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A study of abnormal occurence reports
Taylor, J.R.
Publication date:1975
Document VersionPublisher's PDF, also known as Version of record
Link back to DTU Orbit
Citation (APA):Taylor, J. R. (1975). A study of abnormal occurence reports. Risø National Laboratory. Risø-M No. 1837
A. E.K.Risø R isø -M- [S
I
Titte and author(s)
A STUDY Or ABNORMAL OCCURRENCE REPORT.
«»K» i ' a v i o r
pages -f Ubles -f illustrations
Date S^-pt-r-cer l?v5
Department or group
Group's own registration numbers)
PM
Abstract
A detailed study of fai lu re and occurrence report
froxi five U.S. nuclear r e a c t o r s . Data concerning the
frequency of f a i lu re causer and - u i t i f a i l u r e incident ;
are given.
Methods of f a i lu re c lanr i f ' ico t ic- , a r - difjcunr-a.
Copies to
Available on request from the Library of the Danish Ate nic Energy Commission (Atomenergikommissionens Bit ©tek). Rise, BK-4000 Roskilde, Denmark Telephone; (OS) 35 51 01, est, 334, telex: 43116
LEW 9? 550 0J72 9
TABI£ OF COWTETT5
Page
Foreword • I
Introduction • 1
Choice of data . .* 2
Classification of occurrences 2
Event sequence . — 5
Canton node and coupled failures 11
Failure cause 11
Data l*i
Causes of failure 1*+
Hultifailure incidents 24
Conclusions 26
Appendix 1* Classification of failure events 2&
Appendix 2. Data 32
Foreword
Reports are prepared by engineers in U.S. nuclear power piar.*;: for u vide
range of component failures and similar occurrences. I hi- pr-vcii- reporting re
quirements are specified in the U.S. Nuclear Regulatory Coi?.-issicn£ Regulatory
Guide 1.16. Until 197i*t such reports were termed Abnorssai Occurrence reports.
but since then the term "Abnormal Occurrence reports" has beer, reserved for
those occurrences which have seme safety significance. What were earlier tersed
"Abnormal Occurrences" are now as a general class, t erred Reportable- Occurrences.
The reports are published by the US NRC. The work here ccr.cerr.r a study cf
"Abnormal Occurrence Reports", using the pre 19?** definition and data up tc and
including spring 197^• The data cover a wide range of occurrence types. In. par
ticular they give component failure data mostly of relatively uni-portar.t indi
vidual component failures, such as miscalibration of a single redundant instru-
ment, but in sone cases for failures of sorse engineering significance. The re-
forts are much less forssal than those required for reliability data oanks. They
give considerably more background information concerning t'.ie cause ard natur--
of failures than do reliability data banks; and also much -nore information
concerning consequences and interrelationships between separate failures.
For this reason, the reports provide very valuable information, which is
relevant not only to the study of nuclear power plant reliability, but also
provides insight into the way failures can occur in many different kinds of
process plant.
- 1 -
A STUDY OF ABNGSKAL OCCURRENCE REPORTS
latroductiori
An earlier study (Ris^-K-17^2) attested to evaluate the rcle of design
etr<*rs in nuclear power plant reliability. The results of that study showed
that design error plays a large role ir. cover plant failure: a.-.d the surprising
result that an unexpectedly large proportion of failure Incidents involve sev
eral independent component failures.
Several questions arose as a result of that earlier work. Gr.e would ~x-
ptet- that the role of design error diminishes for individual plants as they
grow older. It was decided to investigate the senier.ce of afcr.or.Tial occurrence
reports from individual reactors over a number of years.
Another question is the extent tc which separate failures in rnulti-failure
incidents contribute significantly to the failure consequence. It is possible
that several of the failures within an incident have no direct bearing on the
extent of failure consequences. They may play an incidental rather than crucial
par* in the failure sequence.
