Frequently Asked Question

Risk logic and controls

Fault-tree analysis works backward from an undesired top event to combinations of lower-level events. Special characteristics identify features whose variation can affect safety, compliance, fit, function, or downstream control. Both tools should connect to requirements, design controls, verification, and manufacturing controls.

These probability forms require independence assumptions. Use approved rating definitions and physical failure evidence.

Worked example

If two independent events have probabilities 0.02 and 0.01, their AND combination is approximately 0.0002. If either event can produce the top event, the OR probability is 1−(0.98×0.99) = 0.0298. The logic changes the control strategy.

Engineering check

For Fault Trees and Causal Risk Analysis, maintain traceability from requirement to risk, design output, evidence, and approval. Record the configuration, acceptance criterion, test or analysis conditions, open actions, and residual risk. A method is not complete when the document is filled in; it is complete when the evidence supports the decision and affected controls are updated.

Traceability note

For Fault Trees and Causal Risk Analysis, maintain a visible link between the requirement, design or process characteristic, risk entry, planned evidence, measured result, and approval record. Identify the configuration and revision used, record deviations and open actions, and state who accepts residual risk. When evidence changes, update the affected requirement, risk analysis, verification or validation plan, and manufacturing control rather than editing only the summary status.