Route 1H is one of two Architectural constraints options made available in the standards IEC 61508-2 and IEC 61511. Route 1H . Both Route 1H and Route 2H are limitations that impose the hardware selected to implement a safety-instrumented function, regardless of the performance calculated for a subsystem. 

Route 1H  is mostly based around the Safe Failure Fraction calculation approach, where the SFF is calculated, and tables in the standards are used to determine minimum Hardware Fault Tolerance (minimum redundancy) for a given SIL level for each Element or subsystem.

Remember: Safe Failure Faction or SFF is:

A measurement of the likelihood of getting a dangerous failure that is NOT detected by automatic self diagnostics.  Shown as the Safe and the Dangerous Detected Failures compared to the Total Failures.

Example: 100% means there are NO failures classified as dangerous that are not detected by automatic diagnostics.  100% is the best that can be achieved. The number goes up for a product that is more inherently fail-safe. The number goes up for a product that has very good automatic diagnostics. 

Fun Fact: Back in the day, (pre-2010, 2nd edition) there was no Route 1H or Route 2H. There was only the SFF equation, and it was slightly different than it is today. It included no effect failures (  in the equation as well. However, companies were “cheating” the calculation by adding in extra parts and possible no effect failures to “improve” their rating. So, when the 2ndedition of the standard was released, the SFF equation changed to no longer include no effect failures. At this point, the SFF was now referred to as 1H, and Route 2H is introduced.

Related Items

Back to Basics 01 - Functional Safety

Back to Basics 02 - Safety Integrity Level (SIL)

Back to Basics 03 - Safety Instrumented Function (SIF)

Back to Basics 04 - Safety Instrumented System (SIS)

Back to Basics 05 - What is a Safety Function?

Back to Basics 06 – IEC 61508

Back to Basics 07– Safety Lifecycle – IEC 61508

Back to Basics 08 – IEC 61511

Back to Basics 09 – Safety Lifecycle – IEC 61511

Back to Basics 10 – How Does a Product Get a SIL?

Back to Basics 11 – How is SIL Used by an End User?

Back to Basics 12 – What is IEC 61508 Certification?

Back to Basics 13 - How Do I Start IEC 61508 Certification?

Back to Basics 14 - Systematic Capability

Back to Basics 15 - Architectural Constraints

Back to Basics 16 - PFDavg

Back to Basics 17 - PFH (Probability of dangerous Failure per Hour)

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