An Introduction to Failure Modes
Effects and Criticality Analysis
FME(C)A
Dr Jane Marshall
Product Excellence using 6 Sigma
Module
PEUSS 2011/2012
FMEA
Page 1
Reliability tool and techniques
• Methods for fault avoidance
• Methods for architectural analysis and
assessment
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1
Methods for fault avoidance
• Parts derating and selection
– Limiting component stress levels to below specified
maxima
– Ratio of applied stress to rated maximum stress
– Applied stress taken as maximum likely to be
applied during worst case operating conditions
• Stress-strength analysis
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Methods for architectural
analysis and assessment
• Bottom-up method
– Event tree analysis (ETA)
– FME(C)A
– Hazard and operability study (HAZOP)
• Top-down method
– Fault tree analysis (FTA)
– Reliability block diagram (RBD)
– Markov analysis
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FME(C)A
•
•
•
•
What is FME(C)A?
Why FME(C)A?
How to perform FME(C)A
FME(C)A Exercise
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Failure Modes and Effects
Analysis (FMEA)
• A qualitative approach that is intended to:
– Recognize and evaluate the potential failures of a product or
process and the effects of that failure
– Identify actions which could eliminate or reduce the chance of
the potential failure occurring
– Document the entire process
• Failure Modes Effects and criticality Analysis (FMECA)
– Extends FMEA to include criticality analysis
– Quantifies failure effects and severity
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3
Definition
• Failure modes effects and criticality analysis (FMECA)
is a step-by-step approach for identifying all possible
failures in a design, a manufacturing or assembly
process, or a product or service.
• “Failure modes” means the ways, or modes, in which
something might fail.
• “Effects and criticality analysis” refers to studying the
consequences of those failures.
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Why is it Important?
• Provides a basis for identifying root failure
causes and developing effective corrective
actions
• Identifies reliability/safety critical components
• Facilitates investigation of design alternatives at
all stages of the design
• Provides a foundation for other maintainability,
safety, testability, and logistics analyses
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4
History/Standards
The FMEA was originally developed by NASA to improve and verify
the reliability of space program hardware.
• MIL-STD-785, Reliability Programs for System and Equipment
Development and Production-Task 204, sets out the procedures
for performing FMECA
• MIL-STD-1629 establishes requirements and procedures for
performing FMECA
• Automotive suppliers may use SAE J1739 FMEAs, or they may
use the Automotive Industry Action Group (AIAG FMEA)
• QS-9000 standard
• IEC 60812 - Analysis techniques for system reliability – Procedure
for failure mode and effects analysis (FMEA)
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Benefits of FME(C)A
• FME(C)A is one of the most important and most widely
used tools of reliability analysis.
• The FME(C)A facilitates identification of potential
design reliability problems
• It can help removing causes for failures or developing
systems that can mitigate the effects of failures.
• Help engineers prioritize and focus on high-risk
components/failures
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5
Benefits of FME(C)A
• It provides detailed insight into the systems
interrelationships and potentials for failure.
• Information and knowledge gained by performing the
FME(C)A can also be used as a basis for trouble
shooting activities, maintenance manual development
and design of effective built-in test techniques.
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Benefits and limitations
•
•
•
•
•
•
•
Systematically identifies cause and effect relationships
Indicates critical failure modes
Identifies outcomes from causes
Framework for identifying mitigating actions
Output may be large even for simple systems
Prioritising may become difficult with competing failure modes
May not easily deal with time sequences, environmental
conditions and maintenance aspects
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6
FME(C)A Applications - 1
• To identify failures which, alone or in combination, have
undesirable or significant effects; to determine the failure
modes which may seriously affect the expected or
required quality.
• To identify safety hazard and liability problem areas, or
non-compliance with regulations.
• To focus development testing on areas of greatest need.
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FME(C)A Applications - 2
• To assist the design of Built-in-Test and failure
indications.
• To assist the preparation of diagnostic flow charts or
fault-finding tables.
• To assist maintenance planning.
• To identify key areas in which to concentrate quality
control, inspection and manufacturing controls.
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7
FME(C)A Applications - 3
• To provide a systematic and rigorous study of
the process and its environment.
– To support the need for standby or alternative
processes or improvements to current processes.
– To identify deficiencies in operator and supervisor
training and practices.
