Connecting safety relay input circuits to PLC inputs

[xyzz]

Our packaging machinery OEM company is using safety relays in dual channel manual reset configuration, with ground fault and short circuit monitoring.
We would like to split signals from certain points of safety switches serial chain, for applying to PLC DC inputs.
Does anybody know about regulations forbidding this in North America?

 

[safetydan]

You might want to look through NFPA 79.

 

[xyzz]

Let's respect each other, guys!
What good is such an answer?
If it was that simple, sure I would read it myself.
From my past experience though, one can spend many days studying such books and still have no answer unless directly refered to specific paragraph.
I asked very specific question in hope to get highly professional advice from fellow engineers.

 

[dpc]

Let's respect people who try to help you. I see nothing wrong with the response you got. Your question is not that specific and gives no indication that you have done anything to try to find an answer on your own.

 

[BJC]

"not that specific"
It's not only not that specific it's unclear and poorly written. A safety switch to most people in the electric business is fused or no-fused device to disconnect and/or lock out a load.
I think he's talking about an E-Stop switch or a shutdown switch ( the same thing only the latter allows for an orderly shut down to avoid damage to equipment).
"Split signals" - is he using a spare set of contact from a E-Stop or is he reading accross as set of NO contact into an isolated PLC input.
I think most people want to help but sometimes figuring out the question is to much effort.

 

[sdl]

xyzz:

If I understand you correctly, you want to monitor points within the the relay circuit and not install secondary monitoring points (i.e. switches). I have seen this done on several relay based systems, and if planned out right, actually provides a very good annunciation system.

I am not aware of anything that specifically does not allow it. You did mention "safety" and "machine" and for this, I would suggest IEC 62061 (a subset from IEC 61508). Best practice would see the safety system separate and distinct from other systems. If you are going to "split signals," I would strongly recommend that you perform your due diligence and make absolutely sure that the additions will not adversly affect the safety function(s). The design should be put through an FMEA with an experienced and knowledgeable team. If you are looking to use the safety sensors as alarms, you will not be allowed to take credit for them as a layer of protection.

I would also check with the OEM as there may be warranty issues. My concern here is that the PLC monitoring may cause problems with the safety functions, i.e. the PLC failing or acting in a way that was unexpected. It maybe unfounded, but it really needs a very detailed examination.

Another thought I had on your question is what kind of equipment is it? For example, a boiler, this would not be allowed in some jurisdictions in the manner you have described (and I interpret).

I am not sure how much I helped. Let me know if there are other questions you have.

 

[ozmosis]

Similar thoughts to sdl. I would have thought that connecting safety relays to a non-safety PLC would cancel out the "safety" part of the relays.

 

[safetydan]

You asked if there is a regulation in the US which forbids doing certain things through the PLC? I am not going to do the leg work for you, other than to point you in a direction to look. If you want me to specifically cite you every regulation I know, I can charge you to same rate I give to everyone else who asks me in my practice.

 

[xyzz]

Sdl, thank you for detailed answer.
As I said, we produce packaging equiment (cartoners).
The circuits mentioned do not compromise the safety functions.
My main concern was whether this may be considered "tampering" with the safety relay, which is directly forbidden in the relay manual.