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:
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:
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
Consequence categories
These are typically combined into a risk class matrix
Where:
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.
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