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FMEA -- Types
System
Concept
FMEA
Design
FMEA
Sub-System
Component
System
Assembly
Process
FMEA
Sub-System
Component
System
Manufacturing
Sub-System
Component
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Design FMEA -- Team
Representatives
from:
Support
Team • Customer Service
Design Engineer
Manufacturing /
Process Engineer
• Suppliers
CORE
Team
• Global Test
Operations
• Corporate Quality
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FMEA
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FMEA Process
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FMEA Procedure
• Identify all potential item failure modes and define their
effects on the immediate function or item, on the system,
and on the mission to be performed
• Evaluate each failure mode in terms of the worst potential
consequence, which may rank severity classification
• Identify failure detection methods and compensating
provision for each failure mode
• Identify corrective design or other actions required to
eliminate the failure or control the risk
• Document the analysis and identify the problems, which
could not be corrected by design
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Setting The Level Of
Analysis
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10
How is it Done?
What are the effects
of box failures on
the system?
What are the effects
of board failures on
the box?
What are the effects
of part failures on
the board?
Note: This is a bottom up example.
Top down examples are possible.
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FMEA Cascade - General
System
Sub-System
Component
Process
Effect
Failure
mode
Cause
Effect
Failure
mode
Cause
Effect
Failure
mode
Effect
Cause
Failure
mode
Cause
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FMEA Cascade - Flipchart Stand
Flip Chart Stand
(System)
Clamp
(Sub-System)
Screw
Assembly
(Assembly)
Screw
(Component)
Effect
Embarrass
Presenter
Failure
mode
Paper falls
out
Effect
Cause
Insufficient
clamping
force
Failure Insufficient
clamping
mode
Effect
Insufficient
clamping
force
Cause
Failure
mode
Screw
failure
Effect
Screw
failure
Cause
Thread
failure
Failure
Thread
failure
Paper falls
out
force
Screw
failure
mode
Cause
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Process
failure
Page 23
Bonnet Release Example
• What can go wrong with the bonnet release on
your car?
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BONNET RELEASE SYSTEM FMEA
FUNCTION
•To release Bonnet for opening
when required
FAILURE MODE
CAUSE
•Cannot release bonnet
EFFECT
1.Cannot operate lever
•Customer annoyance
•Cancelled journey
•Curtailed journey
1.R.H. or L.H. does not release
respective plunger
1.Secondary catch does not
operate
•Difficult to release bonnet
•Difficult to operate lever
1.Customer annoyance
1.Secondary catch difficult to
operate
•To prevent Bonnet releasing or
opening when not required to open
1.Bonnet opens when not required
to open
1.Bonnet liner detaches from
bonnet
•Safety (accident – loss of vision)
1.Primary & secondary catch
failure
•To retain Bonnet in required
closed position (shut lines,
aesthetics) without vibration or
flexing
1.Bonnet releases to safety catch
when not required
1.Primary catch failure
1.Bonnet vibrates
1.L.H. or R.H. plunger not fully
engaged in receptacle
•Hazard (reduced safety)
•Vibration or flexing
1.Inadvertent operation of lever
•High customer annoyance
1.L.H. or R.H. plunger detaches
from liner
1.L.H. or R.H. plunger can move in
receptacle
1.Bonnet flexes
•As 3.1
1.Customer dis-satisfaction
1.Looks awful
1.Incorrect location of L.H. and
R.H. plunger
1.High customer annoyance
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BONNET RELEASE SYSTEM FMEA
FUNCTION
FAILURE MODE
CAUSE
EFFECT
1.To prevent Bonnet being
opened by external means
1.Bonnet can be opened
externally
1.External access to primary
release mechanism
•High customer annoyance
•Theft
1.To enable Bonnet to close and
lock in required position using
minimal force
•Cannot close bonnet
1.Plunger cannot enter
receptacle
1.Cancelled journey
1.Secondary catch cannot enter
secondary receptacle
•Cannot close bonnet in required
position
1.R.H. and/or L.H. plungers
incorrectly adjusted
1.Customer annoyance
•Cannot lock bonnet
1.R.H. and/or L.H. plungers
incorrectly adjusted (length)
•Cancelled journey
1.R.H. and/or L.H. receptacle
failure
•Difficult to close bonnet
•R.H. and/or L.H. plungers
incorrectly adjusted
1.High customer annoyance
•Incorrect plunger spring fitted
•Receptacle stiff to operate
•Difficult to lock bonnet
1.R.H. and/or L.H. plungers
incorrectly adjusted
•Customer dis-satisfaction
1.Receptacle fails open
(intermittent)
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13
FMECA Techniques
• The FMEA can be implemented using a hardware or functional
approach, and often due to system complexity, be performed as
a combination of the two methods.
