IEC 61508-5:2010

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
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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
– 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
only) .

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

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,
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Publication(s)”). Their preparation is entrusted to technical committees; any IEC National Committee interested
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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
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5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity
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6) All users should ensure that they have the latest edition of this publication.
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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.

– 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.
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.
– 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.

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.

– 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

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.
– 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 increasingly important in arriving at tolerable risk criteria for specific
applications;
– the best independent industrial, expert and scientific advice from advisory bodies.
In determining the safety integrity requirements of the E/E/PE safety-related system(s) and
other risk reduction measures, in order to meet the tolerable frequency of a hazardous event,
account needs to be taken of the characteristics of the risk that are relevant to the application.
The tolerable frequency will depend on the legal requirements in the country of application
and on the criteria specified by the user organisation. Issues that may need to be considered
together with how they can be applied to E/E/PE safety-related systems are discussed below.
—————————
In achieving the tolerable risk, the necessary risk reduction will need to be established. Annexes E and G of
this document outline qualitative methods, although in the examples quoted the necessary risk reduction is
incorporated implicitly by specification of the SIL requirement rather than stated explicitly by a numeric value of
risk reduction required.
For example, that the hazardous event, leading to a specific consequence, shall not occur with a frequency
greater than one in 10 h.
61508-5 © IEC:2010 – 11 –
A.2.1 Individual risk
Different targets are usually defined for employees and members of the public. The target for
individual risk for employees is applied to the most exposed individual and may be expressed
as the total risk per year arising from all work activities. The target is applied to a hypothetical
person and therefore needs to take into account the percentage of time that the individual
spends at work. The target applies to all risks to the exposed person and the tolerable risk for
an individual safety function will need to take account of other risks.
Assurance that the total risk is reduced below a specified target can be done in a number of
ways. One method is to consider and sum all risks to the most exposed individual. This may
be difficult in cases where a person is exposed to many risks and early decisions are needed
for system development. An alternative approach is to allocate a percentage of the overall
individual risk target to each safety function under consideration. The percentage allocated
can usually be decided from previous experience of the type of facility under consideration.
The target applied to an individual safety function should also take into account the
conservatism of the method of risk analysis used. All qualitative methods such as risk graphs
involve some evaluation of the critical parameters that contribute to risk. The factors that give
rise to risk are the consequence of the hazardous event and its frequency. In determining
these factors a number of risk parameters may need to be taken into account such as a
vulnerability to the hazardous event, number of people who may be affected by the hazardous
event, the probability that a person is present when the hazardous event occurs (i.e.
occupancy) and probability of avoiding the hazardous event.
Qualitative methods generally involve deciding if a parameter lies within a certain range. The
descriptions of the criteria when using such methods will need to be such that there can be a
high level of confidence that the target for risks is not exceeded. This can involve setting
range boundaries for all parameters so applications with all parameters at the boundary
condition will meet the specified risk criteria for safety. This approach to setting the range
boundaries is very conservative because there will be very few applications where all
parameters will be at the worst case of the range. If members of the public are to be exposed
to risk from failure of a E/E/PE safety-related system then a lower target will normally apply.
A.2.2 Societal risk
This arises where multiple fatalities are likely to arise from single events. Such events are
called societal because they are likely to provoke a socio-political response. There can be
significant public and organisational aversion to high consequence events and this will need
to be taken into consideration in some cases. The criterion for societal risk is often expressed
as a maximum accumulated frequency for fatal injuries to a specified number of persons. The
criterion is normally specified in the form of one or more lines on an F/N plot where F is the
cumulative frequency of hazards and N the number of fatalities arising from the hazards. The
relationship is normally a straight line when plotted on logarithmic scales. The slope of the
line will depend on the extent to which the organisation is risk averse to higher levels of
consequence. The requirement will be to ensure the accumulated frequency for a specified
number of fatalities is lower than the accumulated frequency expressed in the F/N plot. (see
reference [7] in the Bibliography)
A.2.3 Continuous improvement
The principles of reducing risk to as low as reasonably practicable are discussed in Annex C.
A.2.4 Risk profile
In deciding risk criteria to be applied for a specific hazard, the risk profile over the life of the
asset may need to be considered. Residual risk will vary from low just after a proof test or a
repair has been performed to a maximum just prior to proof testing. This may need to be
taken into consideration by organisations that specify the risk criteria to be applied. If proof
test intervals are significant, then it may be appropriate to specify the maximum hazard

