IEC 61508 and IEC 61511 are known as “performance” based standards.  Specific design rules, as found in “prescriptive” standards, are not included, allowing for innovation and new technology, as well as creativity and advancement.  However, it does imply that a certain level of competency is required.

Others would strongly argue that IEC 61511 should eliminate the requirement for performance calculations and replace them with a set of standard designs that must always be implemented the same way.  I have heard people say “You need a PhD in Reliability Engineering to do those calculations.”  Well I do have a PhD in Reliability Engineering, so I understand the comment when thinking about the situation in the 1990s.  But today things are different.  Many, including myself have used that PhD knowledge to help create easy to use SIF verification tools like exSILentia.  A PhD is not necessary with those types of tools.  I readily admit however that a level of competency is still required, just not a PhD in Reliability Engineering.

There are advantages to the cookbook approach using standard designs.  There is no room for engineers to “cheat” and use ridiculously optimistic cycle test failure data.  There is no possibility for semi-competent engineers to “assume” perfect proof testing or perfect maintenance capability in order to meet a SIL level and check the box for a bad design.

However, the world advances only when creative people make breakthroughs with more effective designs, and the performance based approach allows this.  It also allows lifecycle cost optimization, risk based design, and avoids “weak link” designs like an expensive Safety PLC used with a single sensor, and single valve.  These benefits are well worth the tradeoffs.