• Hardware Approach :
– Firstly this method lists individual hardware items analyzes their possible
failure modes.
– This method is used when hardware items can be uniquely identified from
the design schematics and other engineering data.
– The hardware approach is normally used in a bottom-up manner.
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FMECA Techniques
• Functional Approach :
– This approach considers the function of each item. Each
function can be classified and described in terms of having
any number of associated output failure modes.
– The functional method is used when hardware items cannot
uniquely identified.
– Basically, this method should be applied to when the design
process has developed a functional block diagram of the
system, but not yet identified
specific hardware to be used.
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14
Functional Block Diagram
• A functional block diagram is used to show how the different
parts of the system interact with one another to verify the critical
path.
• It is recommended to break the system down to different levels.
• Review schematics and/or other engineering drawings of the
system to show how different parts interface with one another by
their critical support systems to understand the normal functional
flow requirements.
• A list of all functions of the equipment is prepared before
examining the potential failure modes of each of those functions.
• Operating conditions (such as; temperature, loads, and
pressure), and environmental conditions may be included in the
components list.
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Typical FME(C)A Worksheet
Item
Potential
Failure
Mode
Potential
Effect(s) of
Failure
Function
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e
v
C
l
a
s
s
Potential
Cause(s)/
Mechanism(s)
Of Failure
O
c
c
u
r
Current
Design
Controls
Prevent Detect
FMEA
D
e
t
e
c
Action Results
R
P
N
Recommended
Actions
Response &
Traget
Target
Complete
Date
Action
Taken
S
E
V
O
C
C
D
E
T
R
P
N
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15
Failure Definitions
• Failure Mode & Cause – Potential failure modes, for each
function, are determined by examination of the functional
outputs contained on the system functional block diagram. A
bottoms-up approach is used where by analysis begins at the
component level, followed by analysis of subsequent or higher
system levels
• Failure Effects – The consequences of each postulated failure
mode is identified, evaluated, and recorded on the FMEA
worksheets.
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General
Item
Potential
Failure
Mode
Potential
Effect(s) of
Failure
Function
•
•
•
•
S
e
v
C
l
a
s
s
Potential
Cause(s)/
Mechanism(s)
Of Failure
O
c
c
u
r
Current
Design
Controls
Prevent Detect
D
e
t
e
c
Action Results
R
P
N
Recommended
Actions
Response &
Target
Complete
Date
Action
Taken
S
E
V
O
C
C
D
E
T
R
P
N
Assumptions should be included in the header.
Product/part names and numbers must be detailed in the header
All team members must be listed in the header
Revision date, as appropriate, must be documented in the header
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Function
Item
Potential
Failure
Mode
Potential
Effect(s) of
Failure
S
e
v
Function
C
l
a
s
s
Potential
Cause(s)/
Mechanism(s)
Of Failure
O
c
c
u
r
Current
Design
Controls
Prevent Detect
D
e
t
e
c
Action Results
R
P
N
Recommended
Actions
Response &
Target
Complete
Date
S
E
V
Action
Taken
O
C
C
D
E
T
R
P
N
• Function should be written clearly and must be precise so there is no change of
misinterpretation.
• Each function must have an associated measurable metric.
• EXAMPLES
– HVAC system must defog windows and heat or cool cabin to 70 degrees in all operating
conditions (-40 degrees to 100 degrees)
•
•
within 3 to 5 minutes
As specified in functional spec #_______; rev. date_________
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Failure Mode
Item
Potential
Failure
Mode
Potential
Effect(s) of
Failure
Function
S
e
v
C
l
a
s
s
Potential
Cause(s)/
Mechanism(s)
Of Failure
O
c
c
u
r
Current
Design
Controls
Prevent Detect
D
e
t
e
c
Action Results
R
P
N
Recommended
Actions
Response &
Target
Complete
Date
Action
Taken
S
E
V
O
C
C
D
E
T
R
P
N
• Failure modes be written clearly and must be precise so there is no change of
misinterpretation.
