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Operational Safety: SIL Standards

Operational safety is defined as the absence of unacceptable risks, injury or harm to the health of humans, whether direct or indirect, resulting from damage to equipment or the environment.  A risk analysis allows for the determination of how operational safety will allow for a guarantee of adequate protection against any risk that may arise.  These dangers are therefore treated appropriately during the design phase so that the final system is fault-free.

Safety functions result from electrical, electronic or programmable electronic systems, which are usually complex and make it very difficult to determine breakdowns. The objective is therefore to design a system in such a way as to prevent as many breakdowns as possible and control them when they do occur.

Breakdowns may occur for a number of different factors:

  • Software errors,
  • Human error,
  • Environmental influence,
  • Random breakdown of equipment mechanisms,
  • Etc.

Operational safety therefore depends on the proper operations of a global system or equipment in response to system or equipment entries.

For this reason, standard IEC 61508 was created.  It includes the necessary and sufficient requirements to minimize these breakdowns.  All the phases of the equipment and software lifecycle (from the conceptualization through to the design, installation, operations, maintenance and final disposal) are involved.

IEC 61508 was approved by CENELEC as a European Standard (ES)

Standard IEC 61508  presents a generic approach to all activities related to the lifecycle (from the creation to the disposal of the system) of the electrical-electronic-programmable electronic (E/E/PE) elements that are used to realize safety features.

Due to the broad variety of electrical-electronic-programmable electronic applications at very diverse levels of complexity, the IEC 61508 standard defines methods for analysis and development in order to achieve operational safety based on a risk analysis and determine the safety integrity level (SIL) to be reached for a given risk, but no general rules per se.

The SIL or Security Integrity Level is a level of safety integrity. The SIL notion results directly from the IEC 61508 standard. The SIL may be defined as a measurement of operational safety that determines recommendations related to the integrity of the safety features to be assigned to E/E/PE systems.

There are four SIL levels: SIL 4 being the highest level of system security.

This involves an average probability of failure on demand, PFDavg, for a period of 10 years.

SIL 4

Very significant impact on the community leading to a reduction in danger from 10000 to 100000

SIL 3

Very significant impact on the community and employees leading to a reduction in danger from 1000 to 10000

SIL 2

Significant protection of the installation, production and employees leading to a reduction in danger from 100 to 1000.

SIL 1

Low protection of the installation and production leading to a reduction in danger from 10 to 100.

Thanks to significant expertise in formal calculation, operational safety and the use of the B method (broadly used in industry to create proven safety software), ClearSy System Engineering is qualified to conduct projects that require a level SIL2, SIL3 or SIL4 level certification pursuant to Standard 61508.