EN 61124:2006

SLOVENSKI STANDARD
01-januar-2007
Preskušanje zanesljivosti – Ustreznostni preskusi za konstantno pogostost
odpovedi in konstantno intenzivnost odpovedi (IEC 61124:2006)
Reliability testing - Compliance tests for constant failure rate and constant failure
intensity
Prüfungen der Funktionsfähigkeit - Prüfpläne für konstante Ausfallrate und konstante
Ausfalldichte
Essais de fiabilité - Plans d'essais de conformité d'un taux de défaillance constant et
d'une intensité de défaillance constante
Ta slovenski standard je istoveten z: EN 61124:2006
ICS:
19.020 Preskuševalni pogoji in Test conditions and
postopki na splošno procedures in general
21.020 =QDþLOQRVWLLQQDþUWRYDQMH Characteristics and design of
VWURMHYDSDUDWRYRSUHPH machines, apparatus,
equipment
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN STANDARD
EN 61124
NORME EUROPÉENNE
June 2006
EUROPÄISCHE NORM
ICS 03.120.30; 19.020; 21.020
English version
Reliability testing -
Compliance tests for constant failure rate
and constant failure intensity
(IEC 61124:2006)
Essais de fiabilité - Prüfungen der Funktionsfähigkeit -
Plans d'essais de conformité d'un taux de Prüfpläne für konstante Ausfallrate
défaillance constant et d'une intensité de und konstante Ausfalldichte
défaillance constante (IEC 61124:2006)
(CEI 61124:2006)
This European Standard was approved by CENELEC on 2006-05-01. CENELEC members are bound to comply
with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this European Standard
the status of a national standard without any alteration.

Up-to-date lists and bibliographical references concerning such national standards may be obtained on
application to the Central Secretariat or to any CENELEC member.

This European Standard exists in three official versions (English, French, German). A version in any other
language made by translation under the responsibility of a CENELEC member into its own language and notified
to the Central Secretariat has the same status as the official versions.

CENELEC members are the national electrotechnical committees of Austria, Belgium, Cyprus, the Czech
Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia,
Lithuania, Luxembourg, Malta, the Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain,
Sweden, Switzerland and the United Kingdom.

CENELEC
European Committee for Electrotechnical Standardization
Comité Européen de Normalisation Electrotechnique
Europäisches Komitee für Elektrotechnische Normung

Central Secretariat: rue de Stassart 35, B - 1050 Brussels

© 2006 CENELEC - All rights of exploitation in any form and by any means reserved worldwide for CENELEC members.
Ref. No. EN 61124:2006 E
Foreword
The text of document 56/1081/FDIS, future edition 2 of IEC 61124, prepared by IEC TC 56,
Dependability, was submitted to the IEC-CENELEC parallel vote and was approved by CENELEC as
EN 61124 on 2006-05-01.
The following dates were fixed:
– latest date by which the EN has to be implemented
at national level by publication of an identical
(dop) 2007-02-01
national standard or by endorsement
– latest date by which the national standards conflicting
(dow) 2009-05-01
with the EN have to be withdrawn
Annex ZA has been added by CENELEC.
__________
Endorsement notice
The text of the International Standard IEC 61124:2006 was approved by CENELEC as a European
Standard without any modification.
__________
- 3 - EN 61124:2006
Annex ZA
(normative)
Normative references to international publications
with their corresponding European publications

The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced
document (including any amendments) applies.

NOTE  When an international publication has been modified by common modifications, indicated by (mod), the relevant EN/HD
applies.
Publication Year Title EN/HD Year

1)
IEC 60050-191 - International Electrotechnical Vocabulary - -
(IEV)
Chapter 191: Dependability and quality of
service
IEC 60300-3-5 2001 Dependability management - -
Part 3-5: Application guide - Reliability test
conditions and statistical test principles

1)
IEC 60605-2 - Equipment reliability testing - -
Part 2: Design of test cycles
IEC 60605-3 Series Equipment reliability testing - -
Part 3: Preferred test conditions

