IEC 61508 is an international standard of rules applied in industry. It is titled "Functional Safety of Electrical/Electronic/Programmable Electronic Safety-related Systems".
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 sector.
The safety life cycle has 16 phases which roughly can be divided into three groups as follows:
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:
Contents |
The standard requires that hazard and risk assessment should 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:
The safety integrity level is determined primarily from the assessment of three factors. Higher level safety integrity levels require greater compliance in all three areas.
1) Improved reliability. 2) Failure to safety. 3) Management, systematic techniques, verification and validation.
SIL refers to a single method of reducing injury (as determined through risk analysis), not an entire system, nor an individual component.
For systems that operate continuously (continuous mode) the allowable frequency of failure must be determined. For systems that operate more than once a year (high demand) the allowable frequency of failure must be determined. For systems that operate intermittently (less than once a year / low demand) the probability of failure is specified as the probability that the system will fail to respond on 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 |
| 4 | ≥ 10−5 to < 10−4 | ≥ 10−9 to < 10−8 |
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 IEC61508 that specify the level of SFF required for each SIL.
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. IEC61508 specifies techniques that should be used for each phase of the life-cycle.
The development of software for safety related automotive systems is 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 provides the first automotive industry interpretation of the principles of the, then emerging, IEC 61508 standard.
ISO 26262 is an adaptation of IEC 61508 for Automotive Electric/Electronic Systems.
EN 50128 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.
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.
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.
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.
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 coverage criteria is used rather than simple branch coverage. See topic on MCDC code coverage. To obtain the MCDC (modified condition decision coverage) coverage information, one will need a Unit Testing tool, sometimes referred top as a Software Module Testing tool.
M.Medoff, R.Faller, "Functional Safety - An IEC 61508 SIL 3 Compliant Development Process" - www.exida.com
C. O'Brien, "Final Elements and the IEC 61508 and IEC 61511 Functional Safety Standards" - www.exida.com
M.Punch, "Functional Safety for the Mining Industry – An Integrated Approach Using AS(IEC)61508, AS(IEC) 62061 and AS4024.1." (1st Edition, ISBN 978-0-9807660-0-4, in A4 paperback, 150 pages). www.marcuspunch.com
D.Smith, K Simpson, "Safety Critical Systems Handbook: A Straightforward Guide to Functional Safety, IEC 61508 (2010 Edition) And Related Standards, Including Process IEC 61511 and Machinery IEC 62061 and ISO 13849" (3rd Edition ISBN 978-0080967813, Hardcover, 288 Pages).