– 12 – 61508-5 © IEC:2010
probability that can be accepted just prior to proof testing or that the PFD(t) or PFH(t) is lower
than the upper SIL boundary more than a specified percentage of the time (e.g. 90 %).
A.3 Role of E/E/PE safety-related systems
E/E/PE safety-related systems contribute towards providing the necessary risk reduction in
order to meet the tolerable risk.
A safety-related system both
– implements the required safety functions necessary to achieve a safe state for the
equipment under control or to maintain a safe state for the equipment under control; and
– is intended to achieve, on its own or with other E/E/PE safety-related systems or other risk
reduction measures, the necessary safety integrity for the required safety functions (3.5.1
of IEC 61508-4).
NOTE 1 The first part of the definition specifies that the safety-related system must perform the safety functions
which would be specified in the safety functions requirements specification. For example, the safety functions
requirements specification may state that when the temperature reaches x, valve y shall open to allow water to
enter the vessel.
NOTE 2 The second part of the definition specifies that the safety functions must be performed by the safety-
related systems with the degree of confidence appropriate to the application, in order that the tolerable risk will be
achieved.
A person could be an integral part of an E/E/PE safety-related system. For example, a person
could receive information, on the state of the EUC, from a display screen and perform a safety
action based on this information.
E/E/PE safety-related systems can operate in a low demand mode of operation or high
demand or continuous mode of operation (see 3.5.16 of IEC 61508-4).
A.4 Safety integrity
Safety integrity is defined as the probability of a safety-related system satisfactorily
performing the required safety functions under all the stated conditions within a stated period
of time (3.5.4 of IEC 61508-4). Safety integrity relates to the performance of the safety-related
systems in carrying out the safety functions (the safety functions to be performed will be
specified in the safety functions requirements specification).
Safety integrity is considered to be composed of the following two elements.
– Hardware safety integrity; that part of safety integrity relating to random hardware failures
in a dangerous mode of failure (see 3.5.7 of IEC 61508-4). The achievement of the
specified level of safety-related hardware safety integrity can be estimated to a reasonable
level of accuracy, and the requirements can therefore be apportioned between
subsystems using the normal rules for the combination of probabilities. It may be
necessary to use redundant architectures to achieve adequate hardware safety integrity.
– Systematic safety integrity; that part of safety integrity relating to systematic failures in a
dangerous mode of failure (see 3.5.6 of IEC 61508-4). Although the mean failure rate due
to systematic failures may be capable of estimation, the failure data obtained from design
faults and common cause failures means that the distribution of failures can be hard to
predict. This has the effect of increasing the uncertainty in the failure probability
calculations for a specific situation (for example the probability of failure of a safety-
related protection system). Therefore a judgement has to be made on the selection of the
best techniques to minimise this uncertainty. Note that it is not the case that measures to
reduce the probability of random hardware failure will have a corresponding effect on the
probability of systematic failure. Techniques such as redundant channels of identical
hardware, which are very effective at controlling random hardware failures, are of little use
in reducing systematic failures such as software errors.

