AB 1756-OBV8S Faulting

[kekrahulik]

During the "no parts available" era of 2020-2022, we were told that a 1756-OBV8S would have better delivery than PointIO safety modules that we typically used. I got the 1756-OBV8S to work for a direct wire connection to a servo drive safe-off, but could not get it to pull in a 24V Coil Allen-Bradley 700S emergency stop relay. The Card Output just faulted every time it tried to pull in the relay (the relay would give a good "chunk" pull in before the fault). Luckily, the relay was for future use, so I shelved it...until now when I need to replace a bunch of hardwired 120V relays with Safety PLC controlled 24V relays.

As I search the AB knowledgebase, it looks like I'm not the first to struggle. One technote has you adding diodes to prevent faults if you are wiring in Bipolar mode - I am not. I have the card setup for sourcing, so the output wires to the relay coil+ and the relay coil- wires to 24V common.

Another technote makes it sound like you must use a SELV /PELV listed 24V power supply. Interesting that when I search AB for a SELC power supply, I don't seem to find any.

And finally, another technote makes it sound like these modules shouldn't have made it out the door at AB:

Summary
False Overload Faults with OBV8S/OBV8SK in Sourcing mode
Question
Why the OBV8S/OBV8SK module experience Overload Faults when the contactors are switching in Sourcing mode with safety or safety pulse test mode selected but there is no actual overloads observed ?
Environment
• 5069-OBV8S / 5069-OBV8SK V1.011
• 1756-OBV8S / 1756-OBV8SK V1.011
Answer
• 5069-OBV8S / 5069-OBV8SK : Firmware V2.011 and Add-On Profile V2.01 is now available on the Product Compatibility and Download Center to resolve this anomaly.
• 1756-OBV8S / 1756-OBV8SK V1.011 : Use an interposing contactor consuming less current, the anomaly will be resolved in a future firmware.

SO MY QUESTION TO THE GROUP:
1. Has anyone been successful with a 1756-OBV8S card? Any learnings?
2. Do I need to use a SELV power supply for it to function?
3. I just bought some 700-HN123 8-pin safety relays that I can use to pull in the larger 700S relays in case this is a load issue (haven't tried it yet). Has anyone else had to do this?

Of course, the delivery of the 1756-OBV8s ended up being a year delivery - so in hindsight, I should have just gone with PointIO.

 

[jstolaruk]

Problem with using a non-safe interposing relay is that the safety integrity disappears. I use the AB 700-HPS2Z24 (a "Pin" style safety relay), as interposing relays. Its mounted in a 700-HN123 base.

 

[kekrahulik]

I should have said that I bought 700-HPSXZ24 safety relays for my 700-HN123 bases.

My real issue is that the 1756-OBV8S card does not seem capable of pulling in a 700S-DCP1020DZ24 relay. So I'm a little miffed if I have to use a 24Vdc safety relay to pull in my real safety relay.

 

[jstolaruk]

Did you ever figure this out? I reviewed some previous designs and no problem with this module pulling in safety contactors.

 

[kekrahulik]

What do you mean by "reviewed previous designs"?

My coworker just had a discussion with Rockwell. They didn't seem to blink when he mentioned the problem of pulling in a 700S-DCP1020DZ24 relay - just nodded their heads. They suggested their new relay 700S-EF620QJC that has a low current coil.

 

[jstolaruk]

My previous designs used the low current coil version (its not new, been around a while) because I confirmed that the relay holding current was less then what the module could supply. I assumed that the circuit you're having trouble with, the same due diligence had been performed. In the case of a relay's hold current being higher than what the module can supply, an interposing relay is the best fix if a low current version is unavailable.

 

[kekrahulik]

....due diligence missed...

We've been using the other relays with PointIO for years with no issue, so didn't expect the new 1756 card to have less ability than the PointIO.