What is Route 2H approach and what requirements exida established to interpret Route 2H for certification?
An element (e.g. a sensor) is able to meet the requirement of hardware safety integrity architectural constraints in IEC 61508 by implementing one of the two possible routes (IEC 61508-2, 7.4.4):
- 1H, by looking at Safe Failure Fraction (SFF) and Hardware Fault Tolerance (HFT)
- 2H, based on component reliability data from feedback from end users, increased confidence levels and hardware fault tolerance for specified safety integrity levels.
SFF is not examined in Route 2H approach.
Both Type A (IEC 61508-2, 18.104.22.168.2) and Type B (IEC 61508-2, 22.214.171.124.3) elements are allowed to follow Route 2H as long as certain requirements are met. Examples of Type A elements are valves, actuators, etc. Examples of Type B elements are flame detectors, PLC, etc.
This blog article, written by my colleague Loren Stewart, provides an excellent insight on the Route 2H requirements for Type A element.
For Type B element, an additional requirement is “All type B elements used in Route 2H shall have, as a minimum, a diagnostic coverage of not less than 60 %.” (IEC 61508-2, 126.96.36.199.4).
exida has interpreted this to mean not just a simple 90% confidence level in the uncertainty analysis, but a high confidence level in the entire data collection process. As IEC 61508, ed2, 2010 does not give detailed criteria for Route 2H, exida has established the following:
- field unit operational hours of 100,000,000 per each component; and
- a device and all of its components have been installed in the field for one year or more; and
- operational hours are counted only when the data collection process has been audited for correctness and completeness; and
- failure definitions, especially “random” vs. “systematic” are checked by exida; and
- every component used in an FMEDA meets the above criteria.
This set of requirements is chosen to assure high integrity failure data suitable for safety integrity verification.
Examples of Type B element meeting Route 2H criteria can be found at exida SAEL website.