IEC 61123:2019

Reliability testing - Compliance test plans for success ratio

IEC 61123:2019 is intended to define a procedure to verify if a reliability of an item/system complies with the stated requirements. The requirement is assumed to be specified as the percentage of success (success ratio) or the percentage of failures (failure ratio). This document can be used where a number of items are tested (number of trials performed) and classified as passed or failed. It can also be used where one or a number of items are tested repeatedly. The procedures are based on the assumption that the probability of success or failure is the same from trial to trial (statistically independent events). Plans for fixed trial/failure terminated tests as well as truncated sequential probability ratio tests (SPRTs) are included. This document contains extensive tables with ready-to-use SPRT plans and their characteristics for equal and non-equal risks for supplier and customer. In the case of the reliability compliance tests for constant failure rate/intensity, IEC 61124 applies. This second edition cancels and replaces the first edition published in 1991. This edition constitutes a technical revision. This edition includes the following significant technical changes with respect to the previous edition:
The sequential probability ratio test (SPRT) [1, 2][1] has been significantly developed in recent years [3, 4, 5]. This edition contains shorter and accurate tests, a wide range of test plans, and significant additional characteristic data, as follows:
the tests are significantly truncated (the maximum trial numbers are low) without substantially increasing the expected number of trials to decision (ENT);
the true producer’s and consumer’s risks (α', β') are given and very close to the nominal (α, β);
the range of the test parameters is wide (failure ratio, risks and discrimination ratio);
the test plans include various risk ratios (not restricted to equal risks only);
the values of ENT are accurate and given in the relevant region (for practical use);
guidelines for extension of the test sets (interpolation and extrapolation) are included.
In Annex C, the use of the cumulative binomial distribution function of Excel that simplifies the procedure of designing has been added (Clause C.3).
Keywords: verify if a reliability of an item/system complies with the stated requirements

Essais de fiabilité - Plans d'essai de conformité pour une proportion de succès

IEC 61123:2019 vise à définir une procédure pour vérifier si la fiabilité d'une entité/d'un système satisfait aux exigences spécifiées. L'exigence est admise comme étant spécifiée sous forme de pourcentage de succès (proportion de succès) ou de pourcentage de défaillances (proportion de défaillances). Le présent document peut être utilisé lorsqu'un certain nombre d'entités sont soumises à l'essai (nombre d'épreuves réalisées) et classées comme ayant satisfait ou échoué à l'essai. Il peut également être utilisé lorsqu'une ou plusieurs entités sont soumises à l'essai de façon répétée. Les procédures sont fondées sur l'hypothèse que la probabilité de succès ou de défaillance est identique d'une épreuve à l'autre (événements statistiquement indépendants). Le présent document inclut des plans d'essais à nombre fixé d'épreuves et de défaillances, ainsi que des essais de rapport de probabilité progressifs (SPRT). Il contient des tableaux complets décrivant des plans de SPRT prêts à l'emploi et leurs caractéristiques associées, les risques pour le fournisseur et le client étant égaux ou non. Dans le cas d'essais de conformité de fiabilité relatifs à un taux/une intensité constant(e) de défaillance, l'IEC 61124 s'applique. Cette deuxième édition annule et remplace la première édition parue en 1991. Cette édition constitue une révision technique.
Cette édition inclut les modifications techniques majeures suivantes par rapport à l'édition précédente:
les essais sont considérablement tronqués (le nombre maximal d'épreuves est faible) sans pour autant augmenter substantiellement le nombre prévu d'épreuves pour décision (ENT – expected number of trials);
les risques vrais du fournisseur et du client (α', β') sont connus et très proches des valeurs nominales (α, β);
la plage des paramètres d'essai est étendue (proportion de défaillance, risques et rapport de discrimination);
les plans d'essai intègrent différents rapports de risque (qui ne se limitent pas uniquement à des risques égaux);
les valeurs de l'ENT sont précises et données dans la région pertinente (à des fins d'utilisation pratique);
des lignes directrices sont fournies en vue d'étendre les séries d'essais (interpolation et extrapolation);
en Annexe C, l'utilisation de la fonction de distribution binomiale cumulative d'Excel pour simplifier la procédure de calcul a été ajoutée (Article C.3).
Mots clés: vérifier si la fiabilité d'une entité/d'un système satisfait aux exigences spécifiées

General Information

• Status: Published
• Publication Date: 21-Nov-2019
• ICS: 03.120.01 - Quality in general
• ICS: 03.120.30 - Application of statistical methods
• ICS: 21.020 - Characteristics and design of machines, apparatus, equipment
• Technical Committee: TC 56 - Dependability
• Current Stage: PPUB - Publication issued
• Completion Date: 22-Nov-2019

Standards Content (sample)

IEC 61123
Edition 2.0 2019-11
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Reliability testing – Compliance test plans for success ratio
Essais de fiabilité – Plans d'essai de conformité pour une proportion de succès
IEC 61123:2019-11(en-fr)
---------------------- Page: 1 ----------------------
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---------------------- Page: 2 ----------------------
IEC 61123
Edition 2.0 2019-11
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Reliability testing – Compliance test plans for success ratio
Essais de fiabilité – Plans d'essai de conformité pour une proportion de succès
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 03.120.01; 03.120.30; 21.020 ISBN 978-2-8322-7647-1

Warning! Make sure that you obtained this publication from an authorized distributor.

