Safety Integrity Level - SIL

Safety Integrity Level (SIL) is defined as the level of risk reduction guaranteed by a Safety Function (Safety Instrumented Function (SIF), i.e. machine behavior related to the operation of safety components such as interlocking microswitches, light curtains, emergency buttons, …) in the context of Functional Safety Management in the Process Industry.

In practice, the SIL level is a quantification of the integrity level of the safety functions, i.e. the ability and probability that the safety system satisfactorily performs the prescribed functions and that it does so within a certain timeframe to ensure safety: the higher the SIL integrity level of a safety system or function, the lower the probability that the safety system or function will not perform its protective function.

The SIL standards are part of the functional safety standards for electrical and programmable electronic devices and according to them, in particular according to IEC 61508 and IEC 61511, are defined – from SIL1 (the least reliable) to SIL4 (the most reliable).

The SIL levels refer to the value of the probability of a dangerous failure per hour and the data with which a SIL value is assigned are based on the qualitative estimate of the risk, on the frequency and duration of the exposure and on the probability of avoiding or limiting the damage; all in a quite analogous way to what happens for the Performance Level (PL) with which there is a correlation as shown in the figure below.

EN 62061 – SIL on Machines and Plants – Dir. 06/42/CE

For the field of Machinery in particular, from IEC 61508 derives IEC EN 62061, where SIL levels are defined, regarding electrical, electronic and programmable electronic systems related to the safety of machines.
The standard applies to control systems used individually or in combination to perform safety-related control functions on non-mobile and non-portable machinery, including groups of machinery operating together in a coordinated manner. It relates only to functional safety requirements designed to reduce the risk of injury or damage to the health of persons in the immediate vicinity of the machinery or directly involved in its use.
It is limited to risks directly arising from the hazards of the machine or a group of machines operating together in a coordinated manner; it does not specify requirements for the performance of non-electrical control elements, which remain subject exclusively to EN 13849-1, PL.

The purpose of the standard is to define the use of complex electronic systems also for safety-related functions, an eventuality that was frowned upon until not so long ago, also in the light of an uncertainty linked to the technological performance of these systems.
The purpose is achieved by providing a methodology capable of assigning the required safety integrity level for each control function to be implemented.