The remaining area of interest for this study is the problem of common mode
failure. The importance of common sod** failure was established earlier, but the
only datum obtained was a gross figure for the proportion of failures involving
common mode effects. Results of the coisror. 20de failure study performed here
are published separately (Ris2-K-l826).
A major eleaent in motivating this study, was the desire to discover the
weak points in existing techniques of failure sode analys:s of process plant,
and to develop the background information for improving those techniques. There
are some types of failure for which no systematic analysis technique exists,
for erample
- wiring errors involving incorrect interconnections
- "system design" errors in control systems
- mechanical blockage and jamming problems arising from loose parts
- errors in written procedures
- human eri*ors due to confusion between procedures or misinterpretation
of operating situation.
All of these problems involve complex conmon mode effects, and it is
important to discover to what extent they are important ir. practice.
-2-
The procedure in this study has been to take individual incident reports,
and to classify then according to fixed criteria. The nature o; the data prevents
one from obtaining good statistical data with known significance, but it is
felt that qualitative, and "order of Magnitude" conclusions can be drawn from
the results.
Choic* of data
The data used for this study is abnormal occurrence data submitted to the
BSdC by operators of light water reactors. The reason for this choice is that
the iaforaation is readily available, there are consistent criteria for reporting
the data (reporting is required by law}, and the quality of the reporting is
generally excellent. The information differs from that usually available in
reliability data banks, in that complete failure incidents are described, often
involving several individual component failures.
The choice of reactors for this study was determined by the availability
of records for a period of years. Records from the earliest years of reactor
operation were however not available to the author. There has been a change in
style of reporting over a number of years, and this has to some extent negated
the value of the data as a record of "design error" evolution.
Classification of occurrences
For each occurrence report, the date, six month period lumber (from
reactor start up), operating state at the time of occurrence, and method of
failure discovery were recorded. Most failures are detected during surveillance
testing, some via special inspections, but many are discovered as "actual"
failure incidents which interfere with plant operation. In many cases a failed
condition existed over a considerable period, but the plant state recorded
was nevertheless the plant state at the time the failure was discovered. In
virtually all cases it is true that a latent failure, discovered during sur
veillance or special testa, has existed while the plant was operating.
Kach individual component failure was recorded separately for each inci
dent, and in some cases there were several failures contributing to the inci
dent.
-3-
OP Plant operational, generating power.
SU Plant was in start up phase.
SD Plant was in shut down phase.
Cfi Plant was in cold shut down state.
SF Plant was shut down for refuelling.
Table 1. Classification of plant states at the time of failure discovery.
ACT "Actual" incident - occurs during normal operation of
component.
SOB Failure discovered during surveillance testing.
PM Failure discovered during post maintenance testing of
the failed component.
COM Failure discovered during coaaissioning tests.
SI Failure discovered during special inspection, as a
result of suspected incipient failure, or as a result
of information from other plants.
SO Failure discovered during start up testing.
Table 2. Classification of "mode of discovery" of failures.
In a study such as this, which is concerned with the cause of failure, it
is important to define the term component failure carefully. A failure is deemed
to have occurred, if a component is incapable of fulfilling its function, in
spite of the fact that inputs such as power supplies, control signals, mechanical
support, etc. are within the limits specified for the component. Failures due
to incorrect input are judged to be consequent failures, and were recorded for
interest, but were excluded from statistical analyses. Failures due »o environ
mental changes were recorded as component failures, unless the environmental
change was a result of some earlier component failure in which case they were
classified as consequent failures, and again omitted from statistical analyses.
The degree to which a plant is divided into "components" also affects the
number of component failures recorded. In this study, a standard level of div-
- < t -
islon into components was used, as expressed by table J. Hoverer, where a
component was part of a larger component or subsystem, this faet was recorded
by concatenating component and system names.