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® Registered trademark of the International Electrotechnical Commission
Marque déposée de la Commission Electrotechnique Internationale
---------------------- Page: 3 ----------------------
– 2 – IEC 61123:2019 © IEC 2019
CONTENTS

FOREWORD ........................................................................................................................... 4

INTRODUCTION ..................................................................................................................... 6

1 Scope .............................................................................................................................. 8

2 Normative references ...................................................................................................... 8

3 Terms, definitions, abbreviated terms and symbols .......................................................... 8

3.1 Terms and definitions .............................................................................................. 8

3.2 Abbreviated terms and symbols .............................................................................. 9

3.2.1 Abbreviated terms ........................................................................................... 9

3.2.2 Symbols .......................................................................................................... 9

4 General requirements and area of application ................................................................ 10

4.1 Reliability requirement .......................................................................................... 10

4.2 Repair and replacement ........................................................................................ 10

4.3 Types of test plans ................................................................................................ 10

4.3.1 General ......................................................................................................... 10

4.3.2 Features of the test plan types....................................................................... 10

4.4 General test procedure ......................................................................................... 11

4.5 General decision criteria ....................................................................................... 11

5 Truncated sequential probability ratio test plans (SPRT) ................................................ 13

5.1 Characteristics ...................................................................................................... 13

5.2 Decision criteria .................................................................................................... 13

5.3 Operating characteristic curve (OC) ...................................................................... 14

5.4 Expected number of trials to decision (ENT) ......................................................... 15

6 Fixed trial/failure terminated test plans (FTFT) .............................................................. 16

6.1 Characteristics ...................................................................................................... 16

6.2 Decision criteria .................................................................................................... 16

7 Design of fixed trial/failure terminated test plans ........................................................... 17

7.1 Characteristics ...................................................................................................... 17

7.2 Approach .............................................................................................................. 17

7.3 Common case ....................................................................................................... 18

7.4 Other cases .......................................................................................................... 18

7.5 Example of application .......................................................................................... 18

7.6 Procedure to determine D and c or n and c ............................................................ 18

7.6.1 Figure and table readings .............................................................................. 18

7.6.2 Use of figures and tables ............................................................................... 19

7.7 Decision criteria .................................................................................................... 19

Annex A (informative) Additional information on sequential test plans .................................. 20

A.1 Example................................................................................................................ 20

A.2 Extension of the test set (through interpolation and extrapolation) ........................ 21

A.2.1 General ......................................................................................................... 21

A.2.2 Extrapolation for p < 0,001 ........................................................................... 21

A.2.3 Interpolation .................................................................................................. 21

Annex B (informative) Design of fixed trial/failure terminated test plans − Examples ............ 24

B.1 Use of figures and tables ...................................................................................... 24

B.2 Case where number of events, n, is not known, but is predictable ......................... 24

Annex C (informative) Design of fixed trial/failure terminated test plans – Mathematical

procedures and formulas ...................................................................................................... 26

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IEC 61123:2019 © IEC 2019 – 3 –

C.1 General ................................................................................................................. 26

C.2 Symbols ................................................................................................................ 26

C.3 Computation ......................................................................................................... 26

C.3.1 Determination of D versus n ........................................................................... 26

C.3.2 Determination of n and c versus p , D, α, β ..................................................... 27

C.3.3 Test without failures – Determination of n and D versus p , α, β .................... 27

C.3.4 Determination of OC curves ........................................................................... 28

C.3.5 Determination of inverse OC curves .............................................................. 28

C.4 Accuracy ............................................................................................................... 29

C.5 Tables of cumulative normal distribution and its Inverse........................................ 29

Annex D (normative) Truncated sequential test plans .......................................................... 31

Annex E (informative) Design of fixed trial/failure terminated test plans – Figures and

tables to determine D and c or n and c .................................................................................. 40

Bibliography .......................................................................................................................... 49

Figure 1 – Expected and maximal number of trials for SPRT and FTFT with the same risks .. 11

Figure 2 – SPRT diagram...................................................................................................... 14

Figure 3 – OC curve.............................................................................................................. 15

Figure 4 – SPRT – Curve of expected number of trials to decision (ENT) .............................. 15

Figure 5 – Principal layout of Tables E.1 to E.3 ..................................................................... 19

