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SIL Level PFD and RRF Table (IEC 61508)
SIL level table showing the target failure measures for each safety integrity level in low demand and high or continuous demand modes.
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SIL targets
| SIL | PFDavg (low demand) | Risk reduction factor | PFH per hour (high demand) |
|---|---|---|---|
| SIL 4 | ≥ 10−⁵ to < 10−⁴ | > 10,000 to ≤ 100,000 | ≥ 10−⁹ to < 10−⁸ |
| SIL 3 | ≥ 10−⁴ to < 10−³ | > 1,000 to ≤ 10,000 | ≥ 10−⁸ to < 10−⁷ |
| SIL 2 | ≥ 10−³ to < 10−² | > 100 to ≤ 1,000 | ≥ 10−⁷ to < 10−⁶ |
| SIL 1 | ≥ 10−² to < 10−¹ | > 10 to ≤ 100 | ≥ 10−⁶ to < 10−⁵ |
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Minimum hardware fault tolerance (IEC 61511 1:2016)
| SIL | Low demand HFT | High or continuous demand HFT |
|---|---|---|
| SIL 1 | 0 | 0 |
| SIL 2 | 0 | 1 |
| SIL 3 | 1 | 2 |
| SIL 4 | 2 | Not allowed |
About this Table
SIL level table showing the target failure measures for each safety integrity level in low demand and high or continuous demand modes.
Basis: IEC 61508 1 Tables 2 and 3, and IEC 61511 1 (2016) Table 6 minimum hardware fault tolerance for safety instrumented functions.
How to use
- Get the required SIL from the hazard and risk assessment (LOPA, risk graph).
- Verify that the SIF PFDavg falls inside the band.
- Check architecture (HFT) and systematic capability too.
Notes
- Low demand means a demand less than once per year.
- IEC 61511 SIL 4 is rarely used in the process industry.
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Frequently Asked Questions
What PFD is SIL 2?
PFDavg from 10−³ to below 10−², a risk reduction of 100 to 1,000.
What is the difference between PFD and PFH?
PFDavg is for low demand functions; PFH is the dangerous failure rate per hour for high demand.
Is a SIL rated transmitter enough for SIL 2?
No. The whole function (sensor, logic, final element) must meet the target.
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What is SIL (Safety Integrity Level)
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