IEC 61508

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IEC 61508 is an international standard published by the International Electrotechnical Commission (IEC) consisting of methods on how to apply, design, deploy and maintain automatic protection systems called safety-related systems. It is titled Functional Safety of Electrical/Electronic/Programmable Electronic Safety-related Systems (E/E/PE, or E/E/PES).

IEC 61508 is a basic functional safety standard applicable to all industries. 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 fundamental concept is that any safety-related system must work correctly or fail in a predictable (safe) way.

The standard has two fundamental principles:

1. An engineering process called the safety life cycle is defined based on best practices in order to discover and eliminate design errors and omissions.
2. A probabilistic failure approach to account for the safety impact of device failures.

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:

• Parts 1–3 contain the requirements of the standard (normative)
• Part 4 contains definitions
• Parts 5–7 are guidelines and examples for development and thus informative.

Central to the standard are the concepts of probabilistic risk for each 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, associated mechanical devices, or other technologies. Many requirements apply to all technologies but there is strong emphasis on programmable electronics especially in Part 3.

IEC 61508 has the following views on risks:

• Zero risk can never be reached, only probabilities can be reduced
• Non-tolerable risks must be reduced (ALARP)
• Optimal, cost effective safety is achieved when addressed in the entire safety lifecycle

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. The seven parts of the first edition of IEC 61508 were published in 1998 and 2000. The second edition was published in 2010.