61508-5 © IEC:2010 – 13 –
A.5 Modes of operation and SIL determination
The mode of operation relates to the way in which a safety function is intended to be used
with respect to the frequency of demands made upon it which may be either:
– low demand mode: where frequency of demands for operation made on the safety
function is no greater than one per year; or
– high demand mode: where frequency of demands for operation made on the safety
function is greater than one per year; or
– continuous mode: where demand for operation of the safety function is continuous.
Tables 2 and 3 of IEC 61508-1 detail the target failure measures associated with the four
safety integrity levels for each of the modes of operation. The modes of operation are
explained further in the following paragraphs.
A.5.1 Safety integrity and risk reduction for low demand mode applications
The required safety integrity of the E/E/PE safety-related systems and other risk reduction
measures shall be of such a level so as to ensure that:
– the average probability of failure on demand of the safety-related systems is sufficiently
low to prevent the hazardous event frequency exceeding that required to meet the
tolerable risk; and/or
– the safety-related systems modify the consequences of failure to the extent required to
meet the tolerable risk.
Figure A.1 illustrates the general concepts of risk reduction. The general model assumes that:
– there is an EUC and a control system;
– there are associated human factor issues;
– the safety protective features comprise:
– E/E/PE safety-related systems;
– other risk reduction measures.
NOTE Figure A.1 is a generalised risk model to illustrate the general principles. The risk model for a specific
application will need to be developed taking into account the specific manner in which the necessary risk reduction
is actually being achieved by the E/E/PE safety-related systems and/or other risk reduction measures. The
resulting risk model may therefore differ from that shown in Figure A.1.
The various risks indicated in Figure A.1 and A.2 are as follows:
– EUC risk: the risk existing for the specified hazardous events for the EUC, the EUC
control system and associated human factor issues: no designated safety protective
features are considered in the determination of this risk (see 3.1.9 of IEC 61508-4);
– tolerable risk; the risk which is accepted in a given context based on the current values of
society (see 3.1.7 of IEC 61508-4);
– residual risk: in the context of this standard, the residual risk is that remaining for the
specified hazardous events for the EUC, the EUC control system, human factor issues but
with the addition of, E/E/PE safety-related systems and other risk reduction measures (see
also 3.1.7 of IEC 61508-4).
The EUC risk is a function of the risk associated with the EUC itself but taking into account
the risk reduction brought about by the EUC control system. To prevent unreasonable claims
for the safety integrity of the EUC control system, this standard places constraints on the
claims that can be made (see 7.5.2.5 of IEC 61508-1).
The necessary risk reduction is achieved by a combination of all the safety protective
features. The necessary risk reduction to achieve the specified tolerable risk, from a starting

– 14 – 61508-5 © IEC:2010
point of the EUC risk, is shown in Figure A.1 (relevant for a safety function operating in low
demand mode of operation).
Figure A.1 – Risk reduction – general concepts (low demand mode of operation)

Figure A.2 – Risk and safety integrity concept
A.5.2 Safety integrity for high demand mode applications
The required safety integrity of the E/E/PE safety-related systems and other risk reduction
measures shall be of such a level to ensure that:
– the average probability of failure on demand of the safety-related systems is sufficiently
low to prevent the hazardous event frequency exceeding that required to meet the
tolerable risk; and/or
– the average probability of failure per hour of the safety-related system is sufficiently low to
prevent the hazardous event frequency exceeding that required to meet the tolerable risk.
Figure A.3 illustrates the general concepts of high demand applications. The general model
assumes that:
– there is a EUC and a control system;

61508-5 © IEC:2010 – 15 –
– there are associated human factor issues;
– the safety protective features comprise:
– E/E/PE safety-related system operating in high demand mode;
– other risk reduction measures.
Various demands on the E/E/PE safety related systems can occur as follows:
– general demands from the EUC;
– demands arising from failures in the EUC control system;
– demands arising from human failures.
If the total demand rate arising from all the demands on the system exceeds 1 per year then
the critical factor is the dangerous failure rate of the E/E/PE safety-related system. Residual
hazard frequency can never exceed the dangerous failure rate of the E/E/PE safety-related
system. It can be lower if other risk reduction measures reduce the probability of harm.

Figure A.3 – Risk diagram for high demand applications
A.5.3 Safety integrity for continuous mode applications
The required safety integrity of the E/E/PE safety-related systems and any other risk
reduction measures shall be of such a level to ensure that the average probability of a
dangerous failure per hour of the safety-related system is sufficiently low to prevent the
hazardous event frequency exceeding that required to meet the tolerable risk.

– 16 – 61508-5 © IEC:2010
With an E/E/PE safety-related system operating in continuous mode, other risk reduction
measures can reduce the residual hazard frequency according to the risk reduction provided
The model is shown in Figure A.4.

Figure A.4 – Risk diagram for continuous mode operation
A.5.4 Common cause and dependency failures
During the determination of the safety integrity levels it is important to take account of
common cause and dependency failures. The models shown above in Figures A.1, A.2, A.3
and A.4 are drawn on the basis that each safety system relevant to the same hazard is fully
independent. There are many applications where this is not the case. Examples include the
following:
1) Where a dangerous failure of an element within the EUC control system can cause a
demand on a safety-related system and the safety-related system uses an element subject
to failure from the same cause. An example of this could be where the control and
protection system sensors are separate but common cause could lead to failure of both
(see Figure A.5).
2) Where more than one safety-related system is used and some of the same type of
equipment is used within each safety-related system and each is subject to failure from
the same common cause. An example would be where the same type of sensor is used in
two separate protection systems both providing risk reduction for the same hazard (see
Figure A.6).
3) Where more than one protection system is used, the protection systems are diverse but
proof t
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