SIST EN 61508-5:2011

Functional safety of electrical/electronic/programmable electronic safety-related systems - Part 5: Examples of methods for the determination of safety integrity levels (IEC 61508-5:2010)

1.1 This part of IEC 61508 provides information on
– the underlying concepts of risk and the relationship of risk to safety integrity (see Annex A);
– a number of methods that will enable the safety integrity levels for the E/E/PE safetyrelated
systems to be determined (see Annexes C, D, E, F and G).
The method selected will depend upon the application sector and the specific circumstances under consideration. Annexes C, D, E, F and G illustrate quantitative and qualitative approaches and have been simplified in order to illustrate the underlying principles. These annexes have been included to illustrate the general principles of a number of methods but do not provide a definitive account. Those intending to apply the methods indicated in these annexes should consult the source material referenced.
NOTE For more information on the approaches illustrated in Annexes B, and E, see references [5] and [8] in the Bibliography. See also reference [6] in the Bibliography for a description of an  additional approach.
1.2 IEC 61508-1, IEC 61508-2, IEC 61508-3 and IEC 61508-4 are basic safety publications, although this status does not apply in the context of low complexity E/E/PE safety-related systems (see 3.4.3 of IEC 61508-4). As basic safety publications, they are intended for use by technical committees in the preparation of standards in accordance with the principles contained in IEC Guide 104 and ISO/IEC Guide 51. IEC 61508-1, IEC 61508-2, IEC 61508-3 and IEC 61508-4 are also intended for use as stand-alone publications. The horizontal safety function of this international standard does not apply to medical equipment in compliance with the IEC 60601 series.
1.3 One of the responsibilities of a technical committee is, wherever applicable, to make use of basic safety publications in the preparation of its publications. In this context, the requirements, test methods or test conditions of this basic safety publication will not apply unless specifically referred to or included in the publications prepared by those technical committees.
1.4 Figure 1 shows the overall framework of the IEC 61508 series and indicates the role that IEC 61508-5 plays in the achievement of functional safety for E/E/PE safety-related systems.

Funktionale Sicherheit sicherheitsbezogener elektrischer/elektronischer/programmierbarer elektronischer Systeme - Teil 5: Beispiele zur Ermittlung der Stufe der Sicherheitsintegrität (safety integrety level) (IEC 61508-5:2010)

Sécurité fonctionnelle des systèmes électriques/électroniques/électroniques programmables relatifs à la sécurité - Partie 5: Exemples de méthodes pour la détermination des niveaux d'intégrité de sécurité (IEC 61508-5:2010)

La CEI 61508-5:2010 fournit des informations sur les concepts sous-jacents à la notion de risque et les liens entre le risque et l'intégrité de sécurité (voir Annexe A), un certain nombre de méthodes qui permettent de déterminer les niveaux d'intégrité de sécurité des systèmes E/E/PE relatifs à la sécurité (voir Annexes B, C, D et E). La méthode retenue dépend du secteur d'application et des conditions spécifiques à prendre en considération. Les Annexes C, D, E, F et G illustrent les approches quantitatives et qualitatives et ont été simplifiées dans le but d'illustrer les principes sous-jacents. Ces annexes ont été incluses pour illustrer les principes généraux d'un certain nombre de méthodes mais ne fournissent pas une interprétation figée. Il convient que les personnes souhaitant appliquer les méthodes indiquées dans ces annexes consultent les sources documentaires référencées. Cette deuxième édition annule et remplace la première édition publiée en 1998 dont elle constitue une révision technique. Elle a fait l'objet d'une révision approfondie et intègre de nombreux commentaires reçus lors des différentes phases de révision.

