Fault Tree Diagrams: Top Events and AND/OR Logic
Read a fault tree from a precise top event through AND and OR gates. Check backup assumptions and separate logic from probability.
Read a fault tree from a precise top event through AND and OR gates. Check backup assumptions and separate logic from probability.
Open the Fault Tree Diagram Maker to draft the visual, then use the topic-specific checks below to review it.
Original AI-generated fault tree diagram teaching reference. Simplified for explanation; review labels and relationships before use.
Define the top event narrowly
Pump failure is too broad if the analysis concerns no delivered flow during a demanded operation. Use the fault tree diagram maker with a defined operating state and boundary. A precise top event keeps maintenance issues, start-up behavior and running failures from being mixed together.
Read gates as Boolean statements
At an OR gate, any child event is sufficient under the stated assumptions. At an AND gate, all listed child events are required. Read each branch in a sentence before accepting the picture, especially when backup components or shared resources are involved.
Keep qualitative and quantitative analysis separate
A fault tree can explain combinations of failure without assigning likelihood. Probabilities need justified event rates, mission time, dependency treatment and an appropriate model. This page provides a visual draft, not a reliability calculation or safety certification.
Reference key
| Element | Meaning | Review check |
|---|---|---|
| Top event | The unwanted condition being analyzed | Define operating state and system boundary. |
| Intermediate event | A failure explained by lower-level events | Make the causal decomposition explicit. |
| Basic event | A leaf not further decomposed in this model | Give it a testable, specific definition. |
| OR gate | Any input event is sufficient for the output | Confirm each child can cause the parent under the assumptions. |
| AND gate | All input events are required for the output | Account for shared causes and dependencies separately. |
| Assumption | A condition limiting the analysis | Record standby state, coverage and excluded mechanisms. |
Mistakes to check
Using AND to mean then
An AND gate is logical conjunction, not a sequence. If event order matters, describe that limitation or use an appropriate temporal model.
Ignoring common-cause failures
Two supplies may share a power source or environment. Separate leaf icons do not prove independence.
Adding invented probabilities
A plausible percentage changes the claim from qualitative explanation to quantitative analysis. Keep rates out unless they come from a reviewed analysis.
Review the backup-supply branch
The example assumes either supply can independently provide the required drive power. Under that assumption, both supplies must fail to cause loss of supply, so the branch uses AND. If the backup cannot start automatically or shares a failed upstream source, the tree needs additional events. State such assumptions next to the model rather than hiding them in a connector.
Common questions
What does the fault tree diagram maker create?
It drafts a qualitative hierarchy of an unwanted top event and contributing failures connected by explicit logic gates.
Does it compute failure probability?
No. It does not calculate reliability, minimal cut sets or safety integrity levels.
Is this a fishbone diagram?
No. A fault tree uses explicit logical combinations; a fishbone organizes candidate causes into categories.
Can it be used for a safety decision?
Use it only as a drafting aid. A safety decision requires the applicable analysis process, evidence and qualified review.
Sources and editorial notes
Prepared by the AcaDraw product team on November 14, 2026. The reference illustration is original AI-generated artwork. It is a simplified explanatory model, not an experimental result, production specification or independently reviewed analysis.
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