Definition:
Hardware Fault Tolerance (HFT) is the ability of a system to continue performing its safety function in the presence of faults.
HFT is a simple concept to understand, but can be confusing in how it is implemented in a functional safety program. For example, for a SIL 3 SIF, you cannot have a simple 1oo1 instrument, 1oo1 final element, and a logic solver. The SIF needs redundancy somewhere and that is per HFT and IEC 61511-1 table 6.
Key Points:
- Expressed as 0 (no tolerance), 1 (one fault tolerated), etc.
- Drives architecture requirements for higher SIL levels.
Example:
A 1oo2 voting system can tolerate one fault (HFT = 1) without losing safety functionality.
Cited Source:
- IEC 61508-2:2010, Clause 7.4.4.
- Method Functional Safety article here