SIST EN 61508-2:2007
(Main)Functional safety of electrical/electronic/programmable electronic safety-related systems -- Part 2: Requirements for electrical/electronic/programmable electronic safety-related systems
Functional safety of electrical/electronic/programmable electronic safety-related systems -- Part 2: Requirements for electrical/electronic/programmable electronic safety-related systems
This part of IEC 61508 is intended to be used after a thorough understanding of IEC 61508-1, which provides the overall framework for the achievement of functional safety.
Funktionale Sicherheit sicherheitsbezogener elektrischer/elektronischer/programmierbarer elektronischer Systeme -- Teil 2: Anforderungen an sicherheitsbezogene elektrische/elektronische/programmierbare elektronische Systeme
Sécurité fonctionnelle des systèmes électriques/électroniques/électroniques programmables relatifs à la sécurité -- Partie 2: Prescriptions pour les systèmes électriques/électroniques/électroniques programmables relatifs à la sécurité
Destinée à être utilisée uniquement après avoir compris de manière approfondie la EN 61508-1 qui fournit le cadre global permettant de réaliser la sécurité fonctionnelle.
Funkcijska varnost električnih/elektronskih/programirljivih elektronskih varnostnih sistemov - 2. del: Zahteve za električne/elektronske/programirljive elektronske varnostne sisteme (IEC 61508-2:2000)
General Information
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EUROPEAN STANDARD EN 61508-2
NORME EUROPÉENNE
EUROPÄISCHE NORM December 2001
ICS 25.040.40
English version
Functional safety of electrical/electronic/programmable electronic
safety-related systems
Part 2: Requirements for electrical/electronic/programmable electronic
safety-related systems
(IEC 61508-2:2000)
Sécurité fonctionnelle des systèmes Funktionale Sicherheit
électriques/électroniques/électroniques sicherheitsbezogener elektrischer/
programmables relatifs à la sécurité elektronischer/programmierbarer
Partie 2: Prescriptions pour les systèmes elektronischer Systeme
électriques/électroniques/électroniques Teil 2: Anforderungen an
programmables relatifs à la sécurité sicherheitsbezogene elektrische/
(CEI 61508-2:2000) elektronische/programmierbare
elektronische Systeme
(IEC 61508-2:2000)
This European Standard was approved by CENELEC on 2001-07-03. 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, Czech Republic,
Denmark, Finland, France, Germany, Greece, Iceland, Ireland, Italy, Luxembourg, Malta, Netherlands,
Norway, Portugal, Spain, Sweden, Switzerland and 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
© 2001 CENELEC - All rights of exploitation in any form and by any means reserved worldwide for CENELEC members.
Ref. No. EN 61508-2:2001 E
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EN 61508-2:2001 - 2 -
Foreword
The text of the International Standard IEC 61508-2:2000, prepared by SC 65A, System aspects, of
IEC TC 65, Industrial-process measurement and control, was submitted to the Unique Acceptance
Procedure and was approved by CENELEC as EN 61508-2 on 2001-07-03 without any modification.
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) 2002-08-01
– latest date by which the national standards conflicting
with the EN have to be withdrawn (dow) 2004-08-01
Annexes designated "normative" are part of the body of the standard.
In this standard, annexes A, B, C and ZA are normative.
Annex ZA has been added by CENELEC.
IEC 61508 is a basic safety publication covering the functional safety of electrical, electronic and
programmable electronic safety-related systems. The scope states:
"This International Standard covers those aspects to be considered when electrical/electronic/
programmable electronic systems (E/E/PESs) are used to carry out safety functions. A major objective
of this standard is to facilitate the development of application sector international standards by the
technical committees responsible for the application sector. This will allow all the relevant factors
associated with the application, to be fully taken into account and thereby meet the specific needs of
the application sector. A dual objective of this standard is to enable the development of
electrical/electronic/programmable electronic (E/E/PE) safety-related systems where application sector
international standards may not exist".
The CENELEC Report R0BT-004, ratified by 103 BT (March 2000) accepts that some IEC standards,
which today are either published or under development, are sector implementations of IEC 61508. For
example:
� IEC 61511, Functional safety - Safety instrumented systems for the process industry sector;
� IEC 62061, Safety of machinery – Functional safety of electrical, electronic and programmable
electronic control systems;
� IEC 61513, Nuclear power plants – Instrumentation and control for systems important to safety –
General requirements for systems.
The railways sector has also developed a set of European Standards (EN 50126; EN 50128 and
prEN 50129).
NOTE EN 50126 and EN 50128 were based on earlier drafts of IEC 61508. prEN 50129 is based on the principles of the
latest version of IEC 61508.