Amplifier
Anuunciaton
Battery
Battery charger
Cable
Capacitor
Circuit breaker
Magnetic clutch
Control Bwitch
Coil
Diode
Detector
DC power supply
Flow switch
Beater
Input module
Inverter (solidstate)
level switch
lamp
Limit switch
Manual switch
Meter, guage
Motor
AM
AN
BT
BC
CAB
CAP
CB
CL
C£
CO
DI
DE
DC
FS
BC
IM
IN
IVL
UfP
LHT
SV
ac MO
Motor starter
Potentiometer
Recorder
Lightning arrester
Ground switch
Relay
Relay or switch contact
Reset switch
Resistor
Signal comparator
Pressure switch
Torque switch
Temperature switch
Fuse
Generator
Heat tracing
Test button
Thermal overload
Transformer
Transmitter
Wire
Solenoid
MS
POT
SEC
IA
OS
SE
CN
RS
RST
COMP
PS
TQS
TS
F0
OE
HT
SB
OL
TFMR
TKTR
W
SOL
Table 3A. Electrical component coding.
•5-
Accumulator
Blower
Control rod drive
Control rod
Cover plate
Core
Damper
Diesel
Expansion joint
Filter, strainer
Flexible pipe, hose
Fuel
Gas bottle
Oaekat
Heat exchanger
Insulation (thermal)
Ion exchanger
Noggle
Orifice
Pipe
Pipe Cap
Pipe Support
Pressure vessel
Pump
AC
BL
CRD
CR
COV
COS
m DBS
XJ
n. FLEX
F
GB
GK
HEX
TINS
HX
NZ
OS
PP
CP
COP
FV
PM
Refrigeration unit
Sluice
Sump
Tank
Tubing
gate
Turbine
Condenser
Vent
Well
Valve,
Seal
check
explosive
hydraulic
notarised
pneumatic
relief
manual
safety
atop
vacuum relief
main steam isolation
solenoid
Actuating mechanism
RF
SL
SP
TK
TUB
TURB
COND
VT
WL
CV
EV
HV
MV
AV
SV
XV
SV
DV
vv MS IV
CT
SL
ACT
Table 3B. Mechanical component coding.
Event sequence
The structure of event sequences has a dirsot bearing on the way in which
failure records are interpreted and used in later failure node analyses. Failures
were classified as spontaneous, gradual, aiaoperation, latent or consequent.
A spontaneous failure is on* which occurs at the tin of the incident and
serves to start the incident. The classification of "gradual" failures was
introduced because it was difficult te describe some kinds of Initial failures
SPONTANEOUS
FALURE
CONSEQUENT
FAUURE
•FAIURE OF COMPONENT
W OPERATION
MISSILE EFFECT
OVERHEATING
LOSS OF POWER ETC
Fig. 1 Evwit MOJMUCC wrftft ^oMoncow ond consttjntnt foHuras
MITIAL EVENT CAUSE
- SPONTANEOUS fiULURE
-ROUTINE OPERATING EVENT
-SURVELLANCE TEST
fen
INITIAL
EVENT
2 failura
COMPONENT X OPERATES?
NO VES
Fig. 2 EwntMqwnc* wHh latanl faHw*
-5-
FAILURE IS DETECTED
BY INSPECTION
FURTHER CONSEQUENT
FAILURE
Fig. 3 Alternativt gradual '
mMtng from
E.G. START OF MOTOR
OR HUMAN OPERATION
MISOPERATION ACTION
SUCCESSFUL
Pfe4.- fwMI
-lo-
as spontaneous, for example slow leaks which are detected after they tare caused
farther damage.
latent failures occur in components which are callod on to work intermit
tently. Such components may exist in a failed state, which i« "revealed" when
the conponent is tested or is called on to operate.
Kisoperation failures are those which occur when a component or operater
is called on to carry out such operation, and which occur while the conponent
or operator is carrying out the operation.
Consequent failures are those which occur as a direct result of soeje
earlier failure.
