Assignment of a safety integrity requirements - Basic rationale

IEC TR 63161:2022 can be used where a risk assessment according to ISO 12100 has been conducted for a machine or process plant and where a safety related control function has been selected for implementation as a protective measure against specified hazards. This document describes an example basic logical rationale to assign a safety integrity requirement to the selected function.
The description is generic and as far as reasonably possible independent from any specific tool or method that can be used for assignment of a safety integrity requirement. The requirement can be expressed as a safety integrity level (SIL), or performance level (PL).
An example basic rationale is described that is embodied by such methods and tools, as far as they follow a risk based quantitative approach.
Conversely, the logic described in this document can be used as a reference for assessing specific methods or tools for safety integrity assignment. This can clarify how far the respective tool/method is following a risk based quantitative approach, and where deviations from that approach are imposed by other considerations. In real applications, the quantitative risk based approach can be modified or overridden by other considerations in many cases and for good reasons. It is not within the scope of this document to discuss or evaluate such reasons. Usually the reasons for deviations from a given tool or method from a quantitative logic are provided, so that this can be discussed in the proper frame.
Examples for such analyses are provided for common assignment tools in the format of risk graphs and risk matrices.
This document can be used for safety related control functions in all modes of application: continuous mode, high demand mode and low demand mode of application.

General Information

Status
Published
Publication Date
12-Jul-2022
Current Stage
PPUB - Publication issued
Completion Date
13-Jul-2022
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IEC TR 63161
Edition 1.0 2022-07
TECHNICAL
REPORT
colour
inside
Assignment of safety integrity requirements – Basic rationale
IEC TR 63161:2022-07(en)
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---------------------- Page: 2 ----------------------
IEC TR 63161
Edition 1.0 2022-07
TECHNICAL
REPORT
colour
inside
Assignment of safety integrity requirements – Basic rationale
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 13.110 ISBN 978-2-8322-3944-5

Warning! Make sure that you obtained this publication from an authorized distributor.

® Registered trademark of the International Electrotechnical Commission
---------------------- Page: 3 ----------------------
– 2 – IEC TR 63161:2022 © IEC 2022
CONTENTS

FOREWORD ........................................................................................................................... 4

INTRODUCTION ..................................................................................................................... 6

1 Scope .............................................................................................................................. 7

2 Normative references ...................................................................................................... 7

3 Terms and definitions ...................................................................................................... 7

4 Risk based quantitative approach .................................................................................. 10

4.1 General ................................................................................................................. 10

4.2 Sequence of steps in functional safety assignment ............................................... 10

4.3 Reference information ........................................................................................... 12

4.3.1 General ......................................................................................................... 12

4.3.2 Accident scenario .......................................................................................... 13

4.3.3 Hazard zone .................................................................................................. 13

4.3.4 Severity of harm ............................................................................................ 13

4.3.5 Safety control function ................................................................................... 14

5 Quantified parameters of a functional safety assignment ............................................... 14

5.1 General ................................................................................................................. 14

5.2 Parameter types ................................................................................................... 14

5.2.1 General ......................................................................................................... 14

5.2.2 Probability ..................................................................................................... 14

5.2.3 Event rate ...................................................................................................... 14

5.3 Probability of occurrence of harm .......................................................................... 15

5.4 Quantification of risk ............................................................................................. 15

5.5 Target failure measure .......................................................................................... 15

5.6 Probability of occurrence of a hazardous event – P .............................................. 16

5.7 Exposure parameter – F ...................................................................................... 17

5.8 Probability of avoiding or limiting harm – A .......................................................... 18

5.8.1 General ......................................................................................................... 18

5.8.2 Vulnerability (V) ............................................................................................. 18

5.8.3 Avoidability (A) .............................................................................................. 19

5.9 Demand types and related event rates .................................................................. 19

5.9.1 Event classes ................................................................................................ 19

5.9.2 Demand and demand rate .............................................................................. 20

5.9.3 Initiating events and rate of initiating events I ............................................. 20

5.9.4 Safety demands and safety demand rate D ................................................. 21

5.9.5 Tolerable risk limit – Parameter L .............................................................. 22

(S)

5.10 Additional parameters ........................................................................................... 23

6 General principle of functional safety assignment .......................................................... 25

6.1 Basics ................................................................................................................... 25

6.1.1 Applicability to complete functions ................................................................. 25

6.1.2 Risk relation .................................................................................................. 25