Funkcijska varnost električnih/elektronskih/elektronsko programirljivih varnostnih sistemov - 5. del: Primeri metod za ugotavljanje ravni celovite varnosti (IEC 61508-5:2010)

1.1 Ta del IEC 61508 podaja informacije o
– osnovnih konceptih tveganja in odnosu med tveganjem in celovito varnostjo (glej dodatek A);
– številnih metodah, ki omogočajo ugotavljanje ravni celovite varnosti za varnostne sisteme E/E/PE (glej dodatke C, D, E, F in G).
Izbrana metoda je odvisna od obravnavanega aplikacijskega sektorja in okoliščin. Dodatki C, D, E, F in G prikazujejo kvantitativne in kvalitativne pristope in so poenostavljeni, da ponazorijo osnovna načela. Ti dodatki so vključeni za ponazoritev splošnih načel številnih metod, vendar niso dokončni. Kdor namerava uporabiti metode, navedene v teh dodatkih, naj preveri referenčni izvorni material.
OPOMBA: Za več informacij o pristopih, prikazanih v dodatkih B in E, glej referenci [5] in [8] v bibliografiji. Za opis dodatnega pristopa glej tudi referenco [6] v bibliografiji.
1.2 IEC 61508-1, IEC 61508-2, IEC 61508-3 in IEC 61508-4 so osnovne varnostne objave, čeprav ta status ne velja v okviru nezapletenih varnostnih sistemov E/E/PE (glej točko 3.4.3 IEC 61508-4). Kot osnovne varnostne objave so namenjeni temu, da jih uporabljajo tehnični odbori pri pripravi standardov v skladu z načeli, opredeljenimi v Vodilu IEC 104 in Vodilu ISO/IEC 51. IEC 61508-1, IEC 61508-2, IEC 61508-3 in IEC 61508-4 se lahko tudi uporabijo kot samostojne objave. Horizontalno varnostno delovanje tega mednarodnega standarda ne velja za medicinsko opremo v skladu s serijo IEC 60601.
1.3 Ena od odgovornosti tehničnega odbora je, če je primerno, da uporabi temeljne varnostne objave pri pripravi svojih objav. V tem okviru zahteve, preskusne metode ali preskusni pogoji te temeljne varnostne objave ne veljajo, razen če se objave, ki so jih pripravili tehnični odbori, nanje izrecno sklicujejo ali jih vključujejo.
1.4 Slika 1 prikazuje celoten okvir serije IEC 61508 in nakazuje vlogo IEC 61508-5 pri doseganju funkcijske varnosti E/E/PE-varnostnih sistemov.

General Information

• Status: Published
• Publication Date: 11-Apr-2011
• ICS: 25.040.40 - Industrial process measurement and control
• Technical Committee: MOV - Measuring equipment for electromagnetic quantities
• Current Stage: 6060 - National Implementation/Publication (Adopted Project)
• Start Date: 06-Apr-2011
• Due Date: 11-Jun-2011
• Completion Date: 12-Apr-2011
• Ref Project: EN 61508-5:2010 - Functional safety of electrical/electronic/programmable electronic safety-related systems - Part 5: Examples of methods for the determination of safety integrity levels

Relations

• Replaces: SIST EN 61508-5:2007 - Functional safety of electrical/electronic/programmable electronic safety-related systems -- Part 5: Examples of methods for the determination of safety integrity levels
• Effective Date: 01-May-2011
• Revised: SIST EN 61508-5:2007 - Functional safety of electrical/electronic/programmable electronic safety-related systems -- Part 5: Examples of methods for the determination of safety integrity levels
• Effective Date: 07-May-2013
• Replaces: SIST EN 61508-5:2007 - Functional safety of electrical/electronic/programmable electronic safety-related systems -- Part 5: Examples of methods for the determination of safety integrity levels
• Effective Date: 28-Jan-2023

Standards Content (Sample)

