SIST EN 61508-4:2011

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

1.1 This part of IEC 61508 contains the definitions and explanation of terms that are used in parts 1 to 7 of the IEC 61508 series of standards.
1.2 The definitions are grouped under general headings so that related terms can be understood within the context of each other. However, it should be noted that these headings are not intended to add meaning to the definitions.
1.3 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.
1.4 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.
1.5 Figure 1 shows the overall framework of the IEC 61508 series and indicates the role that IEC 61508-4 plays in the achievement of functional safety for E/E/PE safety-related systems.

Funktionale Sicherheit sicherheitsbezogener elektrischer/elektronischer/programmierbarer elektronischer Systeme - Teil 4: Begriffe und Abkürzungen (IEC 61508-4:2010)

Sécurité fonctionnelle des systèmes électriques/électroniques/électroniques programmables relatifs à la sécurité - Partie 4: Définitions et abréviations (CEI 61508-4:2010)

La CEI 61508-4:2010 contient les définitions et explications des termes utilisés dans les parties 1 à 7 de la série de normes CEI 61508. Les définitions sont regroupées sous des titres généraux de telle sorte que les termes qui sont en rapport puissent être compris dans le contexte des autres. Toutefois, il convient de remarquer que ces titres ne sont pas attribués pour ajouter un sens aux définitions. 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. Elle a le statut d'une publication fondamentale de sécurité conformément au Guide CEI 104.

Funkcijska varnost električnih/elektronskih/elektronsko programirljivih varnostnih sistemov - 4. del: Definicije in kratice (IEC 61508-4:2010)

1.1 Ta del IEC 61508 vsebuje definicije in razlago izrazov, ki se uporabljajo v delih od 1 do 7 serije standardov IEC 61508.
1.2 Definicije so razporejene pod splošnimi naslovi, tako da se povezani izrazi lahko razumejo v kontekstu drug drugega. Opozoriti pa je treba, da naslovi definicijam ne dodajajo pomena.
1.3 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.
1.4 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.
1.5 Slika 1 prikazuje celoten okvir serije IEC 61508 in nakazuje vlogo IEC 61508-4 pri doseganju funkcijske varnosti varnostnih sistemov E/E/PE.

General Information

• Status: Published
• Publication Date: 11-Apr-2011
• ICS: 01.040.25 - Manufacturing engineering (Vocabularies)
• 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 61508-4:2010 - Functional safety of electrical/electronic/programmable electronic safety-related systems - Part 4: Definitions and abbreviations

Relations

• Replaces: SIST EN 61508-4:2007 - Functional safety of electrical/electronic/programmable electronic safety-related systems -- Part 4: Definitions and abbreviations
• Effective Date: 01-May-2011
• Revised: SIST EN 61508-4:2007 - Functional safety of electrical/electronic/programmable electronic safety-related systems -- Part 4: Definitions and abbreviations
• Effective Date: 07-May-2013
• Replaces: SIST EN 61508-4:2007 - Functional safety of electrical/electronic/programmable electronic safety-related systems -- Part 4: Definitions and abbreviations
• Effective Date: 28-Jan-2023

Standards Content (Sample)

SLOVENSKI STANDARD
SIST EN 61508-4:2011
01-maj-2011
1DGRPHãþD
SIST EN 61508-4:2007
)XQNFLMVNDYDUQRVWHOHNWULþQLKHOHNWURQVNLKHOHNWURQVNRSURJUDPLUOMLYLKYDUQRVWQLK
VLVWHPRYGHO'HILQLFLMHLQNUDWLFH,(&
Functional safety of electrical/electronic/programmable electronic safety-related systems
- Part 4: Definitions and abbreviations (IEC 61508-4:2010)
Funktionale Sicherheit sicherheitsbezogener
elektrischer/elektronischer/programmierbarer elektronischer Systeme - Teil 4: Begriffe
und Abkürzungen (IEC 61508-4:2010)
Sécurité fonctionnelle des systèmes électriques/électroniques/électroniques
programmables relatifs à la sécurité - Partie 4: Définitions et abréviations (CEI 61508-
4:2010)
Ta slovenski standard je istoveten z: EN 61508-4:2010
ICS:
01.040.25 Izdelavna tehnika (Slovarji) Manufacturing engineering
(Vocabularies)
25.040.40 Merjenje in krmiljenje Industrial process
industrijskih postopkov measurement and control
SIST EN 61508-4:2011 en
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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SIST EN 61508-4:2011