IEC 60605-4 2001 Equipment reliability testing - -
Part 4: Statistical procedures for exponential
distribution - Point estimates, confidence
intervals, prediction intervals and tolerance
intervals
1)
IEC 60605-6 - Equipment reliability testing - -
Part 6: Tests for the validity of the constant
failure rate or constant failure intensity
assumptions
IEC 61123 1991 Reliability testing - Compliance test plans for - -
success ration
1)
Undated reference.
NORME CEI
INTERNATIONALE
IEC
INTERNATIONAL
Deuxième édition
STANDARD
Second edition
2006-03
Essais de fiabilité –
Plan d'essais de conformité d'un taux de
défaillance constant et d'une intensité
de défaillance constante
Reliability testing –
Compliance tests for constant failure rate
and constant failure intensity

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électronique ou mécanique, y compris la photocopie et les photocopying and microfilm, without permission in writing from
microfilms, sans l'accord écrit de l'éditeur. the publisher.
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МеждународнаяЭлектротехническаяКомиссия
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For price, see current catalogue

61124 © IEC:2006 – 3 –
CONTENTS
FOREWORD. 15
1 Scope. 19
2 Normative references. 19
3 Terms, definitions, symbols and acronyms . 21
3.1 Terms and definitions . 21
3.2 Acronyms and symbols . 21
4 General requirements and area of application. 25
4.1 Requirements . 25
4.2 Applicability to replaced and repaired items . 25
4.3 Types of test plans. 27
5 General test procedure. 29
5.1 Test conditions . 29
5.2 General characteristics of the test plans. 29
5.3 Data to be recorded. 31
*
5.4 Calculation of accumulated test time, T . 31
5.5 Number of failures. 33
6 Sequential test plans . 33
6.1 General. 33
6.2 Common test procedure. 33
6.3 Decision criteria. 35
6.4 Overview of test plans . 35
7 Fixed time/failure terminated test plans – Fixed duration test plans. 37
7.1 General. 37
7.2 Common test procedure. 37
7.3 Decision criteria. 37
7.4 Test plans . 39
8 Design of alternative time/failure terminated test plans . 39
8.1 General. 39
8.2 Design procedures . 41
8.3 Common test procedure. 41
8.4 Decision criteria. 41
9 Calendar time/failure terminated test plans for non-replaced items . 43
9.1 General. 43
9.2 Common test procedure. 43
9.3 Decision criteria. 43
9.4 Use of Table 2 of IEC 61123:1991 for fixed calendar time tests . 45
10 Combined test plans . 47
10.1 General. 47
10.2 Common test procedure. 47
10.3 Decision criteria. 47
10.4 Test plans . 49
11 Performing the test . 49
12 Presentation of results . 49

61124 © IEC:2006 – 5 –
Annex A (normative) Tables and graphs for sequential test plans . 51
Annex B (normative) Graphs for fixed time/failure terminated test plans . 89
Annex C (normative) Graphs for alternative time/failure terminated test plans. 107
Annex D (normative) Tables and graphs for combined test plans and additional
sequential test plans. 121
Annex E (informative) Examples and mathematical references for sequential test
plans. 159
Annex F (informative) Design of sequential test plans using a spreadsheet program. 175
Annex G (informative) Examples and mathematical references for fixed duration
time/failure terminated test plans . 197
Annex H (informative) Design of fixed duration time/failure terminated test plans
using a spreadsheet program. 201
Annex I (informative) Examples and mathematical references for the design of
alternative time/failure terminated test plans . 213
Annex J (informative) Examples and mathematical references for the calendar time
terminated test plans . 229
Annex K (informative) Derivation and mathematical reference for the optimized test
plans of GOST 27.402 . 233
Bibliography. 249
Figure A.1 – Accept and reject lines for test plan A.1 . 51
Figure A.2 – Test plan A.1 – Operating characteristic curve . 55
Figure A.3 – Test plan A.1 – Expected accumulated test time to decision. 55
Figure A.4 – Accept and reject lines for test plan A.2 . 57
Figure A.5 – Test plan A.2 – Operating characteristic curve . 59
Figure A.6 – Test plan A.2 – Expected accumulated test time to decision. 59
Figure A.7 – Accept and reject lines for test plan A.3 . 61
Figure A.8 – Test plan A.3 – Operating characteristic curve . 63
Figure A.9 – Test plan A.3 – Expected accumulated test time to decision. 63
Figure A.10 – Accept and reject lines for test plan A.4. 65
Figure A.11 – Test plan A.4 – Operating characteristic curve . 67
Figure A.12 – Test plan A.4 – Expected accumulated test time to decision . 67
Figure A.13 – Accept and reject lines for test plan A.5. 69
Figure A.14 – Test plan A.5 – Operating characteristic curve . 71
Figure A.15 – Test plan A.5 – Expected accumulated test time to decision . 71
Figure A.16 – Accept and reject lines for test plan A.6. 73
Figure A.17 – Test plan A.6 - Operating characteristic curve. 75
Figure A.18 – Test plan A.6 – Expected accumulated test time to decision . 75
Figure A.19 – Accept and reject lines for test plan A.7. 77
Figure A.20 – Test plan A.7 – Operating characteristic curve . 79