This list does not preclude other sector implementations of IEC 61508 which could be currently under
development or published within IEC or CENELEC.
__________
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- 3 - EN 61508-2:2001
Endorsement notice
The text of the International Standard IEC 61508-2:2000 was approved by CENELEC as a European
Standard without any modification.
In the official version, for Bibliography, the following note has to be added for the standard indicated:
IEC 61000-4 NOTE Harmonized in the EN 61000-4 series (not modified).
IEC 60870-5-1 NOTE Harmonized as EN 60870-5-1:1993 (not modified).
__________
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EN 61508-2:2001 - 4 -
Annex ZA
(normative)
Normative references to international publications
with their corresponding European publications
This European Standard incorporates by dated or undated reference, provisions from other
publications. These normative references are cited at the appropriate places in the text and the
publications are listed hereafter. For dated references, subsequent amendments to or revisions of any
of these publications apply to this European Standard only when incorporated in it by amendment or
revision. For undated references the latest edition of the publication referred to applies (including
amendments).
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 60050-371 1984 International electrotechnical vocabulary--
(IEV) - Chapter 371: Telecontrol
IEC 60300-3-2 1993 Dependability management --
Part 3: Application guide
Section 2: Collection of dependability
data from the field
IEC 61000-1-1 1992 Electromagnetic compatibility (EMC)--
Part 1: General
Section 1: Application and interpretation
of fundamental definitions and terms
IEC 61000-2-5 1995 Part 2-5: Environment - Classification of--
electromagnetic environments - Basic
EMC publication
IEC 61508-1 1998 Functional safety of EN 61508-1 2001
+ corr. May 1999 electrical/electronic/programmable
electronic safety-related systems
Part 1: General requirements
IEC 61508-3 1998 Part 3: Software requirements EN 61508-3 2001
+ corr. April 1999
IEC 61508-4 1998 Part 4: Definitions and abbreviations EN 61508-4 2001
+ corr. April 1999
IEC 61508-5 1998 Part 5: Examples of methods for the EN 61508-5 2001
+ corr. April 1999 determination of safety integrity levels
IEC 61508-6 2000 Part 6: Guidelines on the application of EN 61508-6 2001
IEC 61508-2 and IEC 61508-3
IEC 61508-7 2000 Part 7: Overview of techniques and EN 61508-7 2001
measures
IEC Guide 104 1997 The preparation of safety publications--
and the use of basic safety publications
and group safety publications
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- 5 - EN 61508-2:2001
Publication Year Title EN/HD Year
ISO/IEC Guide 51 1990 Guidelines for the inclusion of safety--
aspects in standards
IEEE 352 1987 IEEE guide for general principles of--
reliability analysis of nuclear power
generating station safety systems
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INTERNATIONAL IEC
STANDARD
61508-2
First edition
2000-05
BASIC SAFETY PUBLICATION
Functional safety of electrical/electronic/
programmable electronic safety-related systems –
Part 2:
Requirements for electrical/electronic/
programmable electronic safety-related systems
IEC 2000 Copyright - all rights reserved
No part of this publication may be reproduced or utilized in any form or by any means, electronic or mechanical,
including photocopying and microfilm, without permission in writing from the publisher.