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SIST EN 61508-4:2011

EUROPEAN STANDARD
EN 61508-4

NORME EUROPÉENNE
May 2010
EUROPÄISCHE NORM

ICS 25.040.40; 29.020 Supersedes EN 61508-4:2001


English version


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


Sécurité fonctionnelle des systèmes Funktionale Sicherheit sicherheitsbezogener
électriques/électroniques/électroniques elektrischer/elektronischer/programmierbarer
programmables relatifs à la sécurité - elektronischer Systeme -
Partie 4: Définitions et abréviations Teil 4: Begriffe und Abkürzungen
(CEI 61508-4:2010) (IEC 61508-4:2010)




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 61508-4:2010 E

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SIST EN 61508-4:2011
EN 61508-4:2010 - 2 -
Foreword
The text of document 65A/551/FDIS, future edition 2 of IEC 61508-4, 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 61508-4 on 2010-05-01.
This European Standard supersedes EN 61508-4:2001.
It has the status of a basic safety publication according to IEC Guide 104.
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 61508-4:2010 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 62061:2005 NOTE  Harmonized as EN 62061:2005 (not modified).
[3] IEC 61800-5-2 NOTE  Harmonized as EN 61800-5-2.
[4] IEC 61508-5:2010 NOTE  Harmonized as EN 61508-5:2010 (not modified).
[5] IEC 61508-6:2010 NOTE  Harmonized as EN 61508-6:2010 (not modified).
[6] IEC 61508-7:2010 NOTE  Harmonized as EN 61508-7:2010 (not modified).
[8] IEC 61131-3:2003 NOTE  Harmonized as EN 61131-3:2003 (not modified).
[10] ISO 8402:1994 NOTE  Harmonized as EN ISO 8402:1995 (not modified).
[11] IEC 60601 series NOTE  Harmonized in EN 60601 series (partially modified).
[14] IEC 61508-1:2010 NOTE  Harmonized as EN 61508-1:2010 (not modified).
[15] IEC 61508-2:2010 NOTE  Harmonized as EN 61508-2:2010 (not modified).
[16] IEC 61508-3:2010 NOTE  Harmonized as EN 61508-3:2010 (not modified).
[18] ISO 9000:2005 NOTE  Harmonized as EN ISO 9000:2005 (not modified).
__________

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SIST EN 61508-4:2011
- 3 - EN 61508-4:2010
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 Guide 104 1997 The preparation of safety publications - -
and the use of basic safety publications and
group safety publications


ISO/IEC Guide 51 1999 Safety aspects - Guidelines - -
for their inclusion in standards

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SIST EN 61508-4:2011

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SIST EN 61508-4:2011
IEC 61508-4
®
Edition 2.0 2010-04
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
BASIC SAFETY PUBLICATION
PUBLICATION FONDAMENTALE DE SÉCURITÉ
Functional safety of electrical/electronic/programmable electronic safety-related
systems –
Part 4: Definitions and abbreviations

Sécurité fonctionnelle des systèmes électriques/électroniques/électroniques
programmables relatifs à la sécurité –
Partie 4: Définitions et abréviations

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
V
CODE PRIX
ICS 25.040.40; 29.020 ISBN 978-2-88910-527-4
® Registered trademark of the International Electrotechnical Commission
Marque déposée de la Commission Electrotechnique Internationale

