Safety integrity level

Measure in risk analysis

In functional safety, safety integrity level (SIL) is defined as the relative level of risk reduction provided by a safety instrumented function (SIF), i. e. the measurement of the performance required of the SIF. In the functional safety standards based on the IEC 61508 standard, four SILs are defined, with SIL4 being the most dependable and SIL1 the least. The applicable SIL is determined based on a number of quantitative factors in combination with qualitative factors, such as risk assessments and safety lifecycle management. Other standards, however, may have different SIL number definitions.

Assignment, or allocation of SIL is an exercise in risk analysis where the risk associated with a specific hazard, which is intended to be protected against by a SIF, is calculated without the beneficial risk reduction effect of the SIF. That unmitigated risk is then compared against a tolerable risk target. The difference between the unmitigated risk and the tolerable risk, if the unmitigated risk is higher than tolerable, must be addressed through risk reduction of provided by the SIF. This amount of required risk reduction is correlated with the SIL target. In essence, each order of magnitude of risk reduction that is required correlates with an increase in SIL, up to a maximum of SIL4. Should the risk assessment establish that the required SIL cannot be achieved by a SIL4 SIF, then alternative arrangements must be designed, such as non instrumented safeguards (e. g, a pressure relief valve). SIL assignment processes that use the HSE guidance to ratify assignments developed from Risk Matrices have been certified to meet IEC 61508.

There are several problems inherent in the use of safety integrity levels. These can be summarized as follows:
Poor harmonization of definition across the different standards bodies which utilize SIL. Third parties that can provide certification are CSA Group Testing (previously known as SIRA), TüV, and Exida among others. Self certification is also possible. The requirements of these schemes can be met either by establishing a rigorous development process, or by establishing that the device has sufficient operating history to argue that it has been proven in use. Certification is achieved by proving the functional safety capability (FSC) of the organization, usually by assessment of its functional safety management (FSM) program, and the assessment of the design and life cycle activities of the product to be certified, which is conducted based on specifications, design documents, test specifications and results, failure rate predictions, FMEAs, etc.