Last edit: 03/07/2023
Route 1H uses both the Failure rates and the SFF Parameter.
In the first edition of IEC 61508 (2000), that was the only possible approach.
During the discussion for the second edition of the IEC 61508 series of standards (released in 2010), there were a majority of members, in the Technical Committee, that had more faith in reliability data and considered SFF as being not useful / too conservative. That was the reason why the Route 2H was defined.
Using Route 2H the reliability data look like the following (Rosemount Pressure Transmitter):
- λD = 5·10 -7 [h-1]
- λDU = 1,49·10 -7 [h-1]
- λSU = 7,70·10 -8 [h-1]
- Component Type B
There is no need to calculate the SFF; the architectural constraints are indicated in the following table:
The paragraphs indicated in the table are from IEC 61508-2: 2010
That means, if the pressure transmitter is used in a HFT = 1 subsystem architecture, the maximum reachable SIL is SIL 3.