Safety Integrity Level

Safety Integrity Level (SIL) is defined as a level of risk-reduction provided by a safety function (SIF), 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).

Four SILs are defined, with SIL4 being the most dependable and SIL1 being the least.

A SIL is determined based on a number of quantitative factors in combination with qualitative factors such as development process and safety life cycle management. The requirements for a given SIL are not consistent among all of the functional safety standards.

SIL determination
There are several methods to determine SIL for a SIF. Following are some of the popular ones:

The International Electrotechnical Commission's (IEC) standard IEC 61508 defines SIL using requirements grouped into two broad categories:

A device or system must meet the requirements for both categories to achieve a given SIL.
The SIL requirements for hardware safety integrity are based on a probabilistic analysis of the device.
To achieve a given SIL, the device must have less than the specified probability of dangerous failure (or PFD, probability of failure on demand) and have greater than the specified safe failure fraction (SFF: SFF is calculated as the ratio between the sum of total safe failures plus total dangerous detectable failures divided by the sum of total safe failures plus total dangerous failures) . These failure probabilities are calculated by performing a Failure Modes and Effects Analysis (FMEA) or from the statistical analysis of field data for systems "proven in use". This latter method is the preferred one as it has some architectural advantages according to IEC 61511.

The actual targets required vary depending on the likelihood of a demand, the complexity of the device(s), and types of redundancy used.
PFD (Probability of Failure on Demand) and RRF (Risk Reduction Factor) for different SILs as defined in IEC61508 are as follows:

 

SIL	PFD	RRF
1	0.1-0.01	10-100
2	0.01-0.001	100-1000
3	0.001-0.0001	1000-10000
4	0.001-0.00001	10000-100000

 

 

The SIL requirements for systematic safety integrity define a set of techniques and measures required to prevent systematic failures (bugs) from being designed into the device or system. These requirements can either be met 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.
Electric and electronic devices can be certified for use in functional safety applications according to IEC 61508, providing application developers the evidence required to demonstrate that the application including the device is also compliant.
IEC 61511 is an application specific adaptation of IEC 61508 for the Process Industry sector. This standard is used in the petrochemical and hazardous chemical industries, among others.

 

SIL in other standards
The following standards also use SIL (defined differently) as a measure of reliability and/or risk reduction.