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SIST EN 61508-4:2011
– 2 – 61508-4 © IEC:2010
CONTENTS
FOREWORD.3
INTRODUCTION.5
1 Scope.7
2 Normative references.9
3 Definitions and abbreviations .9
3.1 Safety terms .10
3.2 Equipment and devices.12
3.3 Systems – general aspects .15
3.4 Systems – safety-related aspects.17
3.5 Safety functions and safety integrity.19
3.6 Fault, failure and error (see Figure 4).22
3.7 Lifecycle activities.27
3.8 Confirmation of safety measures.28
Bibliography .32
Index .33

Figure 1 – Overall framework of the IEC 61508 series .8
Figure 2 – Programmable electronic system .16
Figure 3 – Electrical/electronic/programmable electronic system (E/E/PE system) –
structure and terminology.16
Figure 4 – Failure model .23

Table 1 – Abbreviations used in this standard.9

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SIST EN 61508-4:2011
61508-4 © IEC:2010 – 3 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________

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

Part 4: Definitions and abbreviations


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 61508-4 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.
It has the status of a basic safety publication according to IEC Guide 104.

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SIST EN 61508-4:2011
– 4 – 61508-4 © IEC:2010
The text of this standard is based on the following documents:
FDIS Report on voting
65A/551/FDIS 65A/575/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 61508-4:2011
61508-4 © 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 61508-4:2011
– 6 – 61508-4 © 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 61508-4:2011
61508-4 © IEC:2010 – 7 –
FUNCTIONAL SAFETY OF ELECTRICAL/ELECTRONIC/
PROGRAMMABLE ELECTRONIC SAFETY-RELATED SYSTEMS –

Part 4: Definitions and abbreviations



1 Scope
1.1 This part of IEC 61508 contains the definitions and explanation of terms that are used in
parts 1 to 7 of the IEC 61508 series of standards.
1.2 The definitions are grouped under general headings so that related terms can be
understood within the context of each other. However, it should be noted that these headings
are not intended to add meaning to the definitions.
1.3 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.
1.4 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.
1.5 Figure 1 shows the overall framework of the IEC 61508 series and indicates the role that
IEC 61508-4 plays in the achievement of functional safety for E/E/PE safety-related systems.

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SIST EN 61508-4:2011
– 8 – 61508-4 © IEC:2010

Figure 1 – Overall framework of the IEC 61508 series

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SIST EN 61508-4:2011
61508-4 © 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 Guide 104:1997, The preparation of safety publications and the use of basic safety
publications and group safety publications
ISO/IEC Guide 51:1999, Safety aspects – Guidelines for their inclusion in standards
3 Definitions and abbreviations
For the purposes of this document, the definitions and the abbreviations given in Table 1
below, as well as the following apply.
Table 1 – Abbreviations used in this standard
Abbreviation Full expression Definition and/or explanation
of term
ALARP As Low As Reasonably Practicable IEC 61508-5, Annex C
ASIC Application Specific Integrated Circuit 3.2.15
CCF Common Cause Failure 3.6.10
CPLD Complex Programmable Logic Device
DC Diagnostic Coverage 3.8.6
(E)EPLD (Electrically) Erasable Programmable Logic Device
E/E/PE Electrical/Electronic/Programmable Electronic 3.2.13, example: E/E/PE
safety-related system
E/E/PE (system) Electrical/Electronic/Programmable Electronic System 3.3.2
EEPROM Electrically Erasable Programmable Read-Only Memory
EPROM Erasable Programmable Read-Only Memory
EUC Equipment Under Control 3.2.1
FPGA Field Programmable Gate Array
GAL Generic Array Logic
HFT Hardware Fault Tolerance 7.4.4 of IEC 61508-2
MooN M out of N channel architecture IEC 61508-6, Annex B
(for example 1oo2 is 1 out of 2 architecture, where either of
the two channels can perform the safety function)
MooND M out of N channel architecture with Diagnostics IEC 61508-6, Annex B
MTBF Mean Time Between Failures 3.6.19, NOTE 3
MTTR Mean Time To Repair 3.6.21
MRT Mean Repair Time 3.6.22
PAL Programmable Array Logic
PE Programmable Electronic 3.2.12
PE(system) Programmable Electronic 3.3.1
PFD Probability of Dangerous Failure on Demand 3.6.17
PFD Average Probability of dangerous Failure on Demand 3.6.18
avg
-1
PFH Average frequency of dangerous failure [h ] 3.6.19
PLA Programmable Logic Array

