Route 2H (IEC 61508-2:2010 §7.4.4.3) sets a minimum HFT from target SIL and demand mode. It does not use the Route 1H SFF tables. IEC 61511-1:2016 Table 6 uses the same HFT numbers for process SIS, with IEC 61511 evidence — not the 61508 90% confidence gate.
Minimum HFT (SILProof table)
| Target SIL | Low demand | High demand | Continuous |
|---|---|---|---|
| SIL 1 | 0 | 0 | 0 |
| SIL 2 | 0 | 1 | 1 |
| SIL 3 | 1 | 1 | 1 |
| SIL 4 | 2 | 2 | 2 |
SIL 2 with HFT 0 is allowed in low demand only. High demand and continuous require HFT 1 at SIL 2. That is the usual trap when someone copies a low-demand 1oo1 into a high-demand SIF.
What Route 2H still requires
- The HFT cell above for the target SIL and demand mode.
- Type B elements: diagnostic coverage ≥ 60%.
- Field reliability data similar in application and environment, collected to a recognised standard, quantified to greater than 90% confidence (§7.4.4.3). SILProof records that as engineer attestation on the SIF — it does not ingest the raw statistics.
Until that evidence is ticked, SIL verification holds architectural SIL at Below SIL 1 / Review. The calculator tables show the HFT capability only.
How Table 6 differs
Table 6 uses the same minimum HFT grid. It does not apply the IEC 61508 Route 2H >90% confidence criterion. Process-sector evidence sits in IEC 61511 §11.4 / §11.9 records. Do not label a Table 6 claim as Route 2H.
Interactive HFT tables: SFF HFT calculator. Evidence ticks on the SIF: SIL verification. Route 1H SFF tables: Route 1H.