Frequently Asked Question
Design-risk structure
RPN is a prioritization aid. High-severity hazards, regulatory requirements, and single-point failures require explicit treatment even when a multiplied score is not high.
Severity, occurrence, and detection ratings
Scale control: S, O, and D are not universal physical constants. Use the approved rating definitions for the project, customer, industry, and regulatory context. The four anchor points below are an illustrative framework for training and should be expanded into the organization’s controlled 1–10 scale before formal release.
| Rating | Severity (S) | Occurrence (O) | Detection (D) |
|---|---|---|---|
| 1 | No noticeable effect on function or user | Remote design-cause history; prevention is robust | Controls almost certain to detect the cause or failure before release |
| 4 | Minor degradation; performance remains acceptable | Occasional cause in comparable designs or conditions | Controls likely to detect before release with objective evidence |
| 7 | Loss or major degradation of a primary function; significant customer impact | Frequent cause or weak prevention in the intended use range | Detection is uncertain or depends on a late inspection/test |
| 10 | Potential safety hazard or regulatory noncompliance | Failure is effectively inevitable without a design change | No effective detection before the effect reaches the user |
Severity is assigned from the effect, occurrence from the cause and exposure history, and detection from the effectiveness of current controls. Do not reduce a safety or regulatory severity because occurrence is low. RPN is a prioritization aid and must not replace mandatory controls or engineering judgment.
Rating example: for a gasket with a leakage failure mode, a project might assign S = 7 for loss of environmental protection, O = 4 based on comparable history, and D = 6 when detection depends on a late leak test. The traditional RPN is 7×4×6 = 168. The team must still act on high severity or regulatory effects regardless of the number.
Worked example
For a gasket function, “leakage” is the failure mode, loss of environmental protection is an effect, and compression-set or surface damage may be causes. The control should identify compression, surface, and leak evidence—not only a final visual check.