S Spontaneous
G <temdual
L latest
K Misoperation
C Consequeut
table •>. Classification of failures according to how they are
triggered.
farther classification of "consequent" failures is possible - Input
failures (I) fi mi i1 failures), overload or stress failures (secondary fail
ures) (0), and direct effect failures (D). Such classification is often Bade
la failure node analysis studies. Bat such classification was Bade only for
coaaon node failures, 1B this study.
Symbolic description of the different kinds of failure are shown in fig.
1 to *» (see KUe Report Kise-H-17->3) • For each of the different kinds of fall
ure, a different nodel is required to describe failure probability.
as a result of the study, sone of the Initial ideas on classification of fail
ures were revised, and these ideas, which were net used in the study itself.
are given i n Appendix 1.
One of the important objectives of thin study was to ohmene the neafter
of failure events occurring in actual failure sequences. For this purpose,
failures detected by testing were ignored. Mao, several failures occurring
within similar components due to a common node effect, were treated as single
failures. Consequent failures which were certain to occur, given earlier fail
ures, is the sequence were ignored. But consequent failures which involved some
probability factor, such as destruction of component« by Impinging stsaa Jets,
were counted as separate failures.
-11-
Coamon mode and coupled failures
As part of the study of multiple failures, a study was made of common mode and coupled failures in similar plant components. The results of this sttriy are iresented in a separate report.
* Failure cause
The objective in this study has been to come as close as possible to the
original cause of failure. Failure causes are classified at two levels, as
shown in table ->. The second level of classification is much less certain tha:.
the first. In the ease of operator and maintenance errors, the subclaseification
was completely experimental.
To maintain consistency of cause classification it is important to have
clear criteria. An error warn considered a design error if it was explicitly
described as much in the abmrmal occurrence report, if it was one of a long
series of similar failures with very high failure rate, or if the design was
modified as a result of the failure. A similar criterion was used for classi
fying procedural errors. Failures were classed as operator errors if this was
explicitly stated in the abnormal occurrence report, and similarly for main
tenance and installation errors. (This can lead to underestimation of operator errors)
Random component failures were recorded in those cases where a simple
standard mechanism of component failure was involved e.g. bearing leakage,
shorting of a relay coil etc., and in which no excessive grouping of failures
of a similar kind occurred.
In some cases, more than one cause of component failure could be discovered.
In other cases, it was difficult to judge between two alternative failure causes.
In these cases, fractional contributions to failure classes were recorded, an
equal fraction to each contributing cause.
In many cases, the same kind ef failure occurred in the same component
several times in the course of a few years. These cases were counted as single
failures in determining relative importance of different failure causes (though
all incidents were counted in determining coaaon mode failure proportions). If
one does not count failure causes in this way, then sone frequently occurring
failure types come to dominate the distribution of causes. The proportion of
design error failures, in particular, becomes Inflated.
-12-
Cause subclass
C Random component failure
S Design error
0 Operator error
H Mechanical
E Electrical
0 Problem unknown at design time
C Complex system interactions
1 Interdisciplinary problems
0 Oversight
K Coiiiniication problems
Z Calculation, sizing problems
S Component selection problem
0 Omission
X Unnecessary extra operation
*" wrong target of operation
E Error in amount of operation
ft Error in operation sequence
P wrong procedure used
J Judgement of quantity
C Communication problem
R tack of recognition of danger
situation
H Precognition of danger situation
Table *t. Coding for causes of failure.
-13-
Cause Cause subclass
H Maintenance error
I Installation error
P Procedure error
F Fabrication fault
? Cause unknown
A Adjustment (of instruments, switches)
0 Omission of step in installation,
repair
P Positioning of component
H Misuse of component, handling
problem
B activation of other equipment not
under repair
C Choice of component to install.
repair
1 Interchange of two components,
cables etc.
Quality of join
Omission of subprocedure
Extra control, checking required
Procedure open to misinterpretation
Effect unknown before failure
Procedure wrong
Table **. Continued.