• There are 5 types of failure modes:
–
–
–
–
–
complete failure,
partial failure,
intermittent failure,
function out of specification
unintended function
• EXAMPLES
– HVAC system does not heat vehicle or defog windows
– HVAC system takes more than 5 minutes to heat vehicle
– HVAC system does heat cabin to 70 degrees in below zero temperatures
– HVAC system cools cabin to 50 degrees
– HVAC
system activates rear window defogger
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Effect(s) of Failure
Item
Potential
Failure
Mode
Potential
Effect(s) of
Failure
Function
S
e
v
C
l
a
s
s
Potential
Cause(s)/
Mechanism(s)
Of Failure
O
c
c
u
r
Current
Design
Controls
Prevent Detect
D
e
t
e
c
Action Results
R
P
N
Recommended
Actions
Response &
Target
Complete
Date
Action
Taken
S
E
V
O
C
C
D
E
T
R
P
N
• Effects must be listed in a manner customer would describe them
• Effects must include (as appropriate) safety / regulatory body, end user,
internal customers – manufacturing, assembly, service
• EXAMPLES
–
–
–
–
Cannot see out of front window
Air conditioner makes cab too cold
Does not get warm enough
Takes too long to heat up
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Severity Classification
• A qualitative measure of the worst potential
consequences resulting from the item/function
failure.
• It is rated relatively scaled from 1-10.
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Mil-Std-1629 Severity Levels
•
Category I - Catastrophic: A failure which may cause death or weapon
system loss (i.e., aircraft, tank, missile, ship, etc...)
Category II - Critical: A failure which may cause severe injury, major property
damage, or major system damage which will result in mission loss.
Category III - Marginal: A failure which may cause minor injury, minor
property damage, or minor system damage which will result in delay or loss of
availability or mission degradation.
Category IV - Minor: A failure not serious enough to cause injury, property
damage or system damage, but which will result in unscheduled maintenance
or repair.
•
•
•
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Severity
Item
Potential
Failure
Mode
Potential
Effect(s) of
Failure
Function
S
e
v
C
l
a
s
s
Potential
Cause(s)/
Mechanism(s)
Of Failure
O
c
c
u
r
Current
Design
Controls
Prevent Detect
D
e
t
e
c
Action Results
R
P
N
Recommended
Actions
Response &
Target
Complete
Date
Action
Taken
S
E
V
O
C
C
D
E
T
R
P
N
• Severity values should correspond with AIAG, SAE, etc.
• If severity is based upon internally defined criteria or is based upon
standard with specification modifications, a reference to rating tables with
explanation for use must be included in FMEA
• EXAMPLES
–
–
–
–
Cannot see out of front window – severity 9
Air conditioner makes cab too cold – severity 5
Does not get warm enough – severity 5
Takes too long to heat up – severity 4
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Classification
Item
Potential
Failure
Mode
Potential
Effect(s) of
Failure
S
e
v
Function
C
l
a
s
s
Potential
Cause(s)/
Mechanism(s)
Of Failure
O
c
c
u
r
Current
Design
Controls
Prevent Detect
D
e
t
e
c
Action Results
R
P
N
Recommended
Actions
Response &
Target
Complete
Date
Action
Taken
S
E
V
O
C
C
D
E
T
R
P
N
• Classification should be used to define potential critical and significant
characteristics
• Critical characteristics (9 or 10 in severity with 2 or more in occurrence
suggested) must have associated recommended actions
• Significant characteristics (4 thru 8 in severity with 4 or more in occurrence
suggested) should have associated recommended actions
• Classification should have defined criteria for application
• EXAMPLES
– Cannot see out of front window – severity 9 – incorrect vent location – occurrence 2
– Air conditioner makes cab too cold – severity 5 - Incorrect routing of vent hoses (too close
to heat source) – occurrence 6
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Cause(s) of Failure
Item
Potential
Failure
Mode
Potential
Effect(s) of
Failure
Function
S
e
v
C
l
a
s
s
Potential
Cause(s)/
Mechanism(s)
Of Failure
O
c
c
u
r
Current
Design
Controls
Prevent Detect
D
e
t
e
c
Action Results
R
P
N
Recommended
Actions
Response &
Target
Complete
Date
Action
Taken
S
E
V
O
C
C
D
E
T
R
P
N
• Causes should be limited to design concerns
• Analysis must stay within the defined scope (applicable system and interfaces to
adjacent systems)
• Causes at component level analysis should be identified as part or system