International Electrotechnical Commission, 3, rue de Varembé, PO Box 131, CH-1211 Geneva 20, Switzerland
Telephone: +41 22 919 02 11 Telefax: +41 22 919 03 00 E-mail: inmail@iec.ch Web: www.iec.ch
PRICE CODE
XB
Commission Electrotechnique Internationale
International Electrotechnical Commission
Международная Электротехническая Комиссия
For price, see current catalogue
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61508-2 © IEC:2000 – 3 –
CONTENTS
Page
FOREWORD . 7
INTRODUCTION . 11
Clause
1 Scope . 15
2 Normative references. 21
3 Definitions and abbreviations . 23
4 Conformance to this standard . 23
5 Documentation. 23
6 Management of functional safety. 23
7 E/E/PES safety lifecycle requirements . 23
7.1 General. 23
7.2 E/E/PES safety requirements specification. 31
7.3 E/E/PES safety validation planning . 35
7.4 E/E/PES design and development. 37
7.5 E/E/PES integration . 71
7.6 E/E/PES operation and maintenance procedures . 73
7.7 E/E/PES safety validation . 77
7.8 E/E/PES modification. 79
7.9 E/E/PES verification. 79
8 Functional safety assessment . 83
Annex A (normative) Techniques and measures for E/E/PE safety-related systems:
control of failures during operation . 85
A.1 General. 85
A.2 Hardware safety integrity. 87
A.3 Systematic safety integrity. 105
Annex B (normative) Techniques and measures for E/E/PE safety-related systems:
avoidance of systematic failures during the different phases of the lifecycle . 117
Annex C (normative) Diagnostic coverage and safe failure fraction . 137
C.1 Calculation of diagnostic coverage and safe failure fraction of a subsystem . 137
C.2 Determination of diagnostic coverage factors . 139
Bibliography . 143
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61508-2 © IEC:2000 – 5 –
Page
Figure 1 – Overall framework of IEC 61508 . 19
Figure 2 – E/E/PES safety lifecycle (in realisation phase). 25
Figure 3 – Relationship and scope for IEC 61508-2 and IEC 61508-3. 27
Figure 4 – Relationship between the hardware and software architectures of
programmable electronics . 39
Figure 5 – Example limitation on hardware safety integrity for a single-channel
safety function. 49
Figure 6 – Example limitation on hardware safety integrity for a multiple-channel
safety function. 53
Table 1 – Overview – Realisation phase of the E/E/PES safety lifecycle. 29
Table 2 – Hardware safety integrity: architectural constraints on type A safety-related
subsystems . 47
Table 3 – Hardware safety integrity: architectural constraints on type B safety-related
subsystems . 47
Table A.1 – Faults or failures to be detected during operation or to be analysed in
the derivation of safe failure fraction. 89
Table A.2 – Electrical subsystems . 91
Table A.3 – Electronic subsystems. 93
Table A.4 – Processing units . 93
Table A.5 – Invariable memory ranges . 95
Table A.6 – Variable memory ranges. 95
Table A.7 – I/O units and interface (external communication) . 97
Table A.8 – Data paths (internal communication) . 97
Table A.9 – Power supply. 99
Table A.10 – Program sequence (watch-dog) . 99
Table A.11 – Ventilation and heating system (if necessary) . 101
Table A.12 – Clock . 101
Table A.13 – Communication and mass-storage. 103
Table A.14 – Sensors . 103
Table A.15 – Final elements (actuators) . 105
Table A.16 – Techniques and measures to control systematic failures caused by
hardware and software design. 109
Table A.17 – Techniques and measures to control systematic failures caused by
environmental stress or influences . 111
Table A.18 – Techniques and measures to control systematic operational failures . 113
Table A.19 – Effectiveness of techniques and measures to control systematic failures . 115
Table B.1 – Recommendations to avoid mistakes during specification of E/E/PES
requirements (see 7.2) . 121
Table B.2 – Recommendations to avoid introducing faults during E/E/PES design and
development (see 7.4). 12 3
Table B.3 – Recommendations to avoid faults during E/E/PES integration (see 7.5). 125
Table B.4 – Recommendations to avoid faults and failures during E/E/PES operation
and maintenance procedures (see 7.6). 127
Table B.5 – Recommendations to avoid faults during E/E/PES safety validation
(see 7.7) . 129
Table B.6 – Effectiveness of techniques and measures to avoid systematic failures . 131
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61508-2 © IEC:2000 – 7 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
FUNCTIONAL SAFETY OF ELECTRICAL/ELECTRONIC/PROGRAMMABLE
ELECTRONIC SAFETY-RELATED SYSTEMS –
Part 2: Requirements for electrical/electronic/programmable
electronic safety-related systems
FOREWORD
1) The IEC (International Electrotechnical Commission) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of the 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, the IEC publishes International Standards. 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. The 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 the 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 National Committees.
3) The documents produced have the form of recommendations for international use and are published in the form
of standards, technical specifications, technical reports or guides and they are accepted by the National
Committees in that sense.
4) In order to promote international unification, IEC National Committees undertake to apply IEC International
Standards transparently to the maximum extent possible in their national and regional standards. Any
divergence between the IEC Standard and the corresponding national or regional standard shall be clearly
indicated in the latter.
5) The IEC provides no marking procedure to indicate its approval and cannot be rendered responsible for any
equipment declared to be in conformity with one of its standards.
6) Attention is drawn to the possibility that some of the elements of this International Standard may be the subject
of patent rights. The IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 61508-2 has been prepared by subcommittee 65A: System
aspects, of IEC technical committee 65: Industrial-process measurement and control.
It has the status of a basic safety publication according to IEC Guide 104.
The text of this standard is based on the following documents:
FDIS Report on voting
65A/294/FDIS 65A/303/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 3.
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61508-2 © IEC:2000 – 9 –
Annexes A, B, and C form an integral part of this standard.
