Table Of ContentAn engineer's view
of human error
An view
engineer's
of human error
Third edition
Trevor Kletz
The theme of this book:
Try to changes ituations, not people
IChemE
The information int his book is given in good
faith and beliefi n its accuracy, but does not
imply the acceptance of any legal liability or
responsibility whatsoever, by the Institution,
or by the author, for the consequences of its
use or misusei n any particularc ircumstances.
All rightsr eserved. Nop art of this publication
may be reproduced, stored in a retrieval
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Published by
Institutiono f Chemical Engineers (IChemE),
Davis Building,
165—189 Railway Terrace,
Rugby, Warwickshire CV21 3HQ, UK
IChemEi s aR egisteredC harity
© 2001 Trevor Kletz
Firste dition 1985
Second edition 1991
ISBN0 85295 430 1
Printedi n theU nitedK ingdom by Bell & Bain Limited, Glasgow
11
Foreword to the third edition
In this book I set down my views on human error as a cause of accidents and
illustratet hem by describinga numbero f accidents that have occurred, mainly
int he oil and chemicali ndustries. Thought heb ook is particularlya ddressed to
those who work in the process industries, I hope that it will interest all who
design,c onstruct,o peratea nd maintainp lant of all sorts, that itm ay suggest to
some readers new ways of looking ata ccidentp revention and that it will rein-
force the viewso ft hoser eadersw ho alreadyt ry toc hanges ituations rathert han
people. It is intended for practising engineers, especially chemical engineers
rather than experts in human factors and ergonomics, and I hope it will be of
particulari nterestt o students.
Although human factors are not specifically mentionedi n the Institution
of Chemical Engineers' publication Accreditation of University Degree
Courses— A Guide for University Departments (November 1996), they are
essential if the course is to 'develop an awareness of the necessity of safe
design', which is included. Many of the accidents I describe can be used as
the raw material for discussions on the causes of accidents and the action
needed to prevent them happening again, using the methods describedi n the
Institution's safety training packages, collections of notes and slides on acci-
dents that have occurred. Someo f the incidents are includedi n the package
on HumanE rrora nd other packages.
Some readers may wonder why I have added to the existing literatureo n
human error. Whens o much has been written, is there need for more?
I felt that much of the existingl iteraturei s too theoreticalf or the audience
I have in mind, or devoted to particulara spects of the problem. I felt there
was a need for a book which would suggest to engineers how they might
approach the problem of human error and do so by describing accidents
which at first sight seem to be the result of human error. My approach is
therefore pragmatic rather than theoretical, and influencedb y engineering
I
methods.T hus have:
lii
•
the
questioned accepted wisdom;
•
startedw ith situations as they are rather than as we would like them to be;
•
judged possible actionsb y their effectiveness;
•
suggested actions based on the best current information, as in industry we
often cannotp ostponed ecisions until more information is available.
I do not claim any great originality for the ideas in this book. Many of
them are to be found in other books such as Man-Machine Engineeringb y A.
Chapanis (Tavistock, 1965) but while that book is primarily concerned with
mechanical and controle quipment, this one is concernedm ainlyw ith process
equipment.
Not all readers may agree with the actionsI propose. Ify ou do not, may I
suggest that you decide what action you think shouldb e taken. Please do not
ignore the accidents. They happeneda nd will happen again, unless action is
taken to prevent them happening.
Thanks are due to the many friends and colleagues, past and present, who
suggested ideas for this book or commented on the draft — without their
contributionsI could not have produced this book — and to Mr S. Coulson
who prepared the illustrations for Chapter 14. Thanks are also due to the
many companies who allowed me to describe their mistakes and to the
Science and Engineering Research Council for tinancial support for the
earliere ditions.
In the second edition I addedt hree new chapters, on 'Accidents that could
be prevented by better management', on 'Errors in computer-controlled
plants' (as people, not computers make the errors) and on 'Some final
thoughts' and made an umber of additions to the existingc hapters.
In this third edition I have added more examples of accidents caused by
the varioust ypes of human error. I have extended the chapterso n errors made
by managers and designers and now include elTors due to their ignorance of
various options such as inherently safer design. I have also expanded the
chapter on computer control and added an appendix on 'Some myths of
human error', widespread beliefst hat are not whollyt rue. John Doe has been
joined in his adventures by Joe Soap but the reasons for their errors are very
different.
I have often been irritated by authors who use phrases such as, as
discussedi n an earlier chapter', without saying which one. I have therefore
includedc ross-references whenever a topic is discussedu nderm ore than one
heading.
US readers should note that in the UK 'supervisor' is another name for a
foreman while 'manager' is used to describe anyone above the level of
iv
foreman, includingt hose who are known as supervisors or superintendents in
most US companies.
To avoid the clumsy phrases 'he or she' and 'him or her' I have usually
used 'he' and him'. Though there has been a welcome increase in the
number of womene mployed in the process industries, the manager, designer
or accidentv ictimi s still usually a man.
Trevor Kletz
May 2001
Glossary
Some words are used in this book in a specialized sense.
Error
A failure to carry out a task in the wayi ntendedb y the personp erformingi t, in
the way expected by other people or in a way that achieves the desired
objective.
This definitionh as been worded so as to includet he various types of error
listed below. It is consistent with the Shorter Oxford English Dictionajy
which defines error as 'something incorrectly done through ignorance or
inadvertence' and as 'a transgression'. Somew riters use error in a narrower
sense to includeo nly slips and lapses of attention, as defined below.