61124 © IEC:2006 – 7 –
Figure A.21 – Test plan A.7 – Expected accumulated test time to decision . 79
Figure A.22 – Accept and reject lines for test plan A.8. 81
Figure A.23 – Test plan A.8 – Operating characteristic curve . 83
Figure A.24 – Test plan A.8 – Expected accumulated test time to decision . 83
Figure A.25 – Accept and reject lines for test plan A.9. 85
Figure A.26 – Test plan A.9 – Operating characteristic curve . 87
Figure A.27 – Test plan A.9 – Expected accumulated test time to decision . 87
Figure B.1 – Operating characteristic curves for test plans B.1, B.2, B.3 and B.4 . 89
Figure B.2 – Test plan B.1 – Expected test time to decision . 91
Figure B.3 – Test plan B.2 – Expected test time to decision . 91
Figure B.4 – Test plan B.3 – Expected test time to decision . 93
Figure B.5 – Test plan B.4 – Expected test time to decision . 93
Figure B.6 – Operating characteristic curves for test plans B.5, B.6, B.7 and B.8 . 95
Figure B.7 – Test plan B.5 – Expected test time to decision . 95
Figure B.8 – Test plan B.6 – Expected test time to decision . 97
Figure B.9 – Test plan B.7 – Expected test time to decision . 97
Figure B.10 – Test plan B.8 – Expected test time to decision . 99
Figure B.11 – Operating characteristic curves for test plans B.9, B.10 and B.11. 99
Figure B.12 – Test plan B.9 – Expected test time to decision . 101
Figure B.13 – Test plan B.10 – Expected test time to decision. 101
Figure B.14 – Test plan B.11– Expected test time to decision . 103
Figure B.15 – Operating characteristic curves for test plans B.12 and B.13 . 103
Figure B.16 – Test plan B.12 – Expected test time to decision. 105
Figure B.17 – Test plan B.13 – Expected test time to decision. 105
Figure C.1 – Discrimination ratio, D, and the acceptable number of failures, c = 0 to 8,
as a function of the expected number of failures, μ , for recommended values, 2,5%,
5%, 10%, 20%, and 30% of α = β. 113
Figure C.2 – Operation characteristic curves for c = 0 to 8; probability of acceptance
P as a function of the (unknown) true expected number of failures, μ. 115
a
Figure C.3 – Discrimination ratio, D, as a function of the expected number of failures,
μ , for recommended values, 2,5%, 5%, 10%, 15%, 20% and 30% of α = β . 117
Figure C.4 – Acceptable number of failures c minus expected number of failures, μ
Δμ = c − μ as a function of the expected number of failures μ for recommended
0 0 0
values 2,5%, 5%, 10%, 20%, and 30% of α = β. 119
Figure D.1 – Accept and reject lines . 123
∗
Figure D.2 – Expected test time to decision T . 125
e
∗
Figure D.3 – Expected test time to decision of acceptance T (+) . 125
e
Figure D.4 – Operating characteristic P . 125
a
Figure D.5 – Accept and reject lines . 127
∗
T
Figure D.6 – Expected test time to decision . 131
e
61124 © IEC:2006 – 9 –
∗
Figure D.7 – Expected test time to decision of acceptance T (+) . 131
e
Figure D.8 – Operating characteristic P . 131
a
Figure D.9 – Accept and reject lines . 135
∗
Figure D.10 – Expected test time to decision T . 137
e
∗
Figure D.11 – Expected test time to decision of acceptance T (+) . 137
e
Figure D.12 – Operating characteristic P . 137
a
Figure D.13 – Accept and reject lines . 139
∗
Figure D.14 – Expected test time to decision T . 141
e
∗
Figure D.15 – Expected test time to decision of acceptance T (+) . 141
e
Figure D.16 – Operating characteristic P . 141
a
Figure D.17 – Accept and reject lines . 143
∗
Figure D.18 – Expected test time to decision . 145
T
e
∗
Figure D.19 – Expected test time to decision of acceptance T (+) . 145
e
Figure D.20 – Operating characteristic P . 145
a
Figure D.21 – Accept and reject lines . 147
∗