SLOVENSKI STANDARD
SIST EN 61508-5:2011
01-maj-2011
1DGRPHãþD
SIST EN 61508-5:2007
)XQNFLMVNDYDUQRVWHOHNWULþQLKHOHNWURQVNLKHOHNWURQVNRSURJUDPLUOMLYLKYDUQRVWQLK
VLVWHPRYGHO3ULPHULPHWRG]DXJRWDYOMDQMHUDYQLFHORYLWHYDUQRVWL,(&

Functional safety of electrical/electronic/programmable electronic safety-related systems
- Part 5: Examples of methods for the determination of safety integrity levels (IEC 61508-
5:2010)
Funktionale Sicherheit sicherheitsbezogener
elektrischer/elektronischer/programmierbarer elektronischer Systeme - Teil 5: Beispiele
zur Ermittlung der Stufe der Sicherheitsintegrität (safety integrety level) (IEC 61508-
5:2010)
Sécurité fonctionnelle des systèmes électriques/électroniques/électroniques
programmables relatifs à la sécurité - Partie 5: Exemples de méthodes pour la
détermination des niveaux d'intégrité de sécurité (IEC 61508-5:2010)
Ta slovenski standard je istoveten z: EN 61508-5:2010
ICS:
25.040.40 Merjenje in krmiljenje Industrial process
industrijskih postopkov measurement and control
SIST EN 61508-5:2011 en
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

---------------------- Page: 1 ----------------------

SIST EN 61508-5:2011

---------------------- Page: 2 ----------------------

SIST EN 61508-5:2011

EUROPEAN STANDARD
EN 61508-5

NORME EUROPÉENNE
May 2010
EUROPÄISCHE NORM

ICS 25.040.40 Supersedes EN 61508-5:2001


English version


Functional safety of electrical/electronic/programmable electronic safety-
related systems -
Part 5: Examples of methods for the determination of safety integrity
levels
(IEC 61508-5:2010)


Sécurité fonctionnelle des systèmes Funktionale Sicherheit sicherheitsbezogener
électriques/électroniques/électroniques elektrischer/elektronischer/programmierbarer
programmables relatifs à la sécurité - elektronischer Systeme -
Partie 5: Exemples de méthodes Teil 5: Beispiele zur Ermittlung der Stufe
pour la détermination des niveaux der Sicherheitsintegrität (safety integrety
d'intégrité de sécurité level)
(CEI 61508-5:2010) (IEC 61508-5:2010)




This European Standard was approved by CENELEC on 2010-05-01. 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, Bulgaria, Croatia, 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

Management Centre: Avenue Marnix 17, B - 1000 Brussels


© 2010 CENELEC - All rights of exploitation in any form and by any means reserved worldwide for CENELEC members.
Ref. No. EN 61508-5:2010 E

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SIST EN 61508-5:2011
EN 61508-5:2010 - 2 -
Foreword
The text of document 65A/552/FDIS, future edition 2 of IEC 61508-5, prepared by SC 65A, System
aspects, of IEC TC 65, Industrial-process measurement, control and automation, was submitted to the
IEC-CENELEC parallel vote and was approved by CENELEC as EN 61508-5 on 2010-05-01.
This European Standard supersedes EN 61508-5:2001.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN and CENELEC shall not be held responsible for identifying any or all such patent
rights.
The following dates were fixed:
– latest date by which the EN has to be implemented
at national level by publication of an identical
national standard or by endorsement (dop) 2011-02-01
– latest date by which the national standards conflicting
with the EN have to be withdrawn (dow) 2013-05-01
Annex ZA has been added by CENELEC.
__________
Endorsement notice
The text of the International Standard IEC 61508-5:2010 was approved by CENELEC as a European
Standard without any modification.
In the official version, for Bibliography, the following notes have to be added for the standards indicated:
[1] IEC 61511 series NOTE  Harmonized in EN 61511 series (not modified).
[2] IEC 62061 NOTE  Harmonized as EN 62061.
[3] IEC 61800-5-2 NOTE  Harmonized as EN 61800-5-2.
[9] ISO/IEC 31010 NOTE  Harmonized as EN 31010.
[10] ISO 10418:2003 NOTE  Harmonized as EN 10418:2003 (not modified).
[12] ISO 13849-1:2006 NOTE  Harmonized as EN ISO 13849-1:2006 (not modified).
[13] IEC 60601 series NOTE  Harmonized in EN 60601 series (partially modified).
[14] IEC 61508-2 NOTE  Harmonized as EN 61508-2.
[15] IEC 61508-3 NOTE  Harmonized as EN 61508-3.
[16] IEC 61508-6 NOTE  Harmonized as EN 61508-6.
[17] IEC 61508-7 NOTE  Harmonized as EN 61508-7.
[18] IEC 61511-1 NOTE  Harmonized as EN 61511-1.
__________

