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MTL Instruments Group

FAQ - SIL -

FAQ - SIL
Q

What is SIL?
A Safety Instrumented Systems (SIS) are not a new concept and it has long been the practice to fit protective systems to industrial process plant where there is a potential threat to life or the environment should something go wrong. What is new is that IEC61508 provides guidelines to achieving an adequate Safety Integrity Level (SIL) for systems, sub-systems and components. This international standard defines four Safety Integrity Levels: SIL 1 to 4 each corresponding to a range of target likelihood of failures of a safety function.
For further information click here to obtain a copy of the MTL Application Note:
AN9025 ‘An Introduction to Functional Safety and IEC61508’.
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Q

What is IEC61508?
A

IEC 61508 is a ‘generic’ standard, intended to satisfy the needs of all industry sectors. It is a large document, consisting of seven parts and a total of about 400 pages. Ideally it should be used as the basis for writing more specific (e.g. sector- specific and application-specific) standards, but it is also intended to be used directly where these do not exist. It has become a requirement of many customers, and its principles are perceived as defining much of what is considered to be good safety-management practice.
The standard consists of seven parts. The first four are ‘normative’ - i.e. they are mandatory - and the fifth, sixth and seventh are informative - i.e. they provide added information and guidance on the use of the first four.

  • Part 1 (General Requirements) defines the activities to be carried out at each stage of the overall safety lifecycle, as well as the requirements for documentation, conformance to the standard, management and safety assessment.
  • Part 2 (Requirements for Electrical/ Electronic/ Programmable Electronic (E/E/PE) Safety-Related Systems) and Part 3 (Software Requirements) interpret the general requirements of Part 1 in the context of hardware and software respectively. They are specific to phase 9 of the overall safety lifecycle, illustrated in Figure 4.
  • Part 4 (Definitions and Abbreviations) gives definitions of the terms used in the standard.
  • Part 5 (Examples of Methods for the Determination of Safety Integrity Levels) gives risk-analysis examples and demonstrates the allocation of safety integrity levels (SILs).
  • Part 6 (Guidelines on the Application of Parts 2 and 3) offers guidance as per its title.
  • Part 7 (Overview of Techniques and Measures) provides brief descriptions of techniques used in safety and software engineering, as well as references to sources of more detailed information about them. In any given application, it is unlikely that the entire standard would be relevant. Thus, an important initial aspect of use is to define the appropriate part(s) and clauses.
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