RPN is ordinal, not calibrated probability
A change from 240 to 120 means the product of three ratings halved; it does not prove that physical risk or expected loss halved. Keep the underlying ratings visible.
Reliability
Component Failure Risk Calculator
Compare current and residual FMEA risk-priority numbers under explicit controls.
FMEA PRIORITIZATION
Rank a failure mode before and after verified controls
For FMEA teams documenting action priority without converting ordinal scores into false probabilities or bypassing high-severity governance.
Current RPN-
Residual RPN-
RPN reduction-
Threshold margin-
Current band-
Residual band-
CURRENT DECISION RECORD
Current-to-residual FMEA ledger
Every row is regenerated from the active inputs and carried into Copy, TXT, and the page-specific PDF payload.
| Factor | Current | Control | Residual |
|---|
CURRENT CALCULATION PROCESS
Formula, substitution, intermediate values, and reconciliation
RPN=SxOxD; RPNres=Sxmax(1,O(1-rO))xmax(1,D(1-rD))
Waiting for valid inputs.
HOW TO USE
Five steps from failure mode to residual review
- Score severity from the agreed consequence rubric.
- Score occurrence from field evidence.
- Score detection difficulty before release.
- Enter control effects and the action threshold.
- Record residual RPN and owners; never hide a high-severity hazard.
FMEA FUNDAMENTALS
Five ratings and records behind the result
- Severity rating
- A 1-10 ordinal score for the worst credible effect of the failure mode under the team approved rubric.
- Occurrence rating
- A 1-10 evidence-based frequency class tied to field returns, tests, or a documented analogous design.
- Detection rating
- A 1-10 score for how likely the current controls are to miss the cause or failure mode before the effect occurs.
- Risk priority number
- RPN multiplies severity, occurrence, and detection to create a screening rank; it is not a probability or expected loss.
- Residual assessment
- Post-control ratings must describe the verified design after actions, not the hoped-for effect written in an action plan.
DEEP RISK ANALYSIS
Interpret the rank without overstating it
Equal products can hide unequal hazards
S=10, O=3, D=4 and S=5, O=8, D=3 both produce 120, yet the first retains a catastrophic severity that may require action regardless of rank.
Control claims require verification
Occurrence reduction needs prevention evidence, while detection improvement needs inspection capability evidence. Treating one control as both without proof double-counts its benefit.
WORKED DECISION CASES
One control case and one severity boundary
Overheating power connector
A connector scored S=8, O=5, D=6 has RPN 240. A keyed assembly and torque verification reduce modeled occurrence by 50% and detection difficulty by 40%, producing residual RPN 72; the team still preserves the severity-8 escalation review.
Single-point brake sensor boundary
A safety-related sensor with S=10, O=2, D=2 starts at RPN 40, below many numeric thresholds. The low product must not close the issue because a single undetected failure can retain an unacceptable effect.
EVIDENCE RECORD
What must accompany an RPN decision
Retain the FMEA revision, item function, failure chain, scoring rubric, named participants, evidence for each rating, control owner, due date, implementation record, and post-action verification. The residual score is auditable only when reviewers can trace every changed rating to a completed control and its evidence.
MODEL LIMITS
Where this prioritization model stops
- Ordinal FMEA ratings are not calibrated probabilities or monetary losses.
- Multiplicative control reductions are screening assumptions until implementation evidence supports new ratings.
- The model does not replace hazard analysis, fault-tree analysis, regulatory review, or safety acceptance.
- High-severity and single-point failures require separate escalation regardless of the resulting RPN.
FMEA GLOSSARY
Six distinct terms used in the worksheet
- Failure mode
- The specific way an item can fail to meet its intended function, stated at the level being analyzed.
- Failure effect
- The local, next-level, and end effect experienced if the failure mode occurs.
- Failure cause
- The physical, design, process, or use mechanism that produces the failure mode.
- Prevention control
- A design or process measure intended to reduce the chance that the cause occurs.
- Detection control
- A test, inspection, monitor, or diagnostic intended to reveal the cause or mode before the effect.
- Residual risk
- The remaining assessed concern after implemented controls are verified and ratings are reassigned.
FREQUENTLY ASKED QUESTIONS
Questions specific to residual RPN decisions
Can RPN compare unrelated products?
Only when both analyses use the same scoring definitions, evidence windows, and escalation rules. Cross-product ranking without a common rubric creates false precision.
Should severity fall after a prevention control?
Usually no. Prevention changes occurrence; severity changes only when the design changes the consequence experienced after the failure occurs.
Why are residual ratings floored at one?
The 1-10 FMEA scale has no zero category. A control may make failure remote or highly detectable, but the rating framework still records a minimum class of one.
Is an RPN of 100 a universal action threshold?
No. Organizations define their own action priorities, and many require mandatory review for high severity even when RPN is below a numeric threshold.
Can one control reduce occurrence and improve detection?
Possibly, but each effect needs separate evidence. Do not apply the same claimed reduction twice unless the control genuinely prevents causes and independently detects residual defects.
Does a low residual RPN prove the design is safe?
No. It records a prioritization judgment under one rubric. Safety requirements, regulatory duties, single-point failures, and verification evidence remain separate acceptance gates.
RELIABLE SOURCES
Engineering risk and reliability references
IMPORTANT FMEA LIMIT
RPN does not certify safety
Use this page to prioritize investigation and actions. Do not use a low current or residual RPN to waive hazard analysis, statutory requirements, mandatory severity escalation, or verification of safety-critical controls.