Safety Integrity Level (SIL) is an indicator of the degree of risk reduction unit provided by an instrumental safety function (SIF) implemented by a security system (SIS) within a given process. In other words, SIL can be seen as an indicator of the acceptable failure rate for a security function.
It is a key parameter to consider when you want to compare different suppliers’ products, and also an increasingly important requirement in various public and private tender invitations for the supply of mechanical systems and electrical and electronic products.
SIL standards have been developed by the International Electrotechnical Commission (IEC) in the wider range of functional safety. The main regulations are EN IEC 61508 (Functional Safety of electrical/electronic/programmable electronic safety-related systems) and EN IEC 61511 (Functional Safety: safety instrumented systems for the process industry sector).
IEC standards define four SILs, 1 to 4. A Safety Integrity Level is determined on the base of both quantitative and qualitative factors via several methods used in combination, including risk matrix, risk graphs and Layers of protection analysis (LOPA). The higher is the SIL, the more serious is the potential impact of a failure (and the lower the failure rate considered acceptable).
Within a given system, Safety Integrity Levels certification depends on multiple factors, including:
- type of technologies;
- system architecture;
- number of system components;
- probability of failure on demand (PFD) of each component;
- diagnostic test intervals.
A particular device cannot be called “SIL 1” or “SIL 2”. The correct technical expression to use is that the device is “suitable for use within a given SIL environment”, as the entire system must be taken into account.
Further, the SIL depends on the assessment of inherent risk of systems or processes in a given operation. A device suitable for use in SIL 3 may be superfluous if placed in a SIL 2 environment. However, using a product with a SIL level higher than requested can be useful as it automatically increases the T-proof Test Time Intervals up to 10 times; it also reduces the impact which the single instrument has on the probability of total failure of the SIL.