characteristic (a feature that can be controlled at process)
• There is usually more than one cause of failure for each failure mode
• Causes must be identified for a failure mode, not an individual effect
• EXAMPLE
–
–
–
Incorrect location of vents
Incorrect routing of vent hoses (too close to heat source)
Inadequate coolant capacity for application
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Occurrence Classification
Description
10 >= 50% (1 in two)
9 >= 25% (1 in four)
8 >= 10% (1 in ten)
7 >= 5% (1 in 20)
6 >= 2% (1 in 50)
5 >= 1% (1 in 100)
4 >= 0.1% (1 in 1,000)
3 >= 0.01% (1 in 10,000)
2 >= 0.001% (1 in 100,000)
1 Almost Never
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Occurrence
Item
Potential
Failure
Mode
Potential
Effect(s) of
Failure
Function
S
e
v
C
l
a
s
s
Potential
Cause(s)/
Mechanism(s)
Of Failure
O
c
c
u
r
Current
Design
Controls
Prevent Detect
D
e
t
e
c
Action Results
R
P
N
Recommended
Actions
Response &
Target
Complete
Date
Action
Taken
S
E
V
O
C
C
D
E
T
R
P
N
• Occurrence values should correspond with AIAG, SAE
• If occurrence values are based upon internally defined criteria, a reference must be
included in FMEA to rating table with explanation for use
• Occurrence ratings for design FMEA are based upon the likelihood that a cause may
occur, based upon past failures, performance of similar systems in similar
applications, or percent new content
• Occurrence values of 1 must have objective data to provide justification, data or
source of data must be identified in Recommended Actions column
• EXAMPLES
–
–
–
Incorrect location of vents – occurrence 3
Incorrect routing of vent hoses (too close to heat source) – occurrence 6
Inadequate coolant capacity for application – occurrence 2
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Current Design Controls
Item
Potential
Failure
Mode
Potential
Effect(s) of
Failure
Function
S
e
v
C
l
a
s
s
Potential
Cause(s)/
Mechanism(s)
Of Failure
O
c
c
u
r
Current
Design
Controls
Prevent Detect
D
e
t
e
c
Action Results
R
P
N
Recommended
Actions
Response &
Target
Complete
Date
Action
Taken
S
E
V
O
C
C
D
E
T
R
P
N
• Preventive controls are those that help reduce the likelihood that a failure mode
or cause will occur – affects occurrence value
• Detective controls are those that find problems that have been designed into
the product – assigned detection value
• If detective and preventive controls are not listed in separate columns, they
must include an indication of the type of control
• EXAMPLES
–
–
–
–
Engineering specifications (P) – preventive control
Historical data (P) – preventive control
Functional testing (D) – detective control
General vehicle durability (D) – detective control
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Detection rating
• A numerical ranking based on an assessment of
the probability that the failure mode will be
detected given the controls that are in place.
• It is rated relatively scaled from 1-10.
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Detection
Item
Potential
Failure
Mode
Potential
Effect(s) of
Failure
Function
S
e
v
C
l
a
s
s
Potential
Cause(s)/
Mechanism(s)
Of Failure
O
c
c
u
r
Current
Design
Controls
Prevent Detect
D
e
t
e
c
Action Results
R
P
N
Recommended
Actions
Response &
Target
Complete
Date
Action
Taken
S
E
V
O
C
C
D
E
T
R
P
N
• Detection values should correspond with AIAG, SAE
• If detection values are based upon internally defined criteria, a reference must be
included to rating table with explanation for use
• Detection is the value assigned to each of the detective controls
• Detection values of 1 must eliminate the potential for failures due to design
deficiency
• EXAMPLE:
–
–
–
–
Engineering specifications – no detection value
Historical data – no detection value
Functional testing – detection 3
General vehicle durability – detection 5
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Rate the Risks Relatively
• A systematic methodology is used to rate the risks relative to
each other. The RPN is the critical indicator for each failure
mode. The RPN is a function of three factors: The Severity of
the effect, the frequency of Occurrence of the cause of the
failure, and the ability to Detect (or prevent) the failure or effect.
• RPN = Severity rating X Occurrence rating X Detection rating
– The RPN can range from a low of 1 to a high
of 1,000
– Higher RPN higher priority to be improved.
