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

Route 1_H  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 1_H or Route 2_H. 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 1_H, and Route 2_H is introduced.