6.1.3 Logical independence of parameters ............................................................. 25

6.2 High demand or continuous mode of operation ..................................................... 25

6.3 Low demand mode of operation ............................................................................ 26

7 Assignment of the demand mode ................................................................................... 27

7.1 Demand mode – General ...................................................................................... 27

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IEC TR 63161:2022 © IEC 2022 – 3 –

7.2 Assignment criteria ............................................................................................... 30

8 Relation to ISO 12100 ................................................................................................... 30

9 Tools for functional safety assignment ........................................................................... 31

9.1 General ................................................................................................................. 31

9.2 Selection of independent parameters .................................................................... 32

9.3 Logarithmizing parameters .................................................................................... 32

9.4 Discretization of parameters ................................................................................. 32

9.5 Parameter scores .................................................................................................. 33

9.6 Scoring methods in strict sense ............................................................................ 34

Annex A (informative) Examples of SIL assignment tools numerical analysis ....................... 35

A.1 General ................................................................................................................. 35

A.2 Assignment of score values to parameter entries .................................................. 35

A.3 Extraction of tolerable risk limits ........................................................................... 36

A.4 Risk matrix of IEC 62061 ...................................................................................... 38

A.5 Risk graph of ISO 13849 ....................................................................................... 41

A.6 Risk graphs for low demand mode of operation ..................................................... 43

Bibliography .......................................................................................................................... 46

Figure 1 – Sequence of steps in functional safety assignment............................................... 12

Figure 2 – Protection layers, event rates and their relation.................................................... 22

Figure 3 – Hazard rate according to the Henley / Kumamoto equation .................................. 29

Figure 4 – Elements of risk according to ISO 12100 .............................................................. 31

Figure 5 – Discretization of parameters ................................................................................. 33

Figure A.1 – Extraction of tolerable risk limits ....................................................................... 37

Figure A.2 – Risk matrix based on IEC 62061 ....................................................................... 38

Figure A.3 – Maximum allowable PFH as function of the score sum for the different

severity levels ....................................................................................................................... 39

Figure A.4 – Representation by a continuous numerical interpolation .................................... 40

Figure A.5 – Risk graph of ISO 13849-1 ................................................................................ 41

Figure A.6 – Interpolation per severity level .......................................................................... 43

Figure A.7 – Risk graph for low demand mode of operation .................................................. 44

Figure A.8 – Risk graph for low demand mode of operation – from Figure 7 of VDMA

4315-1 .................................................................................................................................. 45

Table 1 – Parameters overview ............................................................................................. 24

Table A.1 – Relation between PLs and ranges in PFH .......................................................... 42

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– 4 – IEC TR 63161:2022 © IEC 2022
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
ASSIGNMENT OF SAFETY INTEGRITY REQUIREMENTS –
BASIC RATIONALE
FOREWORD

1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising

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rights. IEC shall not be held responsible for identifying any or all such patent rights.

IEC TR 63161 has been prepared by IEC technical committee 44: Safety of machinery –

Electrotechnical aspects. It is a Technical Report.
The text of this Technical Report is based on the following documents:
Draft Report on voting
44/935A/DTR 44/954/RVDTR

Full information on the voting for its approval can be found in the report on voting indicated in

the above table.
The language used for the development of this Technical Report is English.

This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in

accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement, available

at www.iec.ch/members_experts/refdocs. The main document types developed by IEC are

described in greater detail at www.iec.ch/standardsdev/publications.
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IEC TR 63161:2022 © IEC 2022 – 5 –

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

stability date indicated on the IEC website under "http://webstore.iec.ch" in the data related to

the specific document. At this date, the document will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.

IMPORTANT – The 'colour inside' logo on the cover page of this publication indicates that it

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contents. Users should therefore print this document using a colour printer.
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– 6 – IEC TR 63161:2022 © IEC 2022
INTRODUCTION

This document describes an example basic logical rationale for assigning a safety integrity

requirement to a safety related control function in a risk based approach. The parameters for

the assignment are explained. It is described how these parameters can relate to the risk

assessment according to ISO 12100 and to the safety integrity requirement.
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IEC TR 63161:2022 © IEC 2022 – 7 –
ASSIGNMENT OF SAFETY INTEGRITY REQUIREMENTS –
BASIC RATIONALE
1 Scope

This document can be used where a risk assessment according to ISO 12100 has been

conducted for a machine or process plant and where a safety related control function has been

selected for implementation as a protective measure against specified hazards. This document

describes an example basic logical rationale to assign a safety integrity requirement to the

selected function.