Figure D.22 – Expected test time to decision T . 149
e
∗
Figure D.23 – Expected test time to decision of acceptance T (+) . 149
e
Figure D.24 – Operating characteristic P . 149
a
Figure D.25 – Accept and reject lines . 151
∗
Figure D.26 – Expected test time to decision T . 153
e
∗
Figure D.27 – Expected test time to decision of acceptance T (+) . 153
e
Figure D.28 – Operating characteristic P . 153
a
Figure D.29 – Accept and reject lines . 155
*
Figure D.30 – Expected test time to decision T . 157
e
∗
Figure D.31 – Expected test time to decision of acceptance T (+) . 157
e
Figure D.32 – Operating characteristic P . 157
a
Figure E.1 – Example of a sequential test using test plan A.3 – α = β = 10 %, D = 3,
*
m = 1,11×10 h; r versus t . 165
Figure F.1 – Beginning of the spreadsheet prepared to obtain a sequential test and
graph . 177
Figure F.2 – Continuation of parameters calculation for the lines necessary for the
SPRT graph. 179
Figure F.3 – Calculations of accept and reject line for the SPRT graph . 179
Figure F.4 – Determination of the test termination time. 181
Figure F.5 – Equations for accept and reject line along with the test termination . 185
Figure F.6 – SPRT spreadsheet graphing example . 187
Figure F.7 – Spreadsheet set-up for construction of the OC curves for the SPRT . 191
Figure F.8 – OC curve for probability of acceptance, P . 193
a
61124 © IEC:2006 – 11 –
Figure H.1 – Set-up of the spreadsheet with embedded equations – Example. 203
Figure H.2 – Equations embedded into the spreadsheet shown in Figure F.1. 205
Figure H.3 – OC curve for the time/failure terminated fixed duration test . 209
Figure H.4 – OC curve plotted from the spreadsheet calculations . 211
Figure K.1 – Test plan types and terminology . 235
Figure K.2 – Principle of test plans. 239
Figure K.3 – Partitioning of the test plan graph . 239
Figure K.4 – Interior nodes and border nodes. 239
Figure K.5 – Paths to the accept line . 241
Figure K.6 – Paths to the reject line. 241
Figure K.7 – Probabilities of paths transfer between nodes. 243
Figure K.8 – The recurrent element – Two cases . 247
Table 1 – Advantages and disadvantages for the different test plan types . 27
Table 2 – Overview of the sequential test plans given in Annex A and D . 35
Table 3 – Fixed time/failure terminated test plans . 39
Table 4 – Combined test plans in Annex D . 49
Table A.1 – Accept and reject lines for test plan A.1 . 53
Table A.2 – Accept and reject lines for test plan A.2 . 57
Table A.3 – Accept and reject lines for test plan A.3 . 61
Table A.4 – Accept and reject lines for test plan A.4 . 65
Table A.5 – Accept and reject lines for test plan A.5 . 69
Table A.6 – Accept and reject lines for test plan A.6 . 73
Table A.7 – Accept and reject lines for test plan A.7 . 77
Table A.8 – Accept and reject lines for test plan A.8 . 81
Table A.9 – Accept and reject lines for test plan A.9 . 85
Table D.1 – Sequential test plans in Annex D . 121
Table D.2 – Combined test plans in Annex D. 121
Table D.3 – Accept and reject lines . 123
Table D.4 – Expected test time to decision and operating characteristic P . 127
a
Table D.5 – Accept and reject lines . 129
Table D.6 – Expected test time to decision and operating characteristic P . 133
a
Table D.7 – Accept and reject lines . 135
Table D.8 – Expected test time to decision and operating characteristic P . 137
a
Table D.9 – Accept and reject lines . 139
Table D.10 – Expected test time to decision and operating characteristic P . 141
a
Table D.11 – Accept and reject lines . 143
Table D.12 – Expected test time to decision and operating characteristic P . 145
a
Table D.13 – Accept and reject lines . 147

