GuardLogix and N.C or N.O. Feedback Contacts

I have a Guardlogix project and see in the majority of the documentation that the normally closed contact of the force guided relay is the feedback to the controller. Can I use the normally open contacts as feedback signal? Is there an advantage to one over the other in a Guardlogix system?

Yes there is. Using the n.c. contacts is "fail-safe", and allows the input module to perform rapid "self-testing" on the input, by momentarily taking the signal driving the contacts low.

This enables the module to determine that the wiring has not gone open-circuit, and that when the relay is energised, it will see a low input.

Daba,

Thank you for replying.

In the past, I have always done diagnostics in the program for the relay by comparing the commanded output to the feedback signal of a normally open contact on the relay. If the rapid self testing occurs while the input is ON and the relay is energized, wouldn't this be be the same? The diagnostics in the program would check both states of the relay. This always seemed fine but I want to use the GuardLogix input diagnostics properly.

Maybe you are saying that by using normally open contacts, the relay would have to be energized first, feedback contacts close, then the fault would be detected and the relay would need to be shut off. When using the normally closed contacts, you would never have to energize the relay because you would already know the input is not capable of going LOW. Does this sound right?

My understanding from similar systems is that the normally closed contact is used with a positively-guided relay to confirm that a relay has correctly opened when required by an E-Stop or guard circuit opening (i.e. 1 = successfully stopped, contactor is not welded open).

This is similar in principle to what a standard emergency stop safety relay does - if the relay does not see the NC contact going true on a shutdown it cannot be reset. While you could probably invert the sense of a normally-open feedback in your code to use, it would break the principle of positive confirmation of correct operation of a safety circuit and cannot be advisable.