IEC 61511-3:2003

INTERNATIONAL IEC
STANDARD
61511-3
First edition
2003-03
Functional safety –
Safety instrumented systems
for the process industry sector –
Part 3:
Guidance for the determination of the required
safety integrity levels
Sécurité fonctionnelle -
Systèmes instrumentés de sécurité pour le secteur
des industries de transformation
Partie 3:
Conseils pour la détermination des niveaux d'intégrité
de sécurité requis
Reference number
Publication numbering
As from 1 January 1997 all IEC publications are issued with a designation in the
60000 series. For example, IEC 34-1 is now referred to as IEC 60034-1.

Consolidated editions
The IEC is now publishing consolidated versions of its publications. For example,

edition numbers 1.0, 1.1 and 1.2 refer, respectively, to the base publication, the
base publication incorporating amendment 1 and the base publication incorporating
amendments 1 and 2.
Further information on IEC publications
The technical content of IEC publications is kept under constant review by the IEC,
thus ensuring that the content reflects current technology. Information relating to
this publication, including its validity, is available in the IEC Catalogue of
publications (see below) in addition to new editions, amendments and corrigenda.
Information on the subjects under consideration and work in progress undertaken
by the technical committee which has prepared this publication, as well as the list
of publications issued, is also available from the following:
• IEC Web Site (www.iec.ch)
• Catalogue of IEC publications
The on-line catalogue on the IEC web site (http://www.iec.ch/searchpub/cur_fut.htm)
enables you to search by a variety of criteria including text searches, technical
committees and date of publication. On-line information is also available on
recently issued publications, withdrawn and replaced publications, as well as
corrigenda.
• IEC Just Published
This summary of recently issued publications (http://www.iec.ch/online_news/
justpub/jp_entry.htm) is also available by email. Please contact the Customer
Service Centre (see below) for further information.
• Customer Service Centre
If you have any questions regarding this publication or need further assistance,
please contact the Customer Service Centre:
Email: custserv@iec.ch
Tel: +41 22 919 02 11
Fax: +41 22 919 03 00
INTERNATIONAL IEC
STANDARD
61511-3
First edition
2003-03
Functional safety –
Safety instrumented systems
for the process industry sector –
Part 3:
Guidance for the determination of the required
safety integrity levels
Sécurité fonctionnelle -
Systèmes instrumentés de sécurité pour le secteur
des industries de transformation
Partie 3:
Conseils pour la détermination des niveaux d'intégrité
de sécurité requis
 IEC 2003  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
Commission Electrotechnique Internationale
XA
International Electrotechnical Commission
Международная Электротехническая Комиссия
For price, see current catalogue

– 2 – 61511-3  IEC:2003(E)
CONTENTS
FOREWORD . 4

INTRODUCTION .6

1 Scope . 9

2 Terms, definitions and abbreviations.10

3 Risk and safety integrity – general guidance .10

3.1 General .10

3.2 Necessary risk reduction.11

3.3 Role of safety instrumented systems .11
3.4 Safety integrity .11
3.5 Risk and safety integrity .13
3.6 Allocation of safety requirements .14
3.7 Safety integrity levels .14
3.8 Selection of the method for determining the required safety integrity level .15
Annex A (informative) As Low As Reasonably Practicable (ALARP) and tolerable
risk concepts.16
Annex B (informative) Semi-quantitative method .19
Annex C (informative) The safety layer matrix method.27
Annex D (informative) Determination of the required safety integrity levels – a semi-
qualitative method: calibrated risk graph .33
Annex E (informative) Determination of the required safety integrity levels –
a qualitative method: risk graph.41
Annex F (informative) Layer of protection analysis (LOPA).46
Figure 1 – Overall framework of this standard . 8
Figure 2 – Typical risk reduction methods found in process plants .10
Figure 3 – Risk reduction: general concepts.13
Figure 4 – Risk and safety integrity concepts .13
Figure 5 – Allocation of safety requirements to the Safety Instrumented Systems,
non-SIS prevention/mitigation protection layers and other protection layers .14
Figure A.1 – Tolerable risk and ALARP .17
Figure B.1 – Pressurized Vessel with Existing Safety Systems.20