61124 © IEC:2006 – 13 –
Table D.14 – Expected test time to decision and operating characteristic P . 149
a
Table D.16 – Expected test time to decision and operating characteristic P . 153
a
Table D.17 – Accept and reject lines . 155
Table D.18 – Expected test time to decision and operating characteristic P . 157
a
Table E.1 – Example for a sequential test using test plan A.3 (with example data) . 163
Table E.2 – Constants for border line equations and their coordinates for sequential
test plans A. 1 to A. 9 . 173
Table I.1 – Cumulative normal distribution for fixed u value . 227
γ
Table I.2 – Inverse cumulative normal distribution for fixed 1–γ values. 227

61124 © IEC:2006 – 15 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
RELIABILITY TESTING –
COMPLIANCE TESTS FOR CONSTANT FAILURE RATE
AND CONSTANT FAILURE INTENSITY
FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote
international co-operation on all questions concerning standardization in the electrical and electronic fields. To
this end and in addition to other activities, IEC publishes International Standards, Technical Specifications,
Technical Reports, Publicly Available Specifications (PAS) and Guides (hereafter referred to as “IEC
Publication(s)”). Their preparation is entrusted to technical committees; any IEC National Committee interested
in the subject dealt with may participate in this preparatory work. International, governmental and non-
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with the International Organization for Standardization (ISO) in accordance with conditions determined by
agreement between the two organizations.
2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
consensus of opinion on the relevant subjects since each technical committee has representation from all
interested IEC National Committees.
3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC
Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any
misinterpretation by any end user.
4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
transparently to the maximum extent possible in their national and regional publications. Any divergence
between any IEC Publication and the corresponding national or regional publication shall be clearly indicated in
the latter.
5) IEC provides no marking procedure to indicate its approval and cannot be rendered responsible for any
equipment declared to be in conformity with an IEC Publication.
6) All users should ensure that they have the latest edition of this publication.
7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
members of its technical committees and IEC National Committees for any personal injury, property damage or
other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and
expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC
Publications.
8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 61124 has been prepared by IEC technical committee 56:
Dependability.
This second edition of IEC 61124 cancels and replaces the first edition published in 1997 and
constitutes a technical revision.
The main changes with respect to the previous edition are as follows:
A number of new test plans have been added based on the Russian standard GOST 27.402
and it is intended to align the new edition of MIL-HDBK-781 with this edition. Algorithms for
optimizing test plans using a spreadsheet program are given and a number of optimized test
plans are listed. Furthermore, emphasis is laid on the fact that the test should be repeated
following design changes.
61124 © IEC:2006 – 17 –
Discrepancies in test plans A, B as well as Annexes A and B that originated in IEC 60605-7
have been corrected so these test plans differ from those given in the first edition of
IEC 61124. As requested by the national committees, mathematical background material and
spreadsheet program information have been moved to informative annexes. In addition, the
symbol lists have been divided, so that some annexes have separate lists of symbols.
Guidance on how to choose test plans has been added as well as guidance on how to use
spreadsheet programs to create them. Test plans A.1 to A.9 and B.1 to B.13 have been
corrected.
Subclauses 8.1, 8.2, 8.3, Clause 9, Annex C, Clauses G.2, I.2, I.3 and Annex J are
unchanged, except for updated terminology and references.
The text of this standard is based on the following documents:
FDIS Report on voting
56/1081/FDIS 56/1095/RVD
Full information on the voting for the approval of this standard can be found in the report on
voting indicated in the above table.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
The committee has decided that the contents of this publication will remain unchanged until
the maintenance result date indicated on the IEC web site under "http://webstore.iec.ch" in
the data related to the specific publication. At this date, the publication will be
• reconfirmed;
• withdrawn;
• replaced by a revised edition, or
• amended.
61124 © IEC:2006 – 19 –
RELIABILITY TESTING –
COMPLIANCE TESTS FOR CONSTANT FAILURE RATE
AND CONSTANT FAILURE INTENSITY
1 Scope
This International Standard gives a number of optimized test plans, the corresponding
operating characteristic curves and expected test times. In addition the algorithms for
designing test plans using a spreadsheet program are also given, together with guidance on
how to choose test plans.
This standard specifies procedures to test whether an observed value of
– failure rate,
– failure intensity,
– mean time to failure (MTTF),
– mean operating time between failures (MTBF),
conforms to a given requirement.
It is assumed, except where otherwise stated, that during the accumulated test time, the times
to failure or the operating times between failures are independent and identically
exponentially distributed. This assumption implies that the failure rate or failure intensity is
constant.
Four types of test plans are described as follows:
– truncated sequential tests;
– time/failure terminated tests;
– fixed calendar time terminated tests without replacement;
– combined test plans.
This standard does not cover guidance on how to plan, perform, analyse and report a test.
This information can be found in IEC 60300-3-5.
This standard does not describe test conditions. This information can be found in IEC 60605-2
and in IEC 60605, Parts 3-1 to 3-6.
2 Normative references
The following referenced documents are indispensable for the application of this document.
For dated references, only the edition cited applies. For undated references, the latest edition
of the referenced document (including any amendments) applies.
IEC 60050(191), International Electrotechnical Vocabulary (IEV) – Chapter 191: Dependability
and quality of service
IEC 60300-3-5:2001, Dependability management – Part 3-5: Application guide – Reliability
test conditions and statistical test principles