Figure A.1 – Example of a truncated sequential test ............................................................. 20

Figure E.1 – Discrimination ratio, D, versus number of events, n for p = 0,05 ....................... 40

Figure E.2 – Discrimination ratio, D, versus number of events, n for p = 0,10 ....................... 41

Figure E.3 – Discrimination ratio, D, versus number of events, n for p = 0,15 ....................... 42

Table 1 – Overview – Maximal number of trials and expected number of trials at p for

SPRT and FTFT .................................................................................................................... 12

Table 2 – Range of the test parameters ................................................................................ 13

Table 3 – OC curve ............................................................................................................... 14

Table 4 – ENT (n ) versus true failure ratio (p) ...................................................................... 15

Table 5 – Fixed trial/failure terminated test plans .................................................................. 16

Table A.1 – Example for interpolation by α and β .................................................................. 23

Table C.1 – Cumulative normal distribution for fixed u values .............................................. 29

Table C.2 – Inverse cumulative normal distribution for fixed 1 − γ values .............................. 30

Table D.1 – Truncated sequential test plans ......................................................................... 31

Table E.1 – Acceptable number of failures, c, versus number of events, n for p = 0,05 ........ 43

Table E.2 – Acceptable number of failures, c, versus number of events, n for p = 0,10 ........ 45

Table E.3 – Acceptable number of failures, c, versus number of events, n for p = 0,15 ........ 47

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– 4 – IEC 61123:2019 © IEC 2019
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
RELIABILITY TESTING –
COMPLIANCE TEST PLANS FOR SUCCESS RATIO
FOREWORD

1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising

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International Standard IEC 61123 has been prepared by IEC technical committee 56,

Dependability.

This second edition cancels and replaces the first edition published in 1991. This edition

constitutes a technical revision.

This edition includes the following significant technical changes with respect to the previous

edition:

a) The sequential probability ratio test (SPRT) [1, 2] has been significantly developed in

recent years [3, 4, 5]. This edition contains shorter and accurate tests, a wide range of test

plans, and significant additional characteristic data, as follows:

– the tests are significantly truncated (the maximum trial numbers are low) without

substantially increasing the expected number of trials to decision (ENT);

– the true producer’s and consumer’s risks (α', β') are given and very close to the nominal

(α, β);
______________
Numbers in square brackets refer to the bibliography.
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IEC 61123:2019 © IEC 2019 – 5 –

– the range of the test parameters is wide (failure ratio, risks and discrimination ratio);

– the test plans include various risk ratios (not restricted to equal risks only);

– the values of ENT are accurate and given in the relevant region (for practical use);

– guidelines for extension of the test sets (interpolation and extrapolation) are included.

b) In Annex C, the use of the cumulative binomial distribution function of Excel that simplifies

the procedure of designing has been added (Clause C.3).
The text of this International Standard is based on the following documents:
FDIS Report on voting
56/1852/FDIS 56/1873/RVD

Full information on the voting for the approval of this International Standard can be found in the

report on voting indicated in the above table.

This document has been drafted in accordance with the ISO/IEC Directives, Part 2.

The committee has decided that the contents of this document will remain unchanged until the

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• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.

IMPORTANT – The 'colour inside' logo on the cover page of this publication indicates

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of its contents. Users should therefore print this document using a colour printer.

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– 6 – IEC 61123:2019 © IEC 2019
INTRODUCTION

A compliance test is an essential part of the reliability assurance system. Reliability is affected

by many random factors, so its prediction is not accurate. The direct way to check if the

item/system meets its reliability specifications is to perform a compliance test.

The test serves to verify the compliance with the specified probability that an item will perform

as required. The outcome of each trial of the test is either success or failure.

The probability of making the correct decision in the test depends on the sample size (number

of trials). The tests require a large sample size and, accordingly, a large consumption of funds

and time. The consumptions are especially high for reliability testing. For this reason, sampling

plans of the tests must be carefully planned in order to reduce the sample size.
This document is dedicated to sampling plans for the tests.

The procedures are based on the assumption that trials of the test are statistically independent

and the probability of success, q in them is constant. This document also applies the probability

of failure p = 1 – q.

The tests are characterized by operating characteristic (OC) and number of trials to decision.

OC is the probability of accepting an item as meeting the requirements. In this document, the

OC is represented by the coordinates of its two points (see ISO 3534-2):
– (p , 1 – α) are the coordinates of the producer’s risk point (PRP);
– (p , β) are the coordinates of the consumer’s risk point (CRP).

The number of trials to reaching a decision regarding the test is a random value and in this

document is usually characterized by its expected (ENT) and maximum (MaxNT) values.

This document contains two types of tests:
– truncated sequential probability ratio test (SPRT);
– fixed trial/failure terminated test (FTFT).