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SIST EN 61508-5:2011
- 3 - EN 61508-5:2010
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

IEC 61508-1 2010 Functional safety of EN 61508-1 2010
electrical/electronic/programmable electronic
safety-related systems -
Part 1: General requirements


IEC 61508-4 2010 Functional safety of EN 61508-4 2010
electrical/electronic/programmable electronic
safety-related systems -
Part 4: Definitions and abbreviations

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SIST EN 61508-5:2011

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SIST EN 61508-5:2011
IEC 61508-5
®
Edition 2.0 2010-04
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE

Functional safety of electrical/electronic/programmable electronic safety-related
systems –
Part 5: Examples of methods for the determination of safety integrity levels

Sécurité fonctionnelle des systèmes électriques/électroniques/électroniques
programmables relatifs à la sécurité –
Partie 5: Exemples de méthodes pour la détermination des niveaux d'intégrité
de sécurité

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
X
CODE PRIX
ICS 25.040.40 ISBN 978-2-88910-528-1
® Registered trademark of the International Electrotechnical Commission
Marque déposée de la Commission Electrotechnique Internationale

---------------------- Page: 7 ----------------------

SIST EN 61508-5:2011
– 2 – 61508-5 © IEC:2010
CONTENTS
FOREWORD.3
INTRODUCTION.5
1 Scope.7
2 Normative references .9
3 Definitions and abbreviations.9
Annex A (informative) Risk and safety integrity – General concepts .10
Annex B (informative) Selection of methods for determining safety integrity level
requirements.21
Annex C (informative) ALARP and tolerable risk concepts .24
Annex D (informative) Determination of safety integrity levels – A quantitative method .27
Annex E (informative) Determination of safety integrity levels – Risk graph methods .30
Annex F (informative) Semi-quantitative method using layer of protection analysis
(LOPA) .38
Annex G (informative) Determination of safety integrity levels – A qualitative method –
hazardous event severity matrix.44
Bibliography.46

Figure 1 – Overall framework of the IEC 61508 series .8
Figure A.1 – Risk reduction – general concepts (low demand mode of operation) .14
Figure A.2 – Risk and safety integrity concept .14
Figure A.3 – Risk diagram for high demand applications .15
Figure A.4 – Risk diagram for continuous mode operation .16
Figure A.5 – Illustration of common cause failures (CCFs) of elements in the EUC
control system and elements in the E/E/PE safety-related system.17
Figure A.6 – Common cause between two E/E/PE safety-related systems .18
Figure A.7 – Allocation of safety requirements to the E/E/PE safety-related systems,
and other risk reduction measures .20
Figure C.1 – Tolerable risk and ALARP.25
Figure D.1 – Safety integrity allocation – example for safety-related protection system.29
Figure E.1 – Risk Graph: general scheme.33
Figure E.2 – Risk graph – example (illustrates general principles only) .34
Figure G.1 – Hazardous event severity matrix – example (illustrates general principles
45
only) .

