While I do fully understand the Catagory 3 standard...
There are apparently contradictory statements under this standard and also a term which leaves the topic of diagnostics somewhat open to interpretation - "reasonably practicable". But when aren't standards open to interpretation?
Without wading into all that, may I first ask a simple question or two?...
If this device has been selected in part to satisfy mitigating an assumed assesed risk, to a minimum Catagory of safety, and comes with in built diagnostics features, and you are not sure should you or should you not include them, then why would you not just use them and remove all doubt?
Does it require more cabling or cost in some way and you are more hoping you don't need to use them?
Of course, just using the feature, without ever being sure whether or why you should use it, will not serve you well in the world of safety and risk mitigation.
I'm just curious (George) as to why you're not just default using the sensors as a possible good practice?
For instance, our company has a multinational minimum standard of dual redundancy for all Emergency Stop safety circuits, both input and output, regardless of the level of assesed risk. If an Emergency Stop is deemed required (it is not always), then it will automatically be minimum redundant. I, personally, hold this same minimum standard, and would apply the same even if the company did not specify it. I see it as a good practice, based on current standard norms and wide availability of redundancy capable devices. It also provides a minimum piece of mind.
Now I do realize that is not practical for all safety function design. Especially for OEM's where the bottom line is always in sharp focus. I have the luxury of running the department in one plant and this good practice has already been sold to the bean counters.
Again, that's without me getting into the requirements of this standard. I'm initially just curious to know your personal opinion?
How we approach safety, that is, our own safety philosophy, can play a big part in how we then go on to interpret safety standards.
Regards,
George