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GB/T 20438 consists of the following seven parts under the general title 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 part is Part 4 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 20438.4-2006 Functional Safety of Electrical/ Electronic/ Programmable Electronic Safety-related Systems - Part 4: Definitions and Abbreviations; the following main technical changes have been made with respect to GB/T 20438.4-2006:
——The terms and definitions of “software on-line and off-line support tools” are added (see 3.2.10 and 3.2.11);
——The term and definition of “application specific integrated circuit” is added (see 3.2.15);
——The terms and definitions of “systematic safety integrity” and “systematic capability” are added (see 3.5.6 and 3.5.9);
——The terms and definitions of “no part failure” and “no effect failure” are added (see 3.6.13 and 3.6.14);
——The terms and definitions of “mean time to restoration” and “mean repair time” are added (see 3.6.21 and 3.6.22).
This part is identical to IEC 61508-4:2010 Functional Safety of Electrical/ Electronic/ Programmable Electronic Safety-related Systems - Part 4: Definitions and Abbreviations by means of translation.
This part was proposed by China Machinery Industry Federation.
This part is under the jurisdiction of the National Technical Committee on Industrial Process Measurement Control and Automation of Standardization Administration of China (SAC/TC124).
The previous edition replaced by this part is as follows:
——GB/T 20438.4-2006.
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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 national standards based on the GB/T 20438 series.
Note: 1: Examples of product and application sector international 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 national 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, 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 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 20438 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 4: Definitions and Abbreviations
1 Scope
1.1 This part of GB/T 20438 contains the definitions and explanation of terms that are used in GB/T 20438.1~GB/T 20438.7.
1.2 The definitions are grouped under general headings so that related terms can be understood within the context of each other. However, it should be noted that these headings are not intended to add meaning to the definitions.
1.3 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 publications. The horizontal safety function of GB/T 20438 does not apply to medical equipment in compliance with the IEC 60601 series.
1.4 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.
1.5 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.
IEC Guide 104:1997 The Preparation of Safety Publications and the Use of Basic Safety Publications and Group Safety Publications
ISO/IEC Guide 51:1999 Safety Aspects - Guidelines for Their Inclusion in Standards
3 Definitions and Abbreviations
The definitions and the abbreviations used in GB/T 20348 are given in Table 1.
Table 1 Abbreviations Used in GB/T 20348
Abbreviation Full expression Definition and/or explanation of term
ALARP As Low As Reasonably Practicable GB/T 20348.5-2017, Annex C
ASIC Application Specific Integrated Circuit 3.2.15
CCF Common Cause Failure 3.6.10
CPLD Complex Programmable Logic Device
DC Diagnostic Coverage 3.8.6
(E)EPLD (Electrically) Erasable Programmable Logic Device
E/E/PE Electrical/Electronic/Programmable Electronic 3.2.13, example: E/E/PE safety-related system
E/E/PE (system) Electrical/Electronic/Programmable Electronic System 3.3.2
EEPROM Electrically Erasable Programmable Read-Only Memory
EPROM Erasable Programmable Read-Only Memory
EUC Equipment Under Control 3.2.1
FPGA Field Programmable Gate Array
GAL Generic Array Logic
HFT Hardware Fault Tolerance 7.4.4 of GB/T 20348.2-2017
MooN M out of N channel architecture (for example 1oo2 is 1 out of 2 architecture, where either of the two channels can perform the safety function) GB/T 20348.6-2017, Annex B
MooND M out of N channel architecture with Diagnostics GB/T 20348.6-2017, Annex B
MTBF Mean Time Between Failures 3.6.19, Note 3
MTTR Mean Time To Repair 3.6.21
MRT Mean Repair Time 3.6.22
PAL Programmable Array Logic
PE Programmable Electronic 3.2.12
PE system Programmable Electronic 3.3.1
PFD Probability of Dangerous Failure on Demand 3.6.17
PFDavg Average Probability of dangerous Failure on Demand 3.6.18
PFH Average frequency of dangerous failure [h-1] 3.6.19
PLA Programmable Logic Array
PLC Programmable Logic Controller GB/T 20348.6-2017, Annex E
PLD Programmable Logic Device
PLS Programmable Logic Sequencer
PML Programmable Macro Logic
RAM Random Access Memory
ROM Read-Only Memory
SFF Safe Failure Fraction 3.6.15
SIL Safety Integrity Level 3.5.8
VHDL Very High Speed Integrated Circuit Hardware Description Language GB/T 20348.2-2017, Annex F, Note 5
3.1 Safety Terms
3.1.1 harm
physical injury or damage to the health of people or damage to property or the environment
[ISO/IEC Guide 51:1999, definition 3.3]
3.1.2
hazard
potential source of harm
[ISO/IEC Guide 51:1999, definition 3.5]
Note: The term includes danger to persons arising within a short time scale (for example, fire and explosion) and also those that have a long-term effect on a person’s health (for example, release of a toxic substance).