26
Table C.1 – Example of risk classification of accidents .
Table C.2 – Interpretation of risk classes .26
Table E.1 – Example of data relating to risk graph (Figure E.2).35
Table E.2 – Example of calibration of the general purpose risk graph .36
Table F.1 – LOPA report .40

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SIST EN 61508-5:2011
61508-5 © IEC:2010 – 3 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________

FUNCTIONAL SAFETY OF ELECTRICAL/ELECTRONIC/
PROGRAMMABLE ELECTRONIC SAFETY-RELATED SYSTEMS –

Part 5: Examples of methods for the determination
of safety integrity levels


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-
governmental organizations liaising with the IEC also participate in this preparation. IEC collaborates closely
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 itself does not provide any attestation of conformity. Independent certification bodies provide conformity
assessment services and, in some areas, access to IEC marks of conformity. IEC is not responsible for any
services carried out by independent certification bodies.
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 61508-5 has been prepared by subcommittee 65A: System
aspects, of IEC technical committee 65: Industrial-process measurement, control and
automation.
This second edition cancels and replaces the first edition published in 1998. This edition
constitutes a technical revision.
This edition has been subject to a thorough review and incorporates many comments received
at the various revision stages.

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SIST EN 61508-5:2011
– 4 – 61508-5 © IEC:2010
The text of this standard is based on the following documents:
FDIS Report on voting
65A/552/FDIS 65A/576/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
A list of all parts of the IEC 61508 series, published under the general title Functional safety
of electrical / electronic / programmable electronic safety-related systems, can be found on
the IEC website.
The committee has decided that the contents of this publication will remain unchanged until
the stability 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.

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SIST EN 61508-5:2011
61508-5 © IEC:2010 – 5 –
INTRODUCTION
Systems comprised of electrical and/or electronic elements have been used for many years to
perform safety functions in most application sectors. Computer-based systems (generically
referred to as programmable electronic systems) are being used in all application sectors to
perform non-safety functions and, increasingly, to perform safety functions. If computer
system technology is to be effectively and safely exploited, it is essential that those
responsible for making decisions have sufficient guidance on the safety aspects on which to
make these decisions.
This International Standard sets out a generic approach for all safety lifecycle activities for
systems comprised of electrical and/or electronic and/or programmable electronic (E/E/PE)
elements that are used to perform safety functions. This unified approach has been adopted
in order that a rational and consistent technical policy be developed for all electrically-based
safety-related systems. A major objective is to facilitate the development of product and
application sector international standards based on the IEC 61508 series.
NOTE 1 Examples of product and application sector international standards based on the IEC 61508 series are
given in the Bibliography (see references [1], [2] and [3]).
In most situations, safety is achieved by a number of systems which rely on many
technologies (for example mechanical, hydraulic, pneumatic, electrical, electronic, programmable
electronic). Any safety strategy must therefore consider not only all the elements within an
individual system (for example sensors, controlling devices and actuators) but also all the
safety-related systems making up the total combination of safety-related systems. Therefore,
while this International Standard is concerned with E/E/PE safety-related systems, it may also
provide a framework within which safety-related systems based on other technologies may be
considered.
It is recognized that there is a great variety of applications using E/E/PE safety-related
systems in a variety of application sectors and covering a wide range of complexity, hazard
and risk potentials. In any particular application, the required safety measures will be
dependent on many factors specific to the application. This International Standard, by being
generic, will enable such measures to be formulated in future product and application sector
international standards and in revisions of those that already exist.
This International Standard
– considers all relevant overall, E/E/PE system and software safety lifecycle phases (for
example, from initial concept, though design, implementation, operation and maintenance
to decommissioning) when E/E/PE systems are used to perform safety functions;
– has been conceived with a rapidly developing technology in mind; the framework is
sufficiently robust and comprehensive to cater for future developments;
– enables product and application sector international standards, dealing with E/E/PE
safety-related systems, to be developed; the development of product and application
sector international standards, within the framework of this standard, should lead to a high
level of consistency (for example, of underlying principles, terminology etc.) both within
application sectors and across application sectors; this will have both safety and economic
benefits;
– provides a method for the development of the safety requirements specification necessary
to achieve the required functional safety for E/E/PE safety-related systems;
– adopts a risk-based approach by which the safety integrity requirements can be
determined;
– introduces safety integrity levels for specifying the target level of safety integrity for the
safety functions to be implemented by the E/E/PE safety-related systems;
NOTE 2 The standard does not specify the safety integrity level requirements for any safety function, nor does it
mandate how the safety integrity level is determined. Instead it provides a risk-based conceptual framework and
example techniques.

