SIL and Functional Safety Cheat Sheet
The key numbers and formulas of IEC 61511 functional safety on one page: SIL bands, PFDavg for common voting, failure fractions, hardware fault tolerance and LOPA.
Download
A one page A4 sheet to print, pin up in the workshop or keep on your phone. Everything on it is also shown below.
Download PDF A4 · 90 KBSIL bands (low demand, IEC 61511)
| SIL | PFDavg | RRF |
|---|---|---|
| 1 | 10⁻² to 10⁻¹ | 10 to 100 |
| 2 | 10⁻³ to 10⁻² | 100 to 1000 |
| 3 | 10⁻⁴ to 10⁻³ | 1000 to 10,000 |
| 4 | 10⁻⁵ to 10⁻⁴ | 10,000 to 100,000 |
High demand (PFH per hour)
| SIL | PFH |
|---|---|
| 1 | 10⁻⁶ to 10⁻⁵ |
| 2 | 10⁻⁷ to 10⁻⁶ |
| 3 | 10⁻⁸ to 10⁻⁷ |
| 4 | 10⁻⁹ to 10⁻⁸ |
High demand: more than one demand per year.
PFDavg (simplified, IEC 61508-6)
| 1oo1 | λDU × TI ÷ 2 |
| 1oo2 | (λDU TI)² ÷ 3 + β λDU TI ÷ 2 |
| 2oo3 | (λDU TI)² + β λDU TI ÷ 2 |
| 2oo2 | λDU × TI |
| RRF | 1 ÷ PFDavg |
| Imperfect test | PTC λDU TI ÷ 2 + (1 − PTC) λDU MT ÷ 2 |
Failure fractions
| Total rate | λ = λSD + λSU + λDD + λDU |
| SFF | (λS + λDD) ÷ λ |
| Diagnostic coverage | λDD ÷ (λDD + λDU) |
| MTBF | 1 ÷ λ |
| 1 FIT | 1 failure per 10⁹ h |
| Spurious trips 1oo1 | STR ≈ λS |
Minimum HFT (IEC 61511 Ed. 2)
| SIL | HFT |
|---|---|
| 1 | 0 |
| 2 (low demand) | 0 |
| 2 (high demand) | 1 |
| 3 | 1 |
| 4 | 2 |
Voting at a glance
| Voting | HFT | Spurious trips |
|---|---|---|
| 1oo1 | 0 | Base |
| 1oo2 | 1 | About double |
| 2oo2 | 0 | Lowest |
| 2oo3 | 1 | Low |
LOPA
| Mitigated frequency | f = fIE × Π PFDIPL × modifiers |
| Required RRF | f ÷ ftolerable |
| Example | 0.2 × 0.1 × 0.1 = 2 × 10⁻³ /yr; target 10⁻⁵ → RRF 200 → SIL 2 |
Typical IPL credits
| Layer | PFD |
|---|---|
| BPCS loop (maximum claim) | 0.1 |
| Operator response to alarm (enough time) | often 0.1 |
| Relief valve | often 0.01 |
| SIF | per SIL |
Use your company LOPA rules; credits must be independent, effective and auditable.
Proof test essentials
- Test interval in the calculation must match real practice.
- Record as found condition before repairs.
- Test final elements fully: stroke, time and tight shut off.
- Bypasses: approved, registered, time limited, with compensating measures.
Related Calculators and Tools
Frequently Asked Questions
What PFDavg does SIL 2 need?
Between 10⁻³ and 10⁻² in low demand mode, which is a risk reduction of 100 to 1000.
Why does β matter so much?
Common cause failures defeat redundancy; with β of a few percent they often dominate 1oo2 PFDavg.
What is HFT 1?
The function still works with one dangerous hardware fault, as in 1oo2 or 2oo3.
Learn more on the blog
What is SIL (safety integrity level)
A practical guide on instrumentationblog.in that explains the theory behind this cheat sheet.
Read Full Article