1
-la-
Beta
Abnormal occurrence reports frem ri«« reactera ven classified for react««
with start up dates in 196a , 196}, 1*7, 1969, and 197o. Abnormal occurrence
reports were generally available to the aether only rroa tk* later years of
reactor operation (fros 1969 onward}.
Both the number and character of abnormal occurrences vane* greatly froa
reactor to reactor. The eariatiaa coald ham aria« froa the differaat emmntity
and type of equipment at the realtor plant, as veil as dir ferene« in reliability
of components. Hoverer, it was hoped that bj concentrating on the eroaortion
of occurrence« of different types, atsnirgful coatlaaions could he draan.
Ås can be seen froa fig. 7 not too aach significance can he attached to
the actual nuabers of abaoraal occurrence reports for successive years.
In addition to abnormal occurrence reports, aoae "unusual m a t " reports
ware included in the analysis, where the reports concern safety related or
pressure boundary equipment (see BSLK safety (aide 13.2. for definition of
abnorasl occurrence, unusual event).
There say be soae Missions of abnormal occurrence reports for the reactors
studied, though where possible records were checked against semiannual operating
reports. On the assumption that omissions are randomly distributed, the effect
on proportions of failure' types should not be too iaportant.
Is all there were 67, 2<t, 33, 1W. 75 abaoraal occurrences for the respect
ive reactors.
Causes of failure
Fig. 8 and 9 show bow the various causes of failure behave as plants grew
older. No significant trends can be detected, the proportion of failures due
to respective causes seen if anything aore or less constant. Bat such more
data would be required, before trends could he detected beneath the nodes)
variations in the data. As has been observed in esrlier studies, design error
seems to be the dominant cause of failure, followed by random component failure.
Table 5 gives the numbers of failures attributed to particular cause classes,
for each of the reactors studied. The pattern ia aore or leas constant, appart
froa an unusually high proportion of failures attributed to eaintanaace errora,
for one of the older reactors, and a coaplete lack of reported operator errors
for one reactor.
The classification of secondary causes for design, operator, procedure, and
aaintenanee errors are shown in tables 6 to 9. Tot classifications bare are
sxptrinental, and should be regarded as indicative rather than definite. Sash
slaaslfieations can be used as a guide to qualitative studies of failure causes.
INSUFFICIENT DATA
I t l i i l 1
• « a u w Fig. > PROPORTION OF OPERATOR AND
PROCEDURE ERRORS
20 22 2* HALF YEAR
* Q P A U '•• I I
I
INSUFFKCNT OAU
D M/J
—»— 10
- 4 — 12 W W W
Fig. S PROPORTION OF COMPONENT, DESIGN, AND, MAINTENANCE/INSTALLATION ERRORS
20 22 I
2* HALF YEAR
20
-18-
6 4*
å
1
2
3
4
5
ALL
b b 09
& •H
3 7 *
1*
48
>4
Jl*
3 5 *
lure
C
8
1 O
6 0
1 PS
u *
11
i i
17
23
1 8 *
M O
fc V b o ** m b
1 1 0 *
l i
0
12
19
12 *
* •o
u
1 B
•M
U
I 1 8 *
11
7
9
7
1 0 *
llati
on
S 3 -*4 b O
* u i ** b c o
•H b
2 .
1 6 *
i£
5
10
6
12 *
'ft b • o
•H
S •H
I i *
0
0
0
3
1 *
b O
•S
1 <-4
S« , • ^ T
°s • o
l i 7 *
18
10
17
7
1 2 *
1 •a -d
m it. •H *
b •** O » b £ a 3
1?
82
28
42
174
96
1.22
Table 5. Cause classes for component failures.