IEC 61508 consists of the following parts, under the general title Functional safety of
electrical/electronic/programmable electronic safety-related systems:
– Part 1: General requirements
– Part 2: Requirements for electrical/electronic/programmable electronic safety-related sys-
tems
– Part 3: Software requirements
– Part 4: Definitions and abbreviations
– Part 5: Examples of methods for the determination of safety integrity levels
– Part 6: Guidelines on the application of parts 2 and 3
– Part 7: Overview of techniques and measures
The committee has decided that the contents of this publication will remain unchanged
until 2006. At this date, the publication will be
• reconfirmed;
• withdrawn;
• replaced by a revised edition, or
• amended.
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61508-2 © IEC:2000 – 11 –
INTRODUCTION
Systems comprised of electrical and/or electronic components have been used for many years
to perform safety functions in most application sectors. Computer-based systems (generically
referred to as programmable electronic systems (PESs)) 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 those 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 components
(electrical/electronic/programmable electronic systems (E/E/PESs)) 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 application sector standards.
In most situations, safety is achieved by a number of protective systems which may 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 electrical/electronic/
programmable electronic (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 recognised that there is a great variety of E/E/PES applications 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 application sector International Standards.
This International Standard
– considers all relevant overall, E/E/PES and software safety lifecycle phases (for example,
from initial concept, through design, implementation, operation and maintenance to
decommissioning) when E/E/PESs 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 application sector International Standards, dealing with safety-related E/E/PESs,
to be developed; the development of application sector International Standards, within the
framework of this International 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;
– uses 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;
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61508-2 © IEC:2000 – 13 –
– adopts a risk-based approach for the determination of the safety integrity level
requirements;
– sets numerical target failure measures for E/E/PE safety-related systems which are linked
to the safety integrity levels;
– sets a lower limit on the target failure measures, in a dangerous mode of failure, that can
be claimed for 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
–5
failure of 10 to perform its design function on demand,
– a high demand or continuous mode of operation, the lower limit is set at a probability
–9
of a dangerous failure of 10 per hour;
NOTE A single E/E/PE safety-related system does not necessarily mean a single-channel architecture.
– adopts a broad range of principles, techniques and measures to achieve functional safety
for E/E/PE safety-related systems, but does not rely on the concept of fail safe which may
be of value when the failure modes are well defined and the level of complexity is relatively
low. The concept of fail safe was considered inappropriate because of the full range of
complexity of E/E/PE safety-related systems that are within the scope of the standard.
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61508-2 © IEC:2000 – 15 –
FUNCTIONAL SAFETY OF ELECTRICAL/ELECTRONIC/PROGRAMMABLE
ELECTRONIC SAFETY-RELATED SYSTEMS –
Part 2: Requirements for electrical/electronic/programmable
electronic safety-related systems
1 Scope
1.1 This part of IEC 61508
a) is intended to be used only after a thorough understanding of IEC 61508-1, which
provides the overall framework for the achievement of functional safety;
b) applies to any safety-related system, as defined by IEC 61508-1, which contains at least
one electrical, electronic or programmable electronic based component;
c) applies to all subsystems and their components within an E/E/PE safety-related system
(including sensors, actuators and the operator interface);
d) specifies how to refine the information developed in accordance with IEC 61508-1,
concerning the overall safety requirements and their allocation to E/E/PE safety-related
systems, and specifies how the overall safety requirements are refined into E/E/PES
safety functions requirements and E/E/PES safety integrity requirements;
e) specifies requirements for activities that are to be applied during the design and
manufacture of the E/E/PE safety-related systems (i.e. establishes the E/E/PES safety
lifecycle model), except for software, which is dealt with by IEC 61508-3 (see figures 2
and 3) – these requirements include the application of techniques and measures, which
are graded against the safety integrity level, for the avoidance of, and control of, faults
and failures;
f) specifies the information necessary for carrying out the installation, commissioning and
final safety validation of the E/E/PE safety-related systems;
g) does not apply to the operation and maintenance phase of the E/E/PE safety-related
systems – this is dealt with in IEC 61508-1 – however, IEC 61508-2 does provide
requirements for the preparation of information and procedures needed by the user for the
operation and maintenance of the E/E/PE safety-related systems;
h) specifies requirements to be met by the organisation carrying out any modification of
the E/E/PE safety-related systems.
NOTE 1 This part of IEC 61508 is mainly directed at suppliers and/or in-company engineering departments, hence
the inclusion of requirements for modification.
NOTE 2 The relationship between IEC 61508-2 and IEC 61508-3 is illustrated in figure 3.
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 3.4.4 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 is also intended for use as a
stand-alone standard.
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