61124 © IEC:2006 – 21 –
IEC 60605-2, Equipment reliability testing – Part 2: Design of test cycles
IEC 60605-3 (all parts), Equipment reliability testing – Part 3: Preferred test conditions
IEC 60605-4:2001, Equipment reliability testing – Part 4: Statistical procedures for
exponential distribution – Point estimates, confidence intervals, prediction intervals and
tolerance intervals
IEC 60605-6, Equipment reliability testing – Part-6: Tests for the validity of the constant
failure rate or constant failure intensity assumptions
IEC 61123:1991, Reliability testing – Compliance test plans for success ratio
3 Terms, definitions, symbols and acronyms
3.1 Terms and definitions
For the purposes of this document the terms and definitions given in IEC 60050(191) apply.
The terms “failure rate” and “f ailure intensity” a re used as meaning constant failure rate and
constant failure intensity.
3.2 Acronyms and symbols
3.2.1 Acronyms
OC Operating characteristic
SPRT Sequential probability ratio test (in some literature called probability ratio sequential
test (PRST)
3.2.2 Symbols
The generic symbol λ is used in the following for failure rate and failure intensity.
The symbol m is used to denote both the following reliability measures:
– mean operating time between failures, MTBF;
– mean time to failure, MTTF.
When used, the relationship between the above quantities, under the given assumptions, is:
λ =
m
Sequential test plans (see Clause 6) and fixed time/failure terminated test plans (see Clause
7) are based on m as a reliability measure, thus in these cases:
m =
λ
61124 © IEC:2006 – 23 –
c
acceptable number of failures during the test
discrimination ratio; D = m m or D = λ λ
D
0 1 1 0
summation variable for failures
k
MTBF mean operating time between failures
MTTF mean time to failure
m true MTBF or MTTF
m specified MTTF or MTBF m = 1 λ (design goal)
0 0 0
m lower limit for MTTF or MTBF m = 1 λ
1 1 1
n number of test items at the beginning of the test
P probability of acceptance
a
p acceptable failure ratio
q acceptable success ratio, q = 1 – p
0 0
R(t) reliability at time t
r observed number of failures during the test
r test truncation failure number for sequential tests (SPRT)
*
T accumulated test time
*
T accumulated test time stated as accept criterion
a
∗
T minimum test time for r = 0 stated as accept criterion
a, min
*
expected accumulated test time to decision
T
e
∗
expected accumulated test time to acceptance
T (+)
e
*
accumulated test time stated as reject criterion
T
r
*
T accumulated test time stated as termination criterion
t
t test time
∗
test truncation time
t
∗
test time for each test item (assumed here to be the same for all test items)
t
t
t
life time of failed item i
i
*
t
calendar test time stated as termination criterion
cal,t
P(r)
probability of r failures
Δμ auxiliary quantity for determination of c, Δμ = c − μ
0 0
nominal producer’s risk (type I risk)
α
α′ true producer’s risk (type I risk)

