Functional safety of electrical/electronic/programmable electronic safety-related systems—Part 5: Examples of methods for the determination of safety integrity levels
GB/T 20438 consists of seven parts under the general title of Functional safety of electrical/electronic/programmable electronic safety-related systems:
——Part 1: General requirements;
——Part 2: Requirements for electrical/electronic/programmable electronic safety-related systems;
——Part 3: Software requirements;
——Part 4: Definitions and abbreviations;
——Part 5: Examples of methods for the determination of safety integrity levels;
——Part 6: Guidelines on the application of GB/T 20438.2 and GB/T 20438.3;
——Part 7: Overview of techniques and measures.
This is Part 5 of GB/T 20438.
This part is developed in accordance with the rules given in GB/T 1.1-2009.
This part replaces GB/T 20438.5-2006 Functional safety of electrical/ electronic/ programmable electronic safety-related systems - Part 5: Examples of methods for the determination of safety integrity levels and the following main technical changes have been made with respect to GB/T 20438.5-2006:
——The selection of methods for determining safety integrity level requirements is added (see Annex B);
——The risk analysis method, i.e., semi-quantitative method using layer of protection analysis (LOPA), is added (see Annex F).
This part, by means of translation, is identical to IEC 61508-5:2010 Functional safety of electrical/electronic/programmable electronic safety-related systems - Part 5: Examples of methods for the determination of safety integrity levels.
This part was proposed by the China Machinery Industry Federation.
This part is under the jurisdiction of SAC/TC 124 National Technical Committee on Industrial Process Measurement and Control of Standardization Administration of China.
The previous edition of this part is as follow:
——GB/T 20438.5-2006.
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.
GB/T 20438 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 standards based on the GB/T 20438 series.
Note 1: Examples of product and application sector standards based on the GB/T 20438 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 GB/T 20438 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. GB/T 20438, by being generic, will enable such measures to be formulated in future product and application sector standards and in revisions of those that already exist.
GB/T 20438
——considers all relevant overall, E/E/PE system and software safety lifecycle phases (for example, from initial concept, thorough 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 standards, dealing with E/E/PE safety-related systems, to be developed; the development of product and application sector standards, within the framework of GB/T 20438, 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: GB/T 20438 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.
——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 dangerous failure on demand of 10-5;
——a high demand or a continuous mode of operation, the lower limit is set at an average frequency of a dangerous failure of 10-9/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 GB/T 20638 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.
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 GB/T 20438 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 shall 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 GB/T 20438.1, GB/T 20438.2, GB/T 20438.3 and GB/T 20438.4 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 GB/T 20438.4-2017). 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. GB/T 20438.1, GB/T 20438.2, GB/T 20438.3 and GB/T 20438.4 are also intended for use as stand-alone standards. The horizontal safety function of GB/T 20438 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 GB/T 20438 series and indicates the role that this part plays in the achievement of functional safety for E/E/PE safety-related systems.
Figure 1 Overall framework of the GB/T 20438 series
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.
GB/T 20438.1-2017 Functional safety of electrical/electronic/programmable electronic safety-related systems - Part 1: General requirements (IEC 61508-1:2010, IDT)
GB/T 20438.4-2017 Functional safety of electrical/electronic/programmable electronic safety-related systems - Part 4: Definitions and abbreviations (IEC 61508-4:2010, IDT)
3 Definitions and abbreviations
For the purposes of this document, the definitions and abbreviations given in GB/T 20438.4-2017 apply.
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 GB/T 20438.4-2017) is the reduction in risk that has to be achieved to meet the tolerable risk for a specific situation (which may be stated either 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.
Foreword I
Introduction III
1 Scope
2 Normative references
3 Definitions and abbreviations
Annex A (Informative) Risk and safety integrity - General concepts
Annex B (Informative) Selection of methods for determining safety integrity level requirements
Annex C (Informative) ALARP and tolerable risk concepts
Annex D (Informative) Determination of safety integrity levels – A quantitative method
Annex E (Informative) Determination of safety integrity levels - Risk graph methods
Annex F (Informative) Semi-quantitative method using layer of protection analysis (LOPA)
Annex G (Informative) Determination of safety integrity levels - A qualitative method - hazardous event severity matrix
Bibliography
Figure 1 Overall framework of the GB/T 20438 series
Figure A.1 Risk reduction - general concepts (low demand mode of operation)
Figure A.2 Risk and safety integrity concept
Figure A.3 Risk diagram for high demand applications
Figure A.4 Risk diagram for continuous mode operation
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
Figure A.6 Common cause between two E/E/PE safety-related systems
Figure A.7 Allocation of safety requirements to the E/E/PE safety-related systems and other risk reduction measures
Figure C.1 Tolerable risk and ALARP
Figure D.1 Safety integrity allocation - example for safety-related protection system
Figure E.1 Risk Graph: general scheme
Figure E.2 Risk graph – example (illustrates general principles only)
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
Table E.1 Example of data relating to risk graph (Figure E.2)
Table E.2 Example of calibration of the general purpose risk graph
Table F.1 LOPA report