The description is generic and as far as reasonably possible independent from any specific tool

or method that can be used for assignment of a safety integrity requirement. The requirement

can be expressed as a safety integrity level (SIL), or performance level (PL).

An example basic rationale is described that is embodied by such methods and tools, as far as

they follow a risk based quantitative approach.

Conversely, the logic described in this document can be used as a reference for assessing

specific methods or tools for safety integrity assignment. This can clarify how far the respective

tool/method is following a risk based quantitative approach, and where deviations from that

approach are imposed by other considerations. In real applications, the quantitative risk based

approach can be modified or overridden by other considerations in many cases and for good

reasons. It is not within the scope of this document to discuss or evaluate such reasons. Usually

the reasons for deviations from a given tool or method from a quantitative logic are provided,

so that this can be discussed in the proper frame.

Examples for such analyses are provided for common assignment tools in the format of risk

graphs and risk matrices.

This document can be used for safety related control functions in all modes of application:

continuous mode, high demand mode and low demand mode of application.
2 Normative references

The following documents are referred to in the text in such a way that some or all of their content

constitutes requirements 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.

ISO 12100:2010, Safety of machinery – General principles for design – Risk assessment and

risk reduction
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.

ISO and IEC maintain terminological databases for use in standardization at the following

addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
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– 8 – IEC TR 63161:2022 © IEC 2022
3.1
probability

real number in the interval 0 to 1 attached to a random event and expressing quantitatively how

likely the occurrence of that event is
Note 1 to entry: See 5.2.2 for more information.

[SOURCE: IEC 60050-103:2009, 103-08-02, modified – Notes 1 and 2 to entry have been

removed and replaced with a new Note 1 to entry.]
3.2
event rate
−1 −1 −1

frequency with the dimension of time , typically given in the units h or year , attached to a

random event and expressing quantitatively how frequently this event is expected to occur

Note 1 to entry: See 5.2.3 for more information.
3.3
tolerable risk

level of risk that is accepted in a given context based on the current values of society

Note 1 to entry: For the purposes of ISO/IEC Guide 51:2014, the terms "acceptable risk" and "tolerable risk" are

considered to be synonymous.
[SOURCE: ISO/IEC Guide 51:2014, 3.15]
3.4
tolerable risk limit

risk which is accepted in the context of a given hazard of machinery or process equipment and

which is quantified as an event rate for the occurrence of harm with a specified level of severity

as a consequence of the hazard
Note 1 to entry: See 5.9.5 for more information.

Note 2 to entry: The harm with the specified level of severity is a necessary attribute of a tolerable risk limit, however

it is not expressed in the limit itself.

Note 3 to entry: This definition adds the element of quantification to the general definition of "tolerable risk", which

is not necessarily implied in the term "tolerable risk" without the modifier "limit".

3.5
hazardous event
event that can cause harm
Note 1 to entry: See 4.3.2 for more information.

[SOURCE: ISO 12100:2010, 3.9, modified – The note to entry has been removed and replaced

by a new one.]
3.6
hazardous situation
circumstance in which a person is exposed to at least one hazard
Note 1 to entry: According to ISO 12100:2010, 3.10.
Note 2 to entry: See 4.3.2 for more information.

[SOURCE: ISO 12100:2010, 3.10, modified – The note to entry has been removed and replaced

by two new ones.]
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IEC TR 63161:2022 © IEC 2022 – 9 –
3.7
demand

event that causes the safety control system to perform the safety

control function
Note 1 to entry: See 5.9.2 for more information.

[SOURCE: IEC 62061:2021, 3.2.25, modified – The abbreviated term "SCS" has been replaced

by the words "safety control system", and "a safety function" has been replaced with "the safety

control function".]
3.8
initiating event

situation which, without the safety function, will result in damage

or harm of any sort or severity
Note 1 to entry: See 5.9.3 for more information.
3.9
safety demand

situation where, unless prevented by the safety control function

under assessment, an accident with a specified level of harm to people would occur

