What is the key difference between Severity, Occurrence and Detection (FMEA)?

There is a lot of difference between listening and hearing: GK Chesterton

Introduction

In our day-to-day life, we often have a problem and a related story around that problem. Many times, we do not distinguish the difference between the problem and the story. We become more entangled in the story rather than understanding the problem and solve it. Once we can detect the real problem, we can easily understand its severity and work on it.  

Objective

The FMEA is a deep dive analysis process of identifying possible internal and external failures. When the organization could identify the criticality of the possible failure, it becomes easy to define its severity, possibility of its occurrence and the mechanism to detect it, when it will occur. By clearly understanding the Severity, Occurrence and Detection, possible actions related to prevention and detection can be established.

Definitions

Severity (S): Severity of the Failure Effect (FE)

Occurrence (O): Occurrence of the Failure Cause (FC)

Detection (D): Detection of the Failure Cause (FE) and/or Mode (FM)

Detailed Information

FMEA (Failure Mode & Effect Analysis) is a team-oriented, systematic, qualitative and analytical method to identify, analyse and mitigate the technical risks related to the product and manufacturing process design.

The FMEA is the ‘Before the Event’ and not the ‘After the Event’ exercise.

Who is Customer:

• Internal (next process)
• External (Tier 2, Tier 1, OEM, Dealer)
• Legal bodies
• Product end-user

Failure Chain:

• Failure Modes (FM): It means the ways, or modes, in which something might fail.
• Failure Effect (FE): It refers to studying the consequences of those failures’ mode.
• Failure Cause (FC): A indication of why failure mode could occur

Risk Analysis:

• Assignment of existing or planned controls and rating of failure
• Assignment of preventive control to the failure causes
• Assignment of detective control to the failure causes or failure modes
• Rating for Severity, Occurrence and Detection for each failure chain
• Evaluation of action priority
• Collaboration between customer and supplier related to the severity
• The basis for the optimization step

Severity (S): It is the measure associated with the most serious Failure Effect (FE) for a given failure mode of the function being evaluated.

• The severity rating is considered without any regard for Occurrence and Detection
• Severity ranking for the identical product for the different teams can be different
• Severity raking should be linked with customer or supplier
• Where the customer impacted for assessing the severity may lie outside the immediate team’s understanding, the consultation with the design engineer team or DFMEA can be taken

Severity Table: Rated from Very High (10) to Very Low (1)

Occurrence (O): It is the measure of the effectiveness of the preventive control by identifying the Failure Causes (FC) to occur in customer operations.

• It is a relative rating and may not reflect the actual occurrence.
• The accuracy of the occurrence rating depends upon the effectiveness of the Preventive Controls

Possible questions to determine to Rate:

• Have prevention controls been put in place?
• Is the product/process completely new or similar to the previous one?
• What is the application of the product/process?
• What are the environmental changes?
• What are the service history and field experience for the similar product/Component/System/subsystem?
• Do standard instructions exist like work instruction, job setup procedures, process monitoring etc. (PFMEA)?
• Are technical error-proofing solutions implemented like SPC study, traceability, machine capability etc. (PFMEA)

Occurrence: Rated from Extremely High (10) to Extremely Low (1)

Detection (D): It is the estimated measure of the effectiveness of the detection control to reliably demonstrate the Failure Causes (FC) or Failure Modes (FM) before the product is released for production.

• Detection rating is associated with the most effective detection control
• Relative rating
• Rating is determined without any linkage with severity or occurrence rating
• Rating is the prediction of the effectiveness of the unproven control and can be reevaluated once detection control is complete

Possible questions to determine to Rate:

• Which test is most effective in detecting the failure causes or modes?
• What sample size is required to detect the failure?
• Is the test procedure proven for detecting this cause/failure mode?

Detection: Rated from Very Low (10) to Very High (1)

Purpose for determining S, O, D Rating:

• Determine ‘Priority of Action’
• Determining the rating as High, Medium, Low
• High: Required to identify appropriate action to improve Prevention and/or Detection Controls, OR justify and document why current controls are adequate
• Medium: Should identify appropriate actions to improve prevention and/or detection controls; OR, at the discretion of management, justify and document why current controls are adequate
• Low: Could identify actions to improve prevention or detection controls

Present Challenges

• How often the cross-functional team is clear about the rating specified for Severity, Occurrence and Detection?
• How often the criteria for selecting the rating are predetermined based on convenience?
• How often recommended actions are based on the ranking and not on convenience?

References:

IATF 16949: 2016

FMEA Handbook (AIAG-VDA 1^st Edition June 2019)

Industry Experts

This is the 121^st article of this Quality Management series. Every weekend, you will find useful information that will make your Management System journey Productive. Please share it with your colleagues too.

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