61124 © IEC:2006 – 25 –
β
nominal consumer’s risk (type II risk)
β ′
true consumer’s risk (type II risk)
λ true failure rate per item
λ
expected failure rate per item (design goal)
λ
upper limit for constant failure rate per item
μ
expected number of failures during the test at the true λ
NOTE μ is not necessarily an integer.
μ
expected number of failures during the test at the specified λ as parameter of the
∗
Poisson distribution μ = λ T
0 0 t
4 General requirements and area of application
4.1 Requirements
It is assumed, except where otherwise stated, that during the accumulated test time the times
to failure or the operating times between failures are independent, and identically
exponentially distributed. This assumption implies that the failure rate or failure intensity is
constant. Under this assumption there is no difference between failure rate and failure
intensity. Therefore they are both called λ and referred to in the following as failure rate̴
It is assumed that the requirement is specified in terms of the acceptable constant failure rate
λ , or the acceptable mean number of failures per time unit, λ or the acceptable mean time
0 0
to failure or mean operating time between failures, m .
If it is necessary to test the constant failure rate/constant failure intensity assumption, the
procedures given in IEC 60605-6 should be used.
4.2 Applicability to replaced and repaired items
The sequential test plans (see Clause 6), the time/failure terminated test plans (see Clause 7)
and the combined test plans (see Clause 10) are applicable to the following:
– replacement of failed items,
– without replacement of failed items,
under the assumption that
– an item can be replaced by repair of the item itself;
– the accumulated test time is calculated as elapsed operating item-time, in accordance
with 5.4;
– replaced items belong to the same population as the original items;
– repaired items can be considered to have the same failure intensity after repair as they
had before they failed.
The calendar time/failure terminated test plans in Clause 9, however, are applicable to cases
where failed items are not replaced and where a fixed number of items are placed on test for
a fixed calendar time. This means that the test is running, even though the number of items
under test may not remain constant because some items may not survive.

61124 © IEC:2006 – 27 –
4.3 Types of test plans
4.3.1 General
Test plans are given for four types of tests:
– truncated sequential tests (see Clause 6);
– time/failure terminated tests (for fixed time/failure terminated tests see Clause 7, and for
alternative time/failure terminated tests, see Clause 8);
– calendar time terminated tests without replacement (see Clause 9);
– combined test plans (see Clause 10).
4.3.2 Advantages and disadvantages of the different test plan types
The different test plan types are shown in Table 1.
Table 1 – Advantages and disadvantages for the different test plan types
Test plan type Advantages Disadvantages
Truncated sequential Assures highest test efficiency Complicated to administer
(shortest test time). For the A test
Test plans in Clause 6 (Table 2), For marginal reliability, the test time
plans the user can develop new test
Annex A and Annex D is longer than for time/failure
plans using a spreadsheet program
terminated and combined test plans
r
The test plans in Annex D are made
The C test plans have true alpha by an iterative process. They cannot
Reject
and beta risks which are closer to be computed using the
the nominal risks than the A test spreadsheets described in Annex F
plans
Accept
∗
T
m
Time/Failure terminated Simple to understand and to For very reliable, or very unreli
...