^»••epsj iPSH
-19-
Conponeat selection 14
Oversight 17
Error due to effect unknown at
design tine 25
Sizing, dimensioning error 13
Error due to lack of recognition of
conplex systea interactions 7
Error due to connunication probleas 1
Error with cause unknown or unrecorded 22 Table 6. Design error secondary causes
( Based on 147 occurrences)
% of error
Omission of a step, operation or
procedure (reason for oaiasion
unknown) **9
Wrong procedure used 16
lack of recognition of situation 7
Misrecognition of situation 2
Error in operation sequence **
Operation applied to wrong target
component *t
Error of judgement of asount 2
Error due to CIIIHMIideations problem,
lack of coBflunication h
Error in aaount of adjustaent 2
Error due to unknown cause 9
Table 7* Secondary causes of operator errors.
(Based on 77 occurrences)
Error due to oadssion of step
H Error due to oaiasion, because
effect was unknown at the tise
the procedure was defined 16
If Efcoeedure was open to Misinterpretation,
unclear 7
F Wrong test frequency specified 2
V Wrong procedure specified 2
C Extra control required - procedure
does not contain sufficient cross
checks 6
? Error with unknown cause 1**
Table 8. Secondary causes of errors in procedures.
(Based on Mt occurrences)
-22-
A Problem with adjustment of instnmnts,
lindt, torque switches ate.
V Wrong operation carried out, or
right operation carried oat wrongly,
due to lack of knowledge or expertise
B Spurious activation of other equipment
while carrying out testa or repair
I Interchange of two cables
0,0 Omission of operation, doe either to
oversight or to ignorance of requirement
P Srror positioning component
<i Probleas of quality in soldering,
welding
B Error due to lack of recognition of
situation
C Krror in choice of which component
to repair
S Breach of safety regulations
1 Error due to unidentified causes 2$
Table 9, Secondary cause« of installation and maintenance
errors.
' (Based on W occurrences)
16
-23-
It is possible to make some qualitative comments on the results.
A lsrge proportion of design errors involve effects which were unknown
before failure occurred. Many failures ot this type occur repeatedly, the
same component sometimes being repaired several times before the failure is
correctly diagnosed. Such incidents underline the value of abnormal occurrence
reporting.
Another large group of design errors involve inappropriate choice, of
materials, or especially, of instruments. Problems of this kind can be reduced
by qualification testing and standardisation, activities which are receiving
a great deal of attention from nuclear engineers.
By far the largest proportion of operator errors involve omission or
oversight, involving just a single type of plant operations. By their nature,
such errors are relatively easy to forsee, and analyse, even in the cases
where several components are affected in a common mode fashion. More serious
are the errors due to lack of recognition of dangerous situations, misrecog-
nition, application of inappropriate procedures or application of correct
procedures to the wrong component. Among installation and maintenance errors,
difficulties in adjusting limit switches and torque switches are outstanding.
Some types of failure are difficult to account for in failure analysis.
It is difficult to identify all of the failures of this kind, but the following
provides a list of errors which occurred, but for which no systematic failure
analysis procedure exists (as yet).
loose parts jamming 5
electrial circuit omissions or miswiring 5
omission of essential procedural steps,
or incorrect steps 3
human decision errors with wide ranging effects 3
established trip levels inappropriate 6
water hammer affeots 3
Also, there were some instances of problems present special difficulties in
failure mode analysis.
common dependence of several components on one
service supply or environment 7
- iå-
lailtifeilnre incidents
ID thi« study, as in the previous one, the number af aaltiple fallere incidents was high when cenpared with esptctatioa. At the level of conaeeaeace repraaantcd by abnorasl occarrancc reporte, there ara at i l l a significant saauer of *, 5, 6, ana 7 fold failar* iocidenta <fig. 8 ) .