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SIST EN 61508-5:2011
– 6 – 61508-5 © IEC:2010
– sets target failure measures for safety functions carried out by E/E/PE safety-related
systems, which are linked to the safety integrity levels;
– sets a lower limit on the target failure measures for a safety function carried out by a
single E/E/PE safety-related system. For E/E/PE safety-related systems operating in
– a low demand mode of operation, the lower limit is set at an average probability of a
–5
dangerous failure on demand of 10 ;
– a high demand or a continuous mode of operation, the lower limit is set at an average

–9 -1
frequency of a dangerous failure of 10 [h ];
NOTE 3 A single E/E/PE safety-related system does not necessarily mean a single-channel architecture.
NOTE 4 It may be possible to achieve designs of safety-related systems with lower values for the target safety
integrity for non-complex systems, but these limits are considered to represent what can be achieved for relatively
complex systems (for example programmable electronic safety-related systems) at the present time.
– sets requirements for the avoidance and control of systematic faults, which are based on
experience and judgement from practical experience gained in industry. Even though the
probability of occurrence of systematic failures cannot in general be quantified the
standard does, however, allow a claim to be made, for a specified safety function, that the
target failure measure associated with the safety function can be considered to be
achieved if all the requirements in the standard have been met;
– introduces systematic capability which applies to an element with respect to its confidence
that the systematic safety integrity meets the requirements of the specified safety integrity
level;
– adopts a broad range of principles, techniques and measures to achieve functional safety
for E/E/PE safety-related systems, but does not explicitly use the concept of fail safe
However, the concepts of “fail safe” and “inherently safe” principles may be applicable and
adoption of such concepts is acceptable providing the requirements of the relevant
clauses in the standard are met.

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SIST EN 61508-5:2011
61508-5 © IEC:2010 – 7 –
FUNCTIONAL SAFETY OF ELECTRICAL/ELECTRONIC/
PROGRAMMABLE ELECTRONIC SAFETY-RELATED SYSTEMS –

Part 5: Examples of methods for the determination
of safety integrity levels



1 Scope
1.1 This part of IEC 61508 provides information on
– the underlying concepts of risk and the relationship of risk to safety integrity (see Annex
A);
– a number of methods that will enable the safety integrity levels for the E/E/PE safety-
related systems to be determined (see Annexes C, D, E, F and G).
The method selected will depend upon the application sector and the specific circumstances
under consideration. Annexes C, D, E, F and G illustrate quantitative and qualitative
approaches and have been simplified in order to illustrate the underlying principles. These
annexes have been included to illustrate the general principles of a number of methods but do
not provide a definitive account. Those intending to apply the methods indicated in these
annexes should consult the source material referenced.
NOTE For more information on the approaches illustrated in Annexes B, and E, see references [5] and [8] in the
Bibliography. See also reference [6] in the Bibliography for a description of an additional approach.
1.2 IEC 61508-1, IEC 61508-2, IEC 61508-3 and IEC 61508-4 are basic safety publications,
although this status does not apply in the context of low complexity E/E/PE safety-related
systems (see 3.4.3 of IEC 61508-4). As basic safety publications, they are intended for use by
technical committees in the preparation of standards in accordance with the principles
contained in IEC Guide 104 and ISO/IEC Guide 51. IEC 61508-1, IEC 61508-2, IEC 61508-3
and IEC 61508-4 are also intended for use as stand-alone publications. The horizontal safety
function of this international standard does not apply to medical equipment in compliance with
the IEC 60601 series.
1.3 One of the responsibilities of a technical committee is, wherever applicable, to make use
of basic safety publications in the preparation of its publications. In this context, the
requirements, test methods or test conditions of this basic safety publication will not apply
unless specifically referred to or included in the publications prepared by those technical
committees.
1.4 Figure 1 shows the overall framework of the IEC 61508 series and indicates the role that
IEC 61508-5 plays in the achievement of functional safety for E/E/PE safety-related systems.