3.1.3
hazardous situation
circumstance in which people, property or the environment are exposed to one or more hazards
[ISO/IEC Guide 51:1999, definition 3.6, modified]
3.1.4
hazardous event
event that may result in harm
Note: Whether or not a hazardous event results in harm depends on whether people, property or the environment are exposed to the consequence of the hazardous event and, in the case of harm to people, whether any such exposed people can escape the consequences of the event after it has occurred.
3.1.5
harmful event
occurrence in which a hazardous situation or hazardous event results in harm
Note: Adapted from ISO/IEC Guide 51, definition 3.4, to allow for a hazardous event.
3.1.6
risk
combination of the probability of occurrence of harm and the severity of that harm
[ISO/IEC Guide 51:1999, definition 3.2]
Note: For more discussion on this concept see Annex A of GB/T 20438.5-2017.
3.1.7
tolerable risk
risk which is accepted in a given context based on the current values of society
[ISO/IEC Guide 51:1999, definition 3.7]
Note: See Annex C of GB/T 20438.5-2017.
3.1.8
residual risk
risk remaining after protective measures have been taken
[ISO/IEC Guide 51:1999, definition 3.9]
3.1.9
EUC risk
risk arising from the EUC or its interaction with the EUC control system
Note 1: The risk in this context is that associated with the specific harmful event in which E/E/PE safety-related systems and other risk reduction measures are to be used to provide the necessary risk reduction, (i.e. the risk associated with functional safety).
Note 2: The EUC risk is indicated in Figure A.1 of GB/T 20438.5-2017. The main purpose of determining the EUC risk is to establish a reference point for the risk without taking into account E/E/PE safety-related systems and other risk reduction measures.
Note 3: Assessment of this risk will include associated human factor issues.
3.1.10
target risk
risk that is intended to be reached for a specific hazard taking into account the EUC risk together with the E/E/PE safety-related systems and the other risk reduction measures
3.1.11
safety
freedom from unacceptable risk
[ISO/IEC Guide 51:1999, definition 3.1]
3.1.12
functional safety
part of the overall safety relating to the EUC and the EUC control system that depends on the correct functioning of the E/E/PE safety-related systems and other risk reduction measures
3.1.13
safe state
state of the EUC when safety is achieved
Note: In going from a potentially hazardous condition to the final safe state, the EUC may have to go through a number of intermediate safe states. For some situations a safe state exists only so long as the EUC is continuously controlled. Such continuous control may be for a short or an indefinite period of time.
3.1.14
reasonably foreseeable misuse
use of a product, process or service in a way not intended by the supplier, but which may result from readily predictable human behaviour
[ISO/IEC Guide 51:1999, definition 3.14]
3.2 Equipment and Devices
3.2.1
equipment under control; EUC
equipment, machinery, apparatus or plant used for manufacturing, process, transportation, medical or other activities
Note: The EUC control system is separate and distinct from the EUC.
Foreword II
Introduction IV
1 Scope
2 Normative References
3 Definitions and Abbreviations
3.1 Safety Terms
3.2 Equipment and Devices
3.3 Systems - General Aspects
3.4 Systems - Safety-related Aspects
3.5 Safety Functions and Safety Integrity
3.6 Fault, Failure and Error (see Figure 4)
3.7 Lifecycle Activities
3.8 Confirmation of Safety Measures
Bibliography
Index
Figure 1 Overall Framework of the GB/T 20438 Series
Figure 2 Programmable Electronic System
Figure 3 Electrical/Electronic/Programmable Electronic System (E/E/PE System) - Structure and Terminology
Figure 4 Failure Model
Table 1 Abbreviations Used in GB/T
