Safety Integrity Level
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Safety Integrity Levels (SILs) are a safety-measurement standard defined by several bodies including the International Electrotechnical Commission in IEC 61508 to quantify the chance of dangerous failures in electrical or electronic safety devices, that is, the probability of the device to fail in performing its safety function.
See also Defence Standard 00-56 Issue 2 for another implementation of SILs.
Four SIL levels are possible, with SIL4 being the most dependable and SIL1 being the least.
For example, for Safety Instrumented Systems acting in low demand mode, the SIL level depends on the average PFD (probability of failure upon demand) as:
| SIL | PFD |
| 4 | 10-5 to 10-4 |
| 3 | 10-4 to 10-3 |
| 2 | 10-3 to 10-2 |
| 1 | 10-2 to 10-1 |
SIL is defined and calculated according to IEC 61508 and other sector-specific standards like IEC 61511, IEC 62061 or ISA S84.01.
Important parts of the safety lifecycle are determining the safety functions required (SIFs) for a process, the target SIL for each one (based on the necessary risk reduction factor), and the design of the Safety Instrumented System to comply with the requested SIL.
Electric and electronic devices can be certified for use in SIL applications of a given SIL level according to IEC 61508, thus providing the market with some degree of confidence in their safety performance and with a means of determine if they can be used in a given application.
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