Ihia patters hold* traa in epite af the fact that
a> failure« to several csapoaents af the aaaa type, dae to
th* aaas caaaa, han been treated aa aiagle failaraa.
b) failaraa ahich ara a direct »naiaaaa« of earlier failar*«, are not coanted in arriving at the naaeer of independent failsrea.
c) failarea ahieh do not contribute significantly to the
consequences of the incident ham been ignored.
asaeining the nature of the aoltiple failara incidenta reveals aereral diatinct type«.
blovdovn incidents, in ahieh steaa i s released, ceasing defects in wuruuuding coaponents to be mea led , in Beat cases, and eaasiaf a large variety of safety esaponeats to be activated.
anltiple haaan error«. It i s clear that i s sos» of these incidents there i s coapling between the errors. Once one haaan being baa aade an error, others tend to parpetaste i t . Boweer, the nature and »egret of interdepeadence of these errors i s difficult to deteraiae.
- because of the »ay that these data are classified, i f single consonant fa i l s to »ark IHIBBBI I t s amliii i s inappropriate and ita operating procedure i s incorrect, the result i s counted as a doable fails**. This "elassifisatioa effect" i s significant in raising the naaber of 2 and J fold failure«. It does not contribute eignificantly to the naaber of •», S, 6 fold failaraa ete.
•y far the largest proportion of failure types in the aultipls failure incidents are latent failures revealed daring special lneideat conditions, or revealed »hen safety equipment i s activated.
HO. OF MODEMS
H
H
a l H
• 3 /
W V vx r UL& 11 10
s . * 7 NO. OP FAHUrW "St MODENT
r% • rstTOOrUM OF MULflrU raatvLW H O B t l a l l
-26-
Couclusions
Conclusions as to the meaning and importance of the kind of results given
here have been presented before (Rise—M-17**2). The additional data collected
hare serve merely to reinforce those conclusions which are
1) That design and other human errors are responsible for a significant
number of failures and abnormal occurrences.
2) Some "design error" type failures cannot be accounted for in tra
ditional types of reliability analysis.
3) Improvement in particular component reliability performance, in
testing, and in standardisation, should be valuable in improving
plant reliability, because a small number of component types are
responsible for a large proportion of failures.
*0 Multiple failure incidents play an unexpectedly large role in
abnormal occurrences. Records of interrelationships between
failures would be a useful addition to failure statistics data
bases.
In addition to these.remarks, some conclusions can be offered concerning
the type of classification study attempted here.
It would be useful to obtain some standardization of the terms used in
classifying different types of failure, according to the way in which failures
reveal themselves, the plant state at «he time of occurrence and/cr discovery,
and the triggering mechanism of the failure. The classification used here is
self consistent, but is different from schemes used elsewhere.
The classification of primary causes of failure seems acceptable, and
is similar to that use by the U8AEC (e.g. CCE-OS-001, 197'*). However the method
of classification used here, attributing a failure to a single class, or using
fractions to represent degree of responsibility for a particular causa, is
•assy. Accepting that a failure may have several causes, and that the per
centages of failures due to different causes may total to more t&an locje,
»erne preferable. But if this method of classification is accepted, sow defi
nition must be made of how important and unusual an effect oust be, before it
is accepted as a contributing cause of failure.
The classification of different failure causes into secondary categories
was not particularly successful. Often there was insufficient information in
the abnormal occurrence reports to make classification precise. And the cat-
-27-
egoriee used here often overtapped. The information in such classification
should not be used to derive percentages to which different causes are respon
sible for failures. The information might be used to perform clustering studies,
with the hope of finding more clearly identifiable types of failure.
Uncertainty as to cause of failure should not be used as an excuse for
assigning two causes to a particular failure. Instead, if there are several
clearly alternative causes, this fact should be recorded explicitly (e.g.
A/B aeans either cause A or cause B is involved, C H D/E means either cause
C and E are together responsible, or cause E is responsible). Failure for which
causes are unknown, should be recorded separately. Only in this way is it
possible to interpret the meaning of failure cause data.
A revised system of classifying different failures according to event
sequences, is given in appendix 1.
