functional safety EN13849-1 wiring categories

[udtwmc]

Hi,

I'd like to understand the difference between the following 'functional safety' wiring methods and how they relate to the EN13849-1 'input - logic - output' block diagrams. It's the 'Output device' that i feel i'm misunderstanding ...







So, is the output device from the first block diagram (according to EN13849-1) referring to the power contactors Q1 and Q2 in figures 2-2 & 2-3 or is it the safety relay 'internal' pair of contacts between terminals 13 & 14?

both schematics state they meet PLe however figure 2-3 obviously looks (and is) more fault tolerant as it uses 2 contactors on 2 separate channels but i would like to save on relay outputs by energizing my Q1 & Q2 with just one output channel as in figure 2-2 ... while i have appeared to have answered my own question, i still have conflicting views from work colleagues and other publications on this matter that tend to only show Cat3/4 as using 2 separate output channels to drive 2 contactors ( btw i am monitoring Q1 and Q2 back into my safety relay)

Also, the block diagram from 13849-1 does appear to me to mean 2 separate output channels as in figure 2-3 whereas if it clearly meant the internal contacts of the safety relay that would make me feel better.

hope that makes some sense and you can shed some light on the matter!

cheers

 

[mk42]

I can't speak to the older Cat standards.

For the newer standards (PL/SIL), it all ends up boiling down to math, and probability of a dangerous fault. As long as you have the redundancy all the way through, the probability could be low enough in the first picture. There is no single point of failure that causes a dangerous fault, even if both contactors are triggered by the same output, because the output itself has two different things that can turn it off.

I know that Siemens safety manuals specifically include it as an allowable option, just like the document you showed.

 

[udtwmc]

Thank you for replying MK42,

what you say makes sense to me. there are 2 outputs 'within' the safety relay that can turn off the 2 external power contactors. The pictures i uploaded certainly state PLe can be achieved using 1 output so i'm sticking to my plan!

It's just my colleagues like to drive these external power contactors from 2 separate safe outputs (unnecessary and costly in my opinion, but the block diagram in the first picture showing the 2 'device outputs' does infer to me that my colleagues are indeed correct! which just left me wondering if there is actually a significant difference that i'm unaware of.

cheers anyway

 

[Timbert]

mk42 has it right. You can do it either way provided that the MTTFd is low enough. Just using that type of architecture is not enough to ensure compliance with PLe.

 

[udtwmc]

Thanks for confirming Timbert.

And yes I haven’t overlooked the additional device data such as MTTFd and diagnostic coverage and the overall system design. The ‘old’ category wiring B,1,2,3 & 4 from EN954 that i refer to is still mentioned in EN13849-1 but just as a means to show the physical wiring arrangement - I just find those particular diagrams confuse me!

Thanks again

 

[mk42]

which just left me wondering if there is actually a significant difference that i'm unaware of.

My guess is that, once you do the math, you might get a slightly better MTTFd with two outputs, but one can get there with good enough components.


Also, historically, in my opinion, safety systems have largely been less engineered and more just throwing HW at the problem in the hopes of making it better. Or ignored completely. Guys knew rules of thumb that had been handed down from earlier, and they are blindly followed because no one actually knows where they came from....