Methods to determine the required HFT

The Second Edition allows 3 different methods to determine the required HFT of a Safety Instrumented System:

1. 1. Route 1_H of IEC 61508-2, based on FMEDA analysis and conformance with the related clauses in IEC 61508-2.
   2. Use of the concept of Prior Use. That means the use of Table 6 of IEC 61511-1, in conjunction with the requirements in IEC 61511-1 clauses 11.5 to 11.9.
   3. Route 2_H of IEC 61508-2, based on product returns to the manufacturer and conformance with related clauses in IEC 61508-2.

​Table clarifies that, in case a SIL 1 has to be reached, no redundancy is necessary; the same is valid in case of SIL 2 in low demand mode. However, if the system is working in high demand mode, in order to reach a SIL 2 level, an HFT=1 is required.