-28-
Affieodtx i. Classification of failure events
Classificatici of the different kind of failure events which can enter
into event sequences is useful and important, because it indicates the relevance
of particular pieces of failure data to different reliability calculation models.
In fact, as reliability models become more complex, more complex failure classi
fications are required. The scheme introduced here ie therefore just a particu
lar example of a range of possible schemes which differ in level of detail.
The first distinction is made between failures which are caused by some
external event or process, and those which arise with no apparent cause or for
which the failure cause is an inherent property of the component. This second
group is the one which has been called "random component failure" earlier in
this note* These are called spontaneous failures here. Examples are the normal
forme of bearing failure, relay contact failure etc., for which no specific cause
can be described, or which cannot be prevented in normal engineering practice.
Examples of typical causes in the first group of failures are design and instal
lation errors, extreme environmental conditions, misoperation by an operator etc.
A distinction which is equally important for obtaining a reliability model
of failure consequences is whether the effects follow instantaneously from the
failure, or whether the effects are gradual (e.g. slow leakage of some valuable
material).
When components operate Intermittently or only occasionally (such as safety
systems) or with intermittent load (such as many pneumatic orhydraulic systems)
a third distinction becomes important - whether the effects of the failure remain
latent, or whether the consequences show themselves inraediately. In the case of
a latent failure, a failure event occurs at some time. The component is reduced
to a state in which it cannot operate according to specifications. When the
component is called upon to operate (the failure is triggered), a "failure to
operate" occurs.
The definitions of the various failure types are illustrated in fig* 7*
-29-
eontinuing failure .
external cause
slow, continuing consequence
caused failures
instantaneous failure!
external cause
•'instantaneous consequence
latent failure \
external cause '
/delayed consequence for plant
internal or "normal" cause
slow continuing consequence for plant
\ x i internal or "normal" cause spontaneous instantaneous consequence for plant failure
^internal or "normal" cause
delayed consequences for plant
Fig. 7* Definitions of failure classes.
For latent failures, the stage at which the failure is discovered becomes
important, especially for systems in which automatic continuous testing and
periodic testing is performed.
A latent failure may remain hidden due to the fact that the particular
type of error is not exercised or triggered by the test inputs applied. These
failures are called "ttntriggered", Equally* a failure may be triggered, but
its effects may not be indicated because the failure alarm outputs and test
measurements performed, are inadequate. These failures are called unmonitored.
(Bee fig. 3 ) .
-Jo-
Stage at which failure pheaotwson occurs
kind of failure involved
latent
issediate
coatiBBOUS operation
failure in operation
during stand-by
latent failure
active failure
on activation
failure to operate
failure on deoand
•isoperation
Table lo. TeraiG used in describing latent and is«ediate failure sequences.
k useful distinction in Judging the effectiveness of testing, i s that
between actual failures and failures found under test . For actual failures
consequences occur which affect the operation of the plant. The following
relation i s true.
-Jl-
latent failure
repealed automatically \ /" untriggered
unrerealed J
1 unnonitored
revealed by test
/ j ontriggered in test
hidden failures J
= revealed by start up I unaonitored in teat
Fig. 8 . Types of latent failure.
Actual failures = hidden failures
+ failures in operation
+ active failures
+ aisoperations. not under test
Finally, it is useful, in classifying oultiple failure sequences, to
indicate whether a failure was initial (initiated the sequence), contributory
(independent, but triggered as a result of the initial failure, or increasing
the consequences of the initial failure), or consequent (caused by the initial
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R e s i s t o r overhes ted * burn -* broke - jairmed timer re lay
Col l a p « of package b o i l e r f l u e Hater hammer/vibration In core spray
Excess demand for rod waste s torage whi l e tank maintenance In progres s
Dump tank (CR. water hold up) l e v e l alarm r e l a y binding
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E x t r a c i r c u i t r y t o p r e v e n t problem
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