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SIST EN 61508-5:2011
– 8 – 61508-5 © IEC:2010
Technical Requirements Other Requirements
Part 4
Part 1
Definitions &
Development of the overall
abbreviations
safety requirements
(concept, scope, definition,
hazard and risk analysis)
7.1 to 7.5
Part 5
Example of methods
for the determination Part 1
of safety integrity Documentation
levels Clause 5 &
Part 1
Annex A
Allocation of the safety requirements
to the E/E/PE safety-related systems
7.6
Part 1
Management of
functional safety
Clause 6
Part 1
Specification of the system safety
requirements for the E/E/PE
safety-related systems
Part 1
Functional safety
7.10 assessm ent
Clause 8
Part 6
Guidelines for the
application of
Parts 2 & 3
Part 2 Part 3
Realisation phase Realisation phase
for E/E/PE for safety-related
safety-related software
systems
Part 7
Overview of
techniques and
measures
Part 1
Installation, commissioning
& safety validation of E/E/PE
safety-related systems
7.13 - 7.14
Part 1
Operation, maintenance,repair,
modification and retrofit,
decommissioning or disposal of
E/E/PE safety-related systems
7.15 - 7.17

Figure 1 – Overall framework of the IEC 61508 series

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SIST EN 61508-5:2011
61508-5 © IEC:2010 – 9 –
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 61508-1:2010, Functional safety of electrical/electronic/programmable electronic safety-
related systems – Part 1: General requirements
IEC 61508-4:2010, Functional safety of electrical/electronic/programmable electronic safety-
related systems – Part 4: Definitions and abbreviations
3 Definitions and abbreviations
For the purposes of this document, the definitions and abbreviations given in IEC 61508-4
apply.

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SIST EN 61508-5:2011
– 10 – 61508-5 © IEC:2010
Annex A
(informative)

Risk and safety integrity –
General concepts

A.1 General
This annex provides information on the underlying concepts of risk and the relationship of risk
to safety integrity.
A.2 Necessary risk reduction
The necessary risk reduction (see 3.5.18 of IEC 61508-4) is the reduction in risk that has to
be achieved to meet the tolerable risk for a specific situation (which may be stated either
1 2
qualitatively or quantitatively ). The concept of necessary risk reduction is of fundamental
importance in the development of the safety requirements specification for the E/E/PE safety-
related systems (in particular, the safety integrity requirements part of the safety requirements
specification). The purpose of determining the tolerable risk for a specific hazardous event is
to state what is deemed reasonable with respect to both the frequency (or probability) of the
hazardous event and its specific consequences. Safety-related systems are designed to
reduce the frequency (or probability) of the hazardous event and/or the consequences of the
hazardous event.
The tolerable risk will depend on many factors (for example, severity of injury, the number of
people exposed to danger, the frequency at which a person or people are exposed to danger
and the duration of the exposure). Important factors will be the perception and views of those
exposed to the hazardous event. In arriving at what constitutes a tolerable risk for a specific
application, a number of inputs are considered. These include:
– legal requirements, both general and those directly relevant to the specific application;
– guidelines from the appropriate safety regulatory authority;
– discussions and agreements with the different parties involved in the application;
– industry standards and guidelines;
– international discussions and agreements; the role of national and international standards
is becoming incre
...