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SIST EN 61508-4:2011
– 10 – 61508-4 © IEC:2010
Abbreviation Full expression Definition and/or explanation
of term
PLC Programmable Logic Controller IEC 61508-6, Annex E
PLD Programmable Logic Device
PLS Programmable Logic Sequencer
PML Programmable Macro Logic
RAM Random Access Memory
ROM Read-Only Memory
SFF Safe Failure Fraction 3.6.15
SIL Safety Integrity Level 3.5.8
VHDL Very High Speed Integrated Circuit Hardware Description IEC 61508-2, Annex F, Note 5
Language
3.1 Safety terms
3.1.1
harm
physical injury or damage to the health of people or damage to property or the environment
[ISO/IEC Guide 51:1999, definition 3.3]
3.1.2
hazard
potential source of harm
[ISO/IEC Guide 51:1999, definition 3.5]
NOTE The term includes danger to persons arising within a short time scale (for example, fire and explosion) and
also those that have a long-term effect on a person’s health (for example, release of a toxic substance).
3.1.3
hazardous situation
circumstance in which people, property or the environment are exposed to one or more
hazards
[ISO/IEC Guide 51:1999, definition 3.6, modified]
3.1.4
hazardous event
event that may result in harm
NOTE Whether or not a hazardous event results in harm depends on whether people, property or the environment
are exposed to the consequence of the hazardous event and, in the case of harm to people, whether any such
exposed people can escape the consequences of the event after it has occurred.
3.1.5
harmful event
occurrence in which a hazardous situation or hazardous event results in harm
NOTE Adapted from ISO/IEC Guide 51, definition 3.4, to allow for a hazardous event.
3.1.6
risk
combination of the probability of occurrence of harm and the severity of that harm
[ISO/IEC Guide 51:1999, definition 3.2]
NOTE For more discussion on this concept see Annex A of IEC 61508-5.

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SIST EN 61508-4:2011
61508-4 © IEC:2010 – 11 –
3.1.7
tolerable risk
risk which is accepted in a given context based on the current values of society
[ISO/IEC Guide 51:1999, definition 3.7]
NOTE See Annex C of IEC 61508-5.
3.1.8
residual risk
risk remaining after protective measures have been taken
[ISO/IEC Guide 51:1999, definition 3.9]
3.1.9
EUC risk
risk arising from the EUC or its interaction with the EUC control system
NOTE 1 The risk in this context is that associated with the specific harmful event in which E/E/PE safety-related
systems and other risk reduction measures are to be used to provide the necessary risk reduction, (i.e. the risk
associated with functional safety).
NOTE 2 The EUC risk is indicated in Figure A.1 of IEC 61508-5. The main purpose of determining the EUC risk is
to establish a reference point for the risk without taking into account E/E/PE safety-related systems and other risk
reduction measures.
NOTE 3 Assessment of this risk will include associated human factor issues.
3.1.10
target risk
risk that is intended to be reached for a specific hazard taking into account the EUC risk
together with the E/E/PE safety-related systems and the other risk reduction measures
3.1.11
safety
freedom from unacceptable risk
[ISO/IEC Guide 51:1999, definition 3.1]
3.1.12
functional safety
part of the overall safety relating to the EUC and the EUC control system that depends on the
correct functioning of the E/E/PE safety-related systems and other risk reduction measures
3.1.13
safe state
state of the EUC when safety is achieved
NOTE In going from a potentially hazardous condition to the final safe state, the EUC may have to go through a
number of intermediate safe states. For some situations a safe state exists only so long as the EUC is continuously
controlled. Such continuous control may be for a short or an indefinite period of time.
3.1.14
reasonably
...