PLC Outputs - 2oo3

Looking to see if anyone has implemented three interposing relays in a hardwired 2oo3 interlock circuit.

We are adding new PLC based interlock to an existing hardwired circuit and 2oo3 has been proposed for perceived reliability. Is there a packaged module with three ice-cubes, already wired as 2oo3? Otherwise we'd just hardwire the interconnecting 2oo3 circuit contacts from the three relays.
 
Not sure what you mean 'pre-packaged module as 2oo3'...
Moreover it's not clear how you would design in a PLC into existing hardware - would this be something similar to Triconex?

I would also be concerned when the wording (maybe by your colleagues) 'perceived reliability' is used. Is anyone undertaking a design study as SIL2 and SIL3 are very much different in how you would approach a highly available system.
 
For this project it was determined that SIL 2 was not required.

GE Mark VI includes relay output modules that are configured 2oo3. (I have not worked with Triconex). I was questioning if anyone manufactured a module with plug-in relays, already configured with 3 inputs/coils, and output contacts configured 2oo3.
 
If Not SIS, you can find the boolean logic for 2oo3 voting logic in a Google (images) search. There's not likely a commercial product because such basic logic that can be implemented in any low end PLC.
 
I have done a poor job in describing the application.

Existing hardwired interlock circuit. Customer wants to integrate PLC interlocks into the hard-wired circuit in a way that will reliably trip when required but will not result in spurious trips. It could be argued that SIL certified equipment should be installed however it is an existing plant with existing hardware so upgrading all is not going to happen.

I was evaluating three DO outputs from PLC, wired to three independent relays, with the contacts on the three independent relays hard-wired in standard 2oo3 logic. But this seems quite klunky and I am not sure that more complexity = higher reliability. Did not know if a better solution was out there. For this particular application, I believe the customer would consider 'spurious trip' less desirable than 'failure to trip'.

The customer further would like for the PLC outputs to be 'energize-to-trip' rather than fail-safe, de-energize to trip. so it is unconventional application.
 
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