IEC TR 63161:2022

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
• ICS: 13.110 - Safety of machinery
• Technical Committee: TC 44 - Safety of machinery - Electrotechnical aspects
• Drafting Committee: PT 63161 - TC 44/PT 63161
• Current Stage: PPUB - Publication issued
• Start Date: 01-Aug-2022
• Completion Date: 13-Jul-2022

Standards Content (Sample)

IEC TR 63161 ®
Edition 1.0 2022-07
TECHNICAL
REPORT
colour
inside
Assignment of safety integrity requirements – Basic rationale

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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

– 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
r
5.7 Exposure parameter – F . 17
r
5.8 Probability of avoiding or limiting harm – A . 18
v
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
R
5.9.4 Safety demands and safety demand rate D . 21
R
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

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

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