Figure B.2 – Fault Tree for Overpressure of the Vessel.23
Figure B.3 – Hazardous Events with Existing Safety Systems .24
Figure B.4 – Hazardous Events with Redundant Protection Layer .25
Figure B.5 – Hazardous Events with SIL 2 SIS Safety Function.26
Figure C.1 – Protection Layers.27
Figure C.2 – Example Safety Layer Matrix.31
Figure D.1 – Risk graph: general scheme.37
Figure D.2 – Risk Graph: Environmental Loss .39
Figure E.1 – DIN V 19250 Risk graph – personnel protection (see Table E.1) .44
Figure E.2 – Relationship IEC 61511, DIN 19250 and VDI/VDE 2180 .45
Figure F.1 – Layer of Protection Analysis (LOPA) Report .47

61511-3  IEC:2003(E) – 3 –
Table A.1 – Example of risk classification of incidents.18

Table A.2 – Interpretation of risk classes.18

Table B.1 – HAZOP analysis results.21

Table C.1 – Frequency of hazardous event likelihood (without considering PLs) .30

Table C.2 – Criteria for rating the severity of impact of hazardous events .30

Table D.1 – Descriptions of process industry risk graph parameters.34

Table D.2 – Example calibration of the general purpose risk graph .37

Table D.3 – General environmental consequences.39

Table E.1 – Data relating to risk graph (see Figure E.1) .44
Table F.1 – HAZOP developed data for LOPA.47
Table F.2 – Impact event severity levels.48
Table F.3 – Typical protection layer (prevention and mitigation) PFDs .49
Table F.4 – Initiation Likelihood.48

– 4 – 61511-3  IEC:2003(E)
INTERNATIONAL ELECTROTECHNICAL COMMISSION

____________
FUNCTIONAL SAFETY–
SAFETY INSTRUMENTED SYSTEMS
FOR THE PROCESS INDUSTRY SECTOR –

Part 3: Guidance for the determination

of the required safety integrity levels

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 61511-3 has been prepared by subcommittee 65A: System
aspects, of IEC technical committee 65: Industrial-process measurement and control.
The text of this standard is based on the following documents:

FDIS Report on voting
65A/367/FDIS 65A/370/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.
IEC 61511 series has been developed as a process sector implementation of IEC 61508
series.
61511-3  IEC:2003(E) – 5 –
IEC 61511 consists of the following parts, under the general title Functional safety – Safety
Instrumented Systems for the process industry sector (see Figure 1):

Part 1: Framework, definitions, system, hardware and software requirements

Part 2: Guidelines for the application of IEC 61511-1

Part 3: Guidance for the determination of the required safety integrity levels

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

2007. At this date, the publication will be

• reconfirmed;
• withdrawn;
• replaced by a revised edition, or
• amended.
The contents of the corrigendum of October 2004 have been included in this copy.

– 6 – 61511-3  IEC:2003(E)
INTRODUCTION
Safety instrumented systems have been used for many years to perform safety instrumented

functions in the process industries. If instrumentation is to be effectively used for safety

instrumented functions, it is essential that this instrumentation achieves certain minimum

standards and performance levels.

This International Standard addresses the application of safety instrumented systems for the

Process Industries. It also requires a process hazard and risk assessment to be carried out to

enable the specification for safety instrumented systems to be derived. Other safety systems

are only considered so that their contribution can be taken into account when considering the

performance requirements for the safety instrumented systems. The safety instrumented
system includes all components and subsystems necessary to carry out the safety
instrumented function from sensor(s) to final element(s).
This International Standard has two concepts which are fundamental to its application; safety
lifecycle and safety integrity levels.
This International Standard addresses safety in
...