Mismatch
An errort hat occurs becauset he task is beyondt hep hysicalo r mentala bilityo f
thep ersona skedt op erformi t, perhapsb eyonda nyone'sa bility (see Chapter4 ).
Mistake
An error that occurs as a result of ignorance of the correctt ask or the correct
way to perform it. The intentioni s fulfilled but the intention is wrong (see
Chapter3 ).
Non-compliance or violation
An error that occurs becauses omeoned ecides not to carry out a task or not to
carryi t outi nt he way instructed or expected. The motivec an range from sabo-
tage,t hrough' can'tb e bothered't o ab elieft hat thei nstructions werei ncorrect.
In such cases the non-compliance may prevent a mistake (as defined above)
(see Chapter5 ).
V
Slips arid lapses of attention
An errort hato ccurs as ar esulto f forgetfulness, habit,f atigueo r similarp sycho-
logical causes. Compared with mistakes (as defined above), the intentioni s
correctb uti t is not fulfilled( see Chapter2 ).
The opening words of Tolstoy's Anna Karenina are, AI1 happy families
resemble one another, each unhappy family is unhappy in its own way.'
Similarly, there is a right way for every task but each error is erroneous in its
own way.
vi
Contents
Foreword tot het hird edition vi
Introduction
1
11 Accept men as we find them
1 2 Meccano or dofs? 3
1 3 Types of humane rror 4
1.4 Twos implee xamples 6
1.5 Accident investigation 8
16 Astory 9
1 7 Research on humane rror 10
2 Accidentsc ausedb y simple slips 11
2 1 Introduction 11
22 Forgetting to open or close a valve 13
23 Operatingt hew rong valve 21
24 Pressing thew rong button 25
2 5 Failures to notice 29
2.6 Wrong connections 31
2 7 Errors in calculations 32
2.8 Otherm edicale rrors 34
2.9 Railways 35
21 0 Otheri ndustries 41
21 1 Everyday life (andt yping) 43
2.12 Fatigue 45
3 Accidentsth at could bep revented by beftert rainingo r instructions 48
3 1 Introduction 48
3 2 Three Mile Island 50
33 Othera ccidents that couldb e prevented by relatively sophisticated training 53
34 Accidents that could be prevented by elementary training 62
3 5 Contradictory instructions 65
3 6 Knowledge ofw hat we don't know 66
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3 7 Some simplew ays ofi mprovingi nstructions 67
3.8 Training or instructions? 72
3.9 Casesw hen trainingi s nott heb esta nswer 74
4 Accidents due to a lack ofp hysical or mental ability 78
4 1 People asked to dot he physically difficult or impossible 78
42 People asked tod ot he mentallyd ifficult or impossible 80
4.3 Individual traits and accident proneness 85
4.4 Mind-sets 88
5 Accidentsd ue to failures to follow instructions 97
5.1 Accidents due to non-compliance by managers 99
5.2 Accidents due to non-compliance by operators 103
5.3 Actions to improvec ompliance 107
54 Alienation 109
5 5 Postscript 110
6. Accidents that could by prevented by better management 112
6.1 An accidentc aused by insularity 114
62 An accidentd ue toa mateurism 115
6.3 The fire at King's Cross railway station 116
64 The Herald of Free Enterprise 117
65 The Clapham Junction railway accident 118
6.6 PiperA lpha 120
67 What morec an senior managers do? 120
6.8 The measurement ofs afety 125
6.9 Conclusions 127
7 The probability ofh umane rror 129
7.1 Why do we needt o know humane rror rates7 130
72 Human error rates — a simplee xample 132
73 A more complexe xample 132
7.4 Othere stimates ofh uman error rates 136
75 Two mores implee xamples 141
7.6 Buttonp ressing 144
7.7 Non-process operations 146
78 Traind river errors 147
7.9 Some pitfalls in usingd ata on humanr eliability 147
7.10 Datao n equipment mayb e data on people 149
Viii
7.11 Who makes thee rrors7 151
7.12 Conclusions 151
8 Somea ccidentsth at could be prevented by better design 154
8.1 Isolation of protective equipment 155
82 Better information display 156
8.3 Pipe failures 156
8.4 Vesself ailures 159
85 The Sellafield leak 159
8.6 Otherd esign errors 161
87 Conceptual shortcomings 162
8.8 Problems of design contractors 165
89 Domestic accidents 166
9 Somea ccidentsth atc ouldb e prevented by better construction 168
91 Pipefailures 168
9.2 Miscellaneous incidents 170
9 3 Prevention ofc onstruction errors 173
10 Some accidents thatc ouldb e prevented by better maintenance 175
10 1 Incidents which occurred because peopled id not understand how
equipmentw orked 175
10.2 Incidents which occurred because ofp oor maintenance practice 178
103 Incidents duet o gross ignorance or incompetence 180
104 Incidents which occurred because peoplet ook short cuts 181
105 Incidents which could be prevented by moref requento r better maintenance 183
10.6 Can we avoidt he needf ors o much maintenance? 185
11 Some accidents thatc ouldb ep revented by better methods
of
operation 187
111 Permits-to-work 187
112 Tanker incidents 189
11 3 Some incidents that could be prevented byb etter instructions 192
4
11 Some incidents involving hoses 193
11.5 Communication failures 194
11.6 Examples from the railways 199
11.7 Simple causes in hight ech industries 201
12 Errors in computer-controlled plants 203
12.1 Hardware failures 204
122 Software errors 205
123 Specification errors 207
12.4 Misjudging responses toa computer 210
ix
