i would recommend
AB's Safebook. There is a lot of information in there. It will tell you about categories, Performance Level (PL), and Safety Integrity Levels (SIL).
Safety is one of those things were a little information is dangerous. I've seen a lot of designs where the "I used safety rated parts so it must be safe" mentality give a completely false sense of security. Yes, you need to use safety-rated components but they need to be used as intended.
Your's in another example of this. It sounds like the correct pieces are there, they just aren't assembled correctly.