paths, not proven causes.

• OR gate: electrical connection failure, firmware or configuration failure, mechanical assembly failure.
• Electrical connection failure (OR): connector not seated, damaged contact, incorrect wiring.

Basic, Intermediate and Undeveloped Events

• Intermediate Event: an event that is decomposed further with another logic gate.
• Basic Event: an event specific enough that it is not decomposed further in this analysis. It is not automatically a verified root cause.
• Undeveloped Event: an event not analysed further because information is missing or it lies outside the chosen scope.

When to use FTA

• Safety-related failures where combinations of conditions matter.
• Complex product failures with several plausible mechanisms.
• Recurring manufacturing defects that previous fixes did not eliminate.
• System-level quality issues spanning several subsystems.
• Risk analysis where redundancy or protective layers exist.

Fault Tree Analysis vs Fishbone Diagram

A Fishbone diagram categorises potential causes broadly across cause families such as People, Machine or Method. A fault tree instead models logically how events combine — using AND and OR — to produce the Top Event. Fishbone is excellent for coverage; FTA is stronger for logical structure.

Fault Tree Analysis vs 5 Why

5 Why follows one causal chain in depth and works well for straightforward, linear problems. FTA represents branched logic where multiple paths, or combinations of events, can produce the same failure.

Fault Tree Analysis in 8D

FTA supports D4 root cause analysis in an 8D investigation, particularly for technically complex issues where several causal paths or event combinations must be examined. See the Qhubio 8D knowledge center for how D4 fits into the discipline sequence.

Frequently asked questions

What is Fault Tree Analysis?

Fault Tree Analysis (FTA) is a top-down, deductive method. It starts from one undesired Top Event and decomposes it into the contributing events that can produce it, connected by logical AND and OR gates.

What is the Top Event in FTA?

The Top Event is the single undesired event being analysed — for example a functional test failure. It should be stated as an observed outcome without embedding an assumed cause.

What is the difference between an AND gate and an OR gate?

An OR gate means any one of the child events is sufficient to produce the parent event. An AND gate means all child events must occur together for the parent event to occur.

What is a Basic Event?

A Basic Event is an event specific enough that it is not decomposed further in this analysis. It is a chosen endpoint of the decomposition, not automatically a verified cause.

Does FTA identify a root cause automatically?

No. A fault tree is a logical causal model. Events and relationships must be verified with evidence before any of them is treated as a confirmed cause.

Is Fault Tree Analysis qualitative or quantitative?

It can be both. The Qhubio FTA Builder is qualitative: it builds the logical structure. It does not calculate probabilities, failure rates or minimal cut set probabilities.

Can FTA be used in manufacturing quality?

Yes. FTA is widely used for recurring defects, system-level quality issues and failures with several interacting causal paths.

Can FTA be used in 8D?

Yes. FTA supports D4 root cause analysis, particularly where multiple causal paths or combinations of events need to be examined systematically.

What is the difference between FTA and Fishbone?

A Fishbone diagram categorises potential causes broadly. A fault tree models logically how events combine — with AND/OR logic — to produce the Top Event.

What is the difference between FTA and 5 Why?

5 Why follows one linear causal chain in depth. FTA builds a branched logical model that can represent several simultaneous or alternative failure paths.

Is the Qhubio FTA Builder free?

Yes. Building the fault tree and viewing the complete result is free, with no registration and no installation.

Do I need an account?

No account is needed to build and view your fault tree. An account is required only if you want to save the analysis to your Qhubio workspace.

Can I save my fault tree?

Yes. Click Save analysis. If you are not signed in, you will be asked to create an account, and your analysis is then saved to your Qhubio dashboard.