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What Is Safety Integrity Level (SIL)

Safety integrity level (SIL)

Safety Integrity Level abbreviated as “SIL” is one of the safety studies carried out to prevent hazard in process industries.

According to wikipedia – Safety integrity level (SIL) is defined as a relative level of risk-reduction provided by a safety function, or to specify a target level of risk reduction. In simple terms, SIL is a measurement of performance required for a safety instrumented function (SIF).


Safety integrity level studies assess the reliability of functional safety systems and gives a level of confidence to the system.

Safety Integrity is defined as: The probability of a Safety Instrumented Function (SIF) satisfactorily performing the required safety functions under all stated conditions within a stated period of time.

SIL required for SIS (Safety Instrumented System) depends on the magnitude of the process risk it needs to address. This will determine the SIS configuration and frequency of functional tests necessary for maintaining the SIL throughout the lifecycle.

There are two basic elements associated with SIL:

  • Hardware safety integrity: Based upon random hardware failures, it can normally be estimated to a reasonable level of accuracy via probability of failure on demand (PFD).
  • Systematic safety integrity: Tends to be harder to quantify, due to the diversity of causes of failures; systematic failures may be introduced during the specification, design, implementation, operational and modification phase and may affect hardware as well as software.

A device or system must meet the requirements for both categories to achieve a given SIL.


Read AlsoWhat is Safety Instrumented System

SIL Analysis Standards

Safety Integrity Level (SIL) comes from two voluntary standards:

  • IEC 61508: Functional Safety of Electrical/Electronic/Programmable Electronic Safety-Related Systems, and
  • IEC 61511: Safety Instrumented Systems for the Process Industry Sector.

As defined in the IEC standards, four SILs are defined “SIL 1 – SIL 4”, with SIL 4 the most dependable and SIL 1 the least.

Higher SIL Level means a greater process hazard and a higher level of protection required from the SIS.

Safety Integrity Level Table

Safety Integrity Level (SIL) Average Probability of Failure on Demand (PFDavg)
SIL 4 10-4 > PFDavg ≧ 10-5
SIL 3 10-3 > PFDavg ≧ 10-4
SIL 2 10-2 > PFDavg ≧ 10-3
SIL 1 10-1 > PFDavg ≧ 10-2

Above are the different SIL which are proportional to different PFD (Probability of Failure on Demand).

However, to determine the SIL for any SIS (Safety Instrumented System), you will need to determine the PFD.

How To Determine Safety Integrity Level 

The SIL level of a product is determined by three things:

  1. The Systematic Capability Rating: If the QMS meets the requirements of 61508 a SIL Capability rating is issued. The rating achieved depends on the effectiveness of your QMS. The certificate is for the systematic capability of a product
  2. The Architectural Constraints for the element: Architectural constraints are established by following Route 1H or Route 2H. Route 1H involves calculating the Safe Failure Fraction for the element. A valve is typically one component of the final element of a safety instrumented function (SIF).
  3. The PFDavg calculation for the product: The PFDavg is based on the dangerous failure rate, system diagnostics, proof test coverage and test intervals. Typically, a final element assembly will have a PFDavg the only meets SIL.


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Methods For Assigning SIL

There are several methods used to assign a SIL. These are normally used in combination, and may include:

  • Risk matrices
  • Risk graphs
  • Layers of protection analysis (LOPA), etc.


Read AlsoFailure analysis – Methods and Steps for carrying it out


Further Reading

Determining Safety Integrity Levels for your Process Application

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