What Should I Design To: Performance Levels Or Safety Categories?

With last year's revision of AS 4024.1:2014 designers of safety control systems now have two options:

1. Design to Safety Categories (AS 4024.1501), or 
2. Performance Levels (AS 4024.1503)

Why does the series have 2 options? Which option should be used? Wouldn't it be much easier if there was one direction for design guidance?

As explained in AS 4024.1100:2014, the standards are in a transition phase and are mimicking the process followed by international standards. In international standards, Performance Levels replaced Safety Categories in 2012 after a 5 year transition period where the two standards ran in parallel. The Australian standards are now entering a similar transition phase. It was decided that an instant changeover would not be achievable because it would take a period of time for the industry to become familiar with Performance Levels and the two methods would run in parallel during this period.

Which design method should you use? 
Most safety control systems can be designed to Performance Levels or Safety Categories, but here are some reasons why you may want to use certain sections of AS 4024.1503.

• Common Cause Failures (CCF). To learn more about CCFs and for guidance, refer to a previous post titled 'New series provides guidance on Common Cause Failure'. I would recommend using the common cause method in Annex F of AS 4024.1503 for any CAT 2, 3 or 4 system
• Guidance for developing safety software. If you are developing/maintaining software for safety programmable devices then section 4.6 of AS 4024.1503 is the only guidance on software development that you will find in the AS 4024.1:2014 series
• Component reliability. If you are designing a CAT 1 system I would recommend calculating a Mean Time To dangerous Failure (MTTFd) for your safety system using section 4.5.2 of AS 4024.1503. CAT 1 is highly dependent on component reliability and thus ensure your CAT 1 system has a MTTFd of HIGH.
• Architecture flexibility. Safety Categories using AS 4024.1501 can be inflexible on the architecture of the safety system and will generally push the design towards conservative architectures with redundancy. By using Performance Levels you will find greater flexibility with the architecture of the safety system; for example observing Table 7 of AS 4024.1503, it can be seen that a CAT 1, 2 or 3 architecture can be used to achieve the same risk reduction level.

So, be aware that the standards are transitioning away from Safety Categories. During this phase Safety Categories and Performance Levels will run in parallel, this should be seen as providing more choice to safety designers, not confusion. As mentioned above, there are some useful sections in AS 4024.1503 that will improve safety systems' design, even if the systems are designed to the requirements of Safety Categories. By using these sections of AS 4024.1503, you will design improved safety systems, have more flexibility in your system design, and be better placed to cope with future changes of the AS 4024.1 series.