What is Failure Mode Effects and Criticality Analysis?

Failure mode effects and criticality analysis is a systematic, proactive method for evaluating a process to identify where and how a potential failure might occur and assess the impact of that failure.  RPN (risk priority number) is used as parameter to determine the impact of that potential failure. RPN itself is formulated as :

Severity x Occurrence x Detection

RPN is the process of prioritizing the impact of the failure. Having determined the RPN, actions can be determined to counteract and avoid that failure. The parameter for severity, occurrence and detection is shown in table 1, 2, and 3.

Failure mode effects and criticality analysis study is first time introduced in US military system in 1940 to study the problems that might arise from the malfunction of US military systems

Before going further, some terms need to be explained:

  • Failure : termination of an equipment to perform its expected function
  • Failure mode : a way in which the equipment is failed. For example : short circuit in transformer bushing make the respective equipment fails or the defective bearing make the motor jammed
  • Failure cause : is the causes that leads to failure mode over certain time. For example : short circuit in transformer can be caused by : environmental (moisture), mechanical stress (fracture), or electrical stress (transient over voltage)
  • Failure effect : consequence that appear from the failure. The consequence can be regarding plant operation or regarding the equipment itself, for example : the effect of short circuit of transformer is the decrease of production capacity and the damage of transformer.
  • Severity : the severity has a strict relationship with the failure effect. It determines the consequences of the system failure
  • Occurrence : refers to the frequency that a failure causes is likely to occur, described in qualitative way
  • Detection : is the likelihood for detecting the root cause before it occurs.

Table 1 Severity Parameter

Value Description Criteria
1 Category IV(Minor) primary function can be done but urgent repair is required.
2 Category III(Marginal) reduction in ability to primary function
3 Category II(Critical)

 

causes a loss of primary function
4 Category I(Catastrophic) product becomes inoperative

 

Table 2 Occurence Parameter

Value Description Criteria
1 Level E (Extremely Unlikely) a single failure mode probability of occurrence is less than 0.001
2 Level D(Remote) a single failure mode probability of occurrence is more than 0.001 but less than 0.01
3 Level C (Occasional) single failure mode probability of occurrence is more than 0.01 but less than 0.10
4 Level B (Reasonably probable) a single failure mode probability of occurrence is more than 0.10 but less than 0.20
5 Level A (Frequent)

a single failure mode probability of occurrence is greater than 0.20

Table 3 Detection Parameter

Value Description Criteria
1 Level F Good identification
2 Level E Fair identification
3 Level D Good detection & rough identification
4 Level C Fair detection
5 Level B Rough detection
6 Level A Complementary test

 

To conduct the Failure mode effects and criticality analysis, follow the steps below :

Define components –> Define function or requirement –> define the failure mode –> define the failure effect –> define the intial severity of the effect –> define the failure causes –> define and classify the current –> define initial occurence and detection rating for cause –> calculate initial RPN –> identify and assign the recommended actions –> specify the action taken –> revise the severity rating for the effect –> revise the occurenece and detection ratings for the cause –> generate report

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Application of FMEA for induction motor analysis is shown in the evaluation spreadsheet

Table 4 FMEA for Inducton Motor

Part Failure Mode Effect Failure Potential Cause of Failure
Sleeve Bearing Corrosion, lubrication failure, electrical pitting Unit failure, unit production capacity decreases Thermal stress, mechanical stress, vibration stress, electrical stress
Stator related Turn-turn short, coil-coil short, phase-phase short, coil-ground short, and open circuit Unit failure, unit production capacity decrease Overloading, thermal aging, voltage variation, voltage unbalance, poor ventilation,
Rotor Rotor bar crack, burn-out rotor Unit failure, unit production capacity decrease Overloading, voltage unbalance, cooling problem

If you want to know more about the root causes of induction motor failure, you can learn more at EASA book : the root cause of induction motor. That book is trully comprehensive for knowing the failure causes of induction motor.

Table 5 FMEA for Transformer

 
Part Failure Mode Effect Failure Potential Cause of Failure
Winding Turn-turn short, coil-coil short, phase-phase short, Unit failure, potential to blackout Construction fault, transient overvoltage, hotspot
Insulation Short circuit Unit failure, potential to blackout Transient overvoltage, aging (deterioration), water content
Bushing Short circuit Unit failure, potential to blackout Dirt, water penetration

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