I'm looking for online articles or printed publications that discuss the different aspects of designing a Fail Safe system. I know there are fail safe sensors, redundant safety systems, etc. But what I'm looking for is wiring and program logic practices for creating a fail safe system for inputs (sensors) outside of safety etc... i.e. every day prox or photoelectric sensors, etc.

 

You need to know how a system (consisting of hardware/firmware/software) may fail (to perform) to design a fail-safe system. Generally FMEDA report from manufacturer is a good help. With increasing awareness of industries toward "functional safety", manufacturers are also forced/encouraged to publish product specific FMEA/FMEDA. Contact vendor or you may refer "www.exida.com" for limited available reports.

Alternative is to fallback on various handbooks to collect failure data/probabilities which are mostly generic and not vendor/model specific.

The website "http://www.safetyusersgroup.com/" is good to start with understanding concepts of functional safety.

Regards.

 

The general rule is that you want the signal active in the good condition. If the signal is off in the good condition, then if a wire is broken or a PLC input fails, you won't know when the bad condition occurs. With signal active = good, you'll find out instantly about any broken wires, failed inputs, etc. Unfortunately, they'll simulate the bad condition and may falsely shut down your process, but that's better than going along complacently thinking everything's OK when you've lost the ability to sense the problem.

 

Just indirectly related (but you mentioned input modules), the new (well not really that new anymore) PLC Producer/Consumer model is something to watch out for with sensors wired failsafe on the hardware side. There needs to be more logic put in place to detect failures in input modules.

Hugo

 

I would suggest reading a very interesting paper, "Loss-Prevention and Risk-Mitigation in Equipment Protection Systems" by Phil Corso. You can contact Phil at cepsicon at aol. com if you can't find his paper. He is very knowledgeable and helpful, especially when discussing Fail Safe Systems.