Note 1 to entry: See 5.9.4 for more information.
3.10
hazard rate

rate of accidents of a specific severity in conjunction with a specific hazard that occurs although

a safety control function has been installed to prevent this type of accident
3.11
probability of avoiding or limiting harm

probability that potentially exposed persons do not suffer harm of the specified level of severity

during a hazardous event
Note 1 to entry: See 5.8 for more information.
3.12
avoidability

probability that potentially exposed persons avoid exposure to the hazard during a hazardous

event
Note 1 to entry: See 5.8 for more information.
3.13
vulnerability

probability that exposed persons in a hazardous situation do suffer harm of the specified level

of severity
Note 1 to entry: See 5.8 for more information.
3.14
hidden failure
hidden fault

failure or fault in hardware or software that does not announce itself and is not detected by

dedicated methods when it occurs

Note 1 to entry: The term "hidden" in the given sense is complementary to the term "revealed" according to

IEC 61511-1:2016, 3.2.13.

Note 2 to entry: A hardware or software failure or fault announces itself, e.g. by a disturbance of the equipment

under control, its working process, or its surroundings.
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– 10 – IEC TR 63161:2022 © IEC 2022

Note 3 to entry: The "hidden status" of a hardware or software failure or fault is terminated when it is either detected

by a dedicated check or method, or when it becomes overt by disturbing the equipment under control, its working

process, or its surroundings. This may be related, e.g. to a change of the operation status or to a person approaching

the equipment. Failures that stay "hidden" without termination are not relevant.
4 Risk based quantitative approach
4.1 General

In a risk based approach, a safety control function can be specified to keep a risk that is caused

by a machine or process below a defined maximum level, the "tolerable risk limit".

The concept of "risk" is defined in ISO 12100:2010, 3.12 as "combination of the probability of

occurrence of harm and the severity of that harm". Although both elements of the definition can

be understood quantitatively, "risk" is not necessarily understood as a quantifiable parameter

in the context of ISO 12100. That holds even more for the "tolerable risk", i.e. the risk which is

accepted in a given context based on the values of society.

On the other hand, the efficiency of a safety control function for mitigating risk, often indicated

as reliability of the control system, is described with the term "safety integrity". This expresses

the degree of reliance that is put on a safety control function. "Safety integrity" has a quantitative

aspect, which is clearly revealed by the complement of safety integrity, the unreliability of a

safety control function. The unreliability is quantified as "target failure measure", i.e. either as

average probability of the function to fail on demand PFD , or as the rate of dangerous

avg
function failures per hour, PFH.

SIL assignment is the process of deriving a target figure for the failure measure of a safety

control function from a risk assessment. As soon as a risk assessment is used as a basis for

specifying a required level of safety integrity, it is implied that elements of this risk assessment

are quantified. After all, a quantitative result is derived as output of the procedure and it is

generally assumed that this is in a logical relation to the assumptions which were used as inputs.

Consequently, there is a basic logical rationale of functional safety assignment, which captures

all relevant aspects of the application of a safety control function in quantified parameters and

sets them in a logical relation to the tolerable risk limit and the target failure measure for the

function.
NOTE Information on risk management can be found in ISO 31000:2018.
4.2 Sequence of steps in functional safety assignment

The following steps can be used to lead to a functional safety assignment in the context of a

risk analysis for a machine or process. In this context, "SIL" is used as generic placeholder for

any type of safety integrity indicator.
1) A hazard is identified by the analysis.

2) Accident scenarios with that hazard can be developed: It is stated which persons could

suffer which type of harm, by which parts or functions of the machine, in which operation

modes of the machine or process, etc. – see 4.3.2 for the elements of an accident scenario.

3) Mitigation measures can be devised conceptually. According to ISO 12100:2010, 6.1, the

priority of measures decreases from inherently safe design measures (step 1) over

safeguarding and/or complementary protective measures (step 2) to information for use

(step 3). Safety functions are a form of "safeguarding and/or complementary protective

measures".

4) The iteration of the overall design of the machine or process leads to the decision that an

instrumented control function will be implemented. At the latest at this point, the

functionalities of the control function are defined.
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IEC TR 63161:2022 © IEC 2022 – 11 –

5) The safety related parts of the instrumented control function can be identified. With respect

to the hazard in step 1 above, the function will be capable of preventing the given hazard

from causing harm, if it works as devised.

NOTE 1 The required SIL is relevant for the functionality according to step 5. With this step 5, the preconditions

for a SIL-assignment can be given. The following steps comprise the assignment in a strict sense. Typically, this

can be done using a graphical tool, table or scoring system. The current description assumes that no such pre-

designed tool is available, but the basic logic of the process can be followed in a "quantitative

...

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