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FMECA

AN OVERVIEW OF BASIC CONCEPTS

Failure Modes, Effects and Criticality Analysis (FMECA) is the method designed to identify potential failure modes for a product or process, to assess the risk associated with those failure modes, to rank the issues in terms of importance and to identify and carry out corrective actions to address the most serious concerns.

This section presents a brief general overview of FMECA analysis techniques and requirements.

AN OVERVIEW OF BASIC CONCEPTS

In general, Failure Modes, Effects and Criticality Analysis (FMECA) requires the identification of the following basic information:

  • Item(s)
  • Function(s)
  • Failure(s)
  • Effect(s) of Failure
  • Cause(s) of Failure
  • Current Control(s)
  • Recommended Action(s)
  • Plus other relevant details

PUBLISHED STANDARDS AND GUIDELINES

There are a number of published guidelines and standards for the requirements and recommended reporting format of failure mode and effects analyses. Some of the main published standards for this type of analysis include SAE J1739, AIAG FMEA-3 and MIL-STD-1629A.

BASIC ANALYSIS PROCEDURE FOR FMECA

The basic steps for performing a Failure Modes, Effects and Criticality Analysis (FMECA) include:

  • Assemble the team.
  • Establish the ground rules.
  • Gather and review relevant information.
  • Identify the item(s) or process(es) to be analyzed.
  • Identify the function(s), failure(s), effect(s), cause(s) and control(s) for each item or process to be analyzed.
  • Evaluate the risk associated with the issues identified by the analysis.
  • Prioritize and assign corrective actions.
  • Perform corrective actions and re-evaluate risk.
  • Distribute, review and update the analysis as appropriate.

RISK EVALUATION METHODS

A typical Failure Modes, Effects and Criticality Analysis incorporate some method to evaluate the risk associated with the potential problems identified through the analysis. The two most common methods, Risk Priority Numbers and Criticality Analysis, are described next.

RISK PRIORITY NUMBERS

To use the Risk Priority Number (RPN) method to assess risk, the analysis team must:

  • Rate the severity of each effect of failure.
  • Rate the likelihood of occurrence for each cause of failure.
  • Rate the likelihood of prior detection for each cause of failure (i.e. the likelihood of detecting the problem before it reaches the end user or customer).
  • Calculate the RPN by obtaining the product of the three ratings:
    • RPN = Severity x Occurrence x Detection
    • The RPN can then be used to compare issues within the analysis and to prioritize problems for corrective action.

CRITICALITY ANALYSIS

The MIL-STD-1629A document describes two types of criticality analysis:

Qualitative and Quantitative.

  • To use qualitative criticality analysis to evaluate risk and prioritize corrective actions, the analysis team must a) rate the severity of the potential effects of failure and b) rate the likelihood of occurrence for each potential failure mode. It is then possible to compare failure modes via a Criticality Matrix, which identifies severity on the horizontal axis and occurrence on the vertical axis.

  • To use quantitative criticality analysis, the analysis team considers the reliability/unreliability for each item at a given operating time and identifies the portion of the item’s unreliability that can be attributed to each potential failure mode. For each failure mode, they also rate the probability that it will result in system failure. The team then uses these factors to calculate a quantitative criticality value for each potential failure and for each item.

APPLICATIONS AND BENEFITS FOR FMECA

The Failure Modes, Effects and Criticality Analysis (FMECA) procedure is a tool that has been adapted in many different ways for many different purposes. It can contribute to improved designs for products and processes, resulting in higher reliability, better quality, increased safety, enhanced customer satisfaction and reduced costs. The tool can also be used to establish and optimize maintenance plans for repairable systems and/or contribute to control plans and other quality assurance procedures. It provides a knowledge base of failure mode and corrective action information that can be used as a resource in future troubleshooting efforts and as a training tool for new engineers. In addition, an FMECA is often required to comply with safety and quality requirements, such as ISO 9001, QS 9000, ISO/TS 16949, Six Sigma, FDA Good Manufacturing Practices (GMPs), Process Safety Management Act (PSM), etc.

You can use something as simple as a paper form or an Excel spreadsheet to record your FMECA analyses. However, if you want to establish consistency among your organization's FMECAs, build a "knowledge base" of lessons learned from past FMECAs, generate other types of reports for FMECA data (e.g. Top 10 Failure Modes by RPN, Actions by Due Date, etc.) and/or track the progress and completion of recommended actions, you may want to use a software tool, to facilitate analysis, data management and reporting for your failure modes and effects analyses.

 

Failure Mode Failure Effects Failure Cause Failure Mechanism Current Design Control Prob1 Sev2 CN3 Det4 RPN5 Recommended Activities
Open grounding wire

1. Bypasses the ground fault detection and ground fault protection
2. Increases the voltage on healthy phase with respect to ground by √3 times
3. High circulating current flows in transformer winding during fault which produces excessive thermodynamic stress on the core
4. Damage lightning arrestor because of high heat in the non linear resistance5. Threats to human as it increases touch potential

1. Smaller than required size of grounding wire
2. Improper connection of grounding element causes spark during fault which in the long run melts the wire

3. Corrosion at the joints, buried under the ground, damage the equipment

The joints of grounding element(resistor or inductor) melt due to high flow of current through it

1. Design the appropriate rating of grounding element
2. All joints should be cad welded used

Currently the grounding system is not tested
3 6 18 8 144 The termination joints of the metallic body should be inspected regularly and tightened every time.
1 Probability
2 Severity
3 Criticality Number
4 Detectability
5 Risk Priority Number

 

 
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