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RPN (Risk Priority Number)
Item
Potential
Failure
Mode
Potential
Effect(s) of
Failure
S
e
v
Function
C
l
a
s
s
Potential
Cause(s)/
Mechanism(s)
Of Failure
O
c
c
u
r
Current
Design
Controls
Prevent Detect
D
e
t
e
c
Action Results
R
P
N
Recommended
Actions
Response &
Target
Complete
Date
S
E
V
Action
Taken
O
C
C
D
E
T
R
P
N
• Risk Priority Number is a multiplication of the severity,
occurrence and detection ratings
• Lowest detection rating is used to determine RPN
• RPN threshold should not be used as the primary trigger for
definition of recommended actions
• EXAMPLE
–
–
–
–
Cannot see out of front window – severity 9,
incorrect vent location – occurrence 2,
Functional testing – detection 3,
RPN - 54
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Recommended Actions
Item
Potential
Failure
Mode
Potential
Effect(s) of
Failure
Function
S
e
v
C
l
a
s
s
Potential
Cause(s)/
Mechanism(s)
Of Failure
O
c
c
u
r
Current
Design
Controls
Prevent Detect
D
e
t
e
c
Action Results
R
P
N
Recommended
Actions
Response &
Target
Complete
Date
Action
Taken
S
E
V
O
C
C
D
E
T
R
P
N
• All critical or significant characteristics must have recommended actions
associated with them
• Recommended actions should be focused on design, and directed toward
mitigating the cause of failure, or eliminating the failure mode
• If recommended actions cannot mitigate or eliminate the potential for
failure, recommended actions must force characteristics to be forwarded to
process FMEA for process mitigation
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Responsibility & Target Completion Date
Item
Potential
Failure
Mode
Potential
Effect(s) of
Failure
S
e
v
Function
C
l
a
s
s
Potential
Cause(s)/
Mechanism(s)
Of Failure
O
c
c
u
r
Current
Design
Controls
Prevent Detect
D
e
t
e
c
Action Results
R
P
N
Recommended
Actions
Response &
Target
Complete
Date
Action
Taken
S
E
V
O
C
C
D
E
T
R
P
N
• All recommended actions must have a person assigned
responsibility for completion of the action
• Responsibility should be a name, not a title
• Person listed as responsible for an action must also be listed as a
team member
• There must be a completion date accompanying each
recommended action
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FMEA
Page 49
Action Results
Item
Potential
Failure
Mode
Potential
Effect(s) of
Failure
Function
S
e
v
C
l
a
s
s
Potential
Cause(s)/
Mechanism(s)
Of Failure
O
c
c
u
r
Current
Design
Controls
Prevent Detect
D
e
t
e
c
Action Results
R
P
N
Recommended
Actions
Response &
Target
Complete
Date
Action
Taken
S
E
V
O
C
C
D
E
T
R
P
N
• Action taken must detail what actions occurred, and the results of those
actions
• Actions must be completed by the target completion date
• Unless the failure mode has been eliminated, severity should not change
• Occurrence may or may not be lowered based upon the results of actions
• Detection may or may not be lowered based upon the results of actions
• If severity, occurrence or detection ratings are not improved, additional
recommended actions must to be defined
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25
Criticality – Mil-Std-1629
Approach
• Occurrence is a measure of the frequency of an
event.
– May be based on qualitative judgment or
– May be based on failure rate data (most common)
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FMEA
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Criticality Analysis
• Qualitative analysis:
– Used when specific part or item failure rates are not
available.
• Quantitative analysis:
– Used when sufficient failure rate data is available to
calculate criticality numbers.
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Quantitative Criticality
Analysis
• Define the reliability/unreliability for each item, at a given operating
time.
• Identify the portion of the items unreliability that can be attributed to
each potential failure mode.
• Rate the probability of loss (or severity) that will result from each
failure mode that may occur.
•
•
– Calculate the criticality for each potential failure mode by obtaining the product of
the three factors:
– Mode Criticality = Item Unreliability x Mode Ratio of Unreliability x
Probability of Loss
Calculate the criticality for each item by obtaining the sum of the criticalities for each
failure mode that has been identified for the item.
Item Criticality = SUM of Mode Criticalities
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FMEA
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Quantitative Analysis
• Calculate the expected number
of occurrences over a specific time interval.
