IEC 61508

IEC 61508 is an international standard published by the International Electrotechnical Commission of rules applied in industry. It is titled Functional Safety of Electrical/Electronic/Programmable Electronic Safety-related Systems (E/E/PE, or E/E/PES).

IEC 61508 is intended to be a basic functional safety standard applicable to all kinds of industry. It defines functional safety as: “part of the overall safety relating to the EUC (Equipment Under Control) and the EUC control system which depends on the correct functioning of the E/E/PE safety-related systems, other technology safety-related systems and external risk reduction facilities.”

The standard covers the complete safety life cycle, and may need interpretation to develop sector specific standards. It has its origins in the process control industry.

The safety life cycle has 16 phases which roughly can be divided into three groups as follows:

  1. Phases 1-5 address analysis
  2. Phases 6-13 address realisation
  3. Phases 14-16 address operation.

All phases are concerned with the safety function of the system.

The standard has seven parts:

Central to the standard are the concepts of risk and safety function. The risk is a function of frequency (or likelihood) of the hazardous event and the event consequence severity. The risk is reduced to a tolerable level by applying safety functions which may consist of E/E/PES and/or other technologies. While other technologies may be employed in reducing the risk, only those safety functions relying on E/E/PES are covered by the detailed requirements of IEC 61508.

IEC 61508 has the following views on risks:

Hazard and Risk Analysis

The standard requires that hazard and risk assessment be carried out: 'The EUC (equipment under control) risk shall be evaluated, or estimated, for each determined hazardous event'.

The standard advises that 'Either qualitative or quantitative hazard and risk analysis techniques may be used' and offers guidance on a number of approaches. One of these, for the qualitative analysis of hazards, is a framework based on 6 categories of likelihood of occurrence and 4 of consequence.

Categories of likelihood of occurrence

Category Definition Range (failures per year)
Frequent Many times in system lifetime > 10−3
Probable Several times in system lifetime 10−3 to 10−4
Occasional Once in system lifetime 10−4 to 10−5
Remote Unlikely in system lifetime 10−5 to 10−6
Improbable Very unlikely to occur 10−6 to 10−7
Incredible Cannot believe that it could occur < 10−7

Consequence categories

Category Definition
Catastrophic Multiple loss of life
Critical Loss of a single life
Marginal Major injuries to one or more persons
Negligible Minor injuries at worst

These are typically combined into a risk class matrix

Consequence
Likelihood Catastrophic Critical Marginal Negligible
Frequent I I I II
Probable I I II III
Occasional I II III III
Remote II III III IV
Improbable III III IV IV
Incredible IV IV IV IV

Where:

Safety integrity level

The safety integrity level (SIL) provides a target to attain in regards to a system's development. A risk assessment effort yields a target SIL, which thus becomes a requirement for the final system. The requirement informs how to set up the development process (using appropriate quality control, management processes, validation and verification techniques, failure analysis etc.) so that one can reasonably justify that the final system attains the required SIL. Part 2 and 3 of IEC 61508 give guidance on activities to perform in order to attain a SIL.

Improved reliability

The meaning of the SIL varies depending on whether the functional component will be exposed to high or low demand:

SIL Low demand mode:
average probability of failure on demand 		High demand or continuous mode:
probability of dangerous failure per hour
1 ≥ 10−2 to < 10−1 ≥ 10−6 to < 10−5
2 ≥ 10−3 to < 10−2 ≥ 10−7 to < 10−6
3 ≥ 10−4 to < 10−3 ≥ 10−8 to < 10−7 (1 dangerous failure in 1140 years)
4 ≥ 10−5 to < 10−4 ≥ 10−9 to < 10−8

Failure to safety

Calculation of safe failure fraction (SFF) determines how Fail-safe the system is. This compares the likelihood of safe failures with dangerous failures. Reliability by itself is not sufficient to claim a SIL level. There are charts in IEC 61508 that specify the level of SFF required for each SIL.

Management, systematic techniques, verification and validation

Specific techniques ensure that mistakes and errors are avoided across the entire life-cycle. Errors introduced anywhere from the initial concept, risk analysis, specification, design, installation, maintenance and through to disposal could undermine even the most reliable protection. IEC 61508 specifies techniques that should be used for each phase of the life-cycle.

Industry/application specific variants

Automotive software

ISO 26262 is an adaptation of IEC 61508 for Automotive Electric/Electronic Systems. It is being widely adopted by the major car manufacturers.

Before the launch of ISO 26262, the development of software for safety related automotive systems was predominantly covered by the Motor Industry Software Reliability Association guidelines.[1] The MISRA project was conceived to develop guidelines for the creation of embedded software in road vehicle electronic systems. In November 1994 Development a set of guidelines for vehicle based software were published. This document provided the first automotive industry interpretation of the principles of the, then emerging, IEC 61508 standard.

Today MISRA is most widely known for its guidelines on how to use the C and C++ languages. MISRA C has gone on to become the de facto standard for embedded C programming in the majority of safety-related industries, and is also used to improve software quality even where safety is not the main consideration. MISRA has also developed guidelines for the use of model based development.

Rail software

IEC 62279 provides a specific interpretation of IEC 61508 for railway applications. It is intended to cover the development of software for railway control and protection including communications, signaling and processing systems.

Process industries

The process industry sector includes many types of manufacturing processes, such as refineries, petrochemical, chemical, pharmaceutical, pulp and paper, and power. IEC 61511 is a technical standard which sets out practices in the engineering of systems that ensure the safety of an industrial process through the use of instrumentation.

Nuclear power plants

IEC 61513 provides requirements and recommendations for the instrumentation and control for systems important to safety of nuclear power plants. It indicates the general requirements for systems that contain conventional hardwired equipment, computer-based equipment or a combination of both types of equipment.

Machinery

IEC 62061 is the machinery-specific implementation of IEC 61508. It provides requirements that are applicable to the system level design of all types of machinery safety-related electrical control systems and also for the design of non-complex subsystems or devices.

Testing software

Software written in accordance with IEC 61508 may need to be unit tested, depending up on the SIL level it needs to achieve. The main requirement in Unit Testing is to ensure that the software is fully tested at the function level and that all possible branches and paths are taken through the software. In some higher SIL level applications, the software code coverage requirement is much tougher and an MCDC code coverage criterion is used rather than simple branch coverage. To obtain the MCDC (modified condition decision coverage) coverage information, one will need a Unit Testing tool, sometimes referred to as a Software Module Testing tool.

See also

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