The FTFT is characterized by decision rules for accepting or rejecting compliance when the

termination trials number n (MaxNT) has been reached, or the acceptable number of failures c

has been exceeded. This test has the smallest n among all tests with specified PRP and CRP.

When testing objects with p ≤ p , ENT is close to n , and for p > p , ENT decreases significantly.

0 f 0

Another advantage of the FTFT is the ability to conduct all trials simultaneously, but ENT

increases and becomes equal to n .

In the SPRT, the decision is made after each trial: accept or reject compliance, or continue

testing. This document contains a truncated SPRT with MaxNT = n . This n is 1,1 to 1,2 times

t t

greater than n of the FTFT with the same PRP and CRP. However, the ENT of the SPRT is

significantly smaller than that of the corresponding FTFT, and for p ≤ p it can be 1,4 to 1,8

times smaller. This is a great advantage of the SPRT. If it is necessary to shorten the calendar

time of the SPRT, it is possible to run the trials by small portions of n , while the OC and ENT

will not change significantly.

The planning of the SPRT is quite complicated so this document contains extensive tables with

ready-to-use test plans and their characteristics. Tests are listed for α = β as well as for α ≠ β.

The tables also allow the design of additional tests by simple interpolation and, for small p , by

extrapolation.
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IEC 61123:2019 © IEC 2019 – 7 –

Some of the tests have a very large sample size, which will probably be used rarely. However,

the data allow the user of this document to assess the economic benefit of the OC test

requirements and, in general, to assess the advisability of performing the test.

The test is used for reliability testing; for example, to check compliance of the reliability of a

non-repairable item for a given time interval (warranty period or designed lifetime). The test

makes no assumption on whether the failure rate is constant or non-constant. IEC 61124

assumes a constant failure rate and is more statistically efficient since it takes the accumulated

operating time into account.

Clause 4 presents the types of tests and recommendations for their selection. It also discusses

the ability to reuse items during the test. Clause 5 explains the parameters of the stopping

boundaries and the characteristics of the SPRT (their values are given in Annex D). Clause 6

is devoted to the FTFT, a table with parameters of stopping boundaries and characteristics is

given. Annex A is devoted to the SPRT and provides examples of choosing a test by cost-

benefit considerations, extension of the test set of Clause 5 by extra- and interpolation.

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– 8 – IEC 61123:2019 © IEC 2019
RELIABILITY TESTING –
COMPLIANCE TEST PLANS FOR SUCCESS RATIO
1 Scope

This international standard is intended to define a procedure to verify if a reliability of an

item/system complies with the stated requirements. The requirement is assumed to be specified

as the percentage of success (success ratio) or the percentage of failures (failure ratio).

This document can be used where a number of items are tested (number of trials performed)

and classified as passed or failed. It can also be used where one or a number of items are

tested repeatedly. The procedures are based on the assumption that the probability of success

or failure is the same from trial to trial (statistically independent events). Plans for fixed

trial/failure terminated tests as well as truncated sequential probability ratio tests (SPRTs) are

included. This document contains extensive tables with ready-to-use SPRT plans and their

characteristics for equal and non-equal risks for supplier and customer.

In the case of the reliability compliance tests for constant failure rate/intensity, IEC 61124

applies.
2 Normative references

The following documents are referred to in the text in such a way that some or all of their content

constitutes requirements 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 60050-192:2015, International Electrotechnical Vocabulary – Part 192: Dependability

(available at http://www.electropedia.org)

IEC 60300-3-5:2001, Dependability management – Part 3-5: Application guide – Reliability test

conditions and statistical test principles
3 Terms, definitions, abbreviated terms and symbols
3.1 Terms and definitions
For the purposes of this document, the following terms and definitions apply.

ISO and IEC maintain terminological databases for use in standardization at the following

addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
3.1.1
success ratio

probability that an item will perform as required or that a trial will be successful under stated

conditions

Note 1 to entry: An observed success ratio is the ratio of the number of non-faulty items or of successful trials at

the completion of testing, to the total number of test items or number of trials.

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IEC 61123:2019 © IEC 2019 – 9 –
3.1.2
failure ratio

probability that an item will fail or that a trial will be unsuccessful under stated conditions

Note 1 to entry: An observed failure ratio is the ratio of the number of faulty items or of unsuccessful trials at the

completion of testing, to the total number of test items or number of trials.
3.2 Abbreviated terms and symbols
3.2.1 Abbreviated terms
AQL acceptable quality level
CRP consumer’s risk point
ENT expected number of trials to decision
FTFT fixed trial/failure terminated test
LQ limiting quality
MaxNT maximal number of trials to decision
OC operating characteristic
PRP producer’s risk point

SPRT (truncated) sequential probability ratio test (in some literature called probability ratio

sequential test (PRST))
3.2.2 Symbols
α nominal producer's risk (type I risk)
...