• Many different methods are used
– Use handbook reliability data
– Use past experience
– Uses various Bayesian combinations of past
experience data and expert
judgement
– Uses other analysis methods (RBD, FTA etc.)
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Qualitative criticality analysis
• To use the method to evaluate risk and prioritize
corrective actions, the analysis team must:
– Rate the severity of the potential effects of failure.
– Rate the likelihood of occurrence for each potential
failure mode.
– Compare failure modes via a Criticality Matrix, which
identifies severity on the horizontal axis and
occurrence on the vertical axis.
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FMEA
Page 55
Qualitative Analysis
•
•
•
Because failure rate data is not available, failure mode ratios and failure
mode probability are not used.
The probability of occurrence of each failure is grouped into discrete levels
that establish the qualitative failure probability level for each entry based on
the judgment of the analyst.
The failure mode probability levels of occurrence are:
–
–
–
–
–
Level A - Frequent
Level B - Probable
Level C - Occasional
Level D - Remote
Level E - Extremely Unlikely
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FME(C)A Checklist
•
•
•
•
•
•
•
•
System description/specification
Ground rules
Block Diagram
Identify failure modes
Failure effect analysis
Worksheet (RPN ranking)
Recommendations (Corrective action)
Reporting
PEUSS 2011/2012
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Page 57
The results of the FME(C)A
• Highlight single point failures requiring corrective
action
• Rank each failure mode.
• Identify reliability, safety critical components
• FMECA is a living document
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Integrated FMECA
• FMECAs are often used by other functions such as
Maintainability, Safety, Testability, and Logistics.
– Coordinate effort with other functions up front
– Integrate as many other tasks into the FMECA as possible
and as make sense (Testability, Safety, Maintainability, etc.)
• Integrating in this way can save considerable cost over doing the
efforts separately and will usually produce a better product.
• If possible, use the same analyst to accomplish these tasks for the
same piece of hardware. This can be a huge cost saver.
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FMEA
Page 59
FMECA Facts and Tips
• FMECAs should begin as early as possible
– This allows the analyst to affect the design before it is set in
stone.
– If you start early (as you should) expect to have to redo portions
as the design is modified.
• FMECAs take a lot of time to complete.
• FMECAs require considerable knowledge of system operation
necessitating extensive discussions with software/hardware Design
Engineering and System Engineering.
• Spend time developing ground rules with your customer up front.
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30
Exercise : Flashlight
This flashlight is for use by fire and rescue operative involved in
emergency operation to rescue people from fires, floods and other disasters.
Perform an FMECA on the torch.
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Flashlight (cont.)
How can it fail?
What is the effect? Note
that Next Higher Effect =
End Effect in this case.
Part
Item
Failure Mode
End Effect
bulb
dim light
no light
flashlight output dim
no flashlight output
switch
stuck closed
stuck open
interm ittent
constant flashlight output
no flashlight output
flashlight sometimes will not turn on
contact
poor contact
no contact
interm ittent
flashlight output dim
no flashlight output
flashlight sometimes will not turn on
battery
low power
no power
flashlight output dim
no flashlight output
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31
Simple Example: Flashlight
(cont.)
• Severity
– Severity I
– Severity II
– Severity III
– Severity IV
Light stuck in the “on” condition
Light will not turn on
Degraded operation
No effect
PEUSS 2011/2012
FMEA
Page 63
Simple Example: Flashlight
(cont.)
Item
Failure Mode
End Effect
bulb
dim light
no light
flashlight output dim
no flashlight output
III
II
switch
stuck closed
stuck open
interm ittent
constant flashlight output
no flashlight output
flashlight sometimes will not turn on
I
II
III
contact
poor contact
no contact
interm ittent
flashlight output dim
no flashlight output
flashlight sometimes will not turn on
III
II
III
battery
low power
no power
flashlight output dim
no flashlight output
III
II
PEUSS 2011/2012
Severity
FMEA
Page 64
32
Simple Example: Flashlight
(cont.)
PEUSS 2011/2012
FMEA
Page 65
Simple Example: Flashlight
(cont.)
Can circled items be designed out or mitigated?
(There may be others that need to addressed also.)
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FMEA
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33
Summary
•
•
•
•
Defined FMEA
Difference between FMEA and FMECA
Standard approach and pro-forma
Applications
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34