David’s current sensor is a valid feedback source, but most contactors already have an auxiliary contact block, which is cheaper.
Wire an NO auxiliary contact to a PLC input (or use the current relay if the contactor can’t take an aux block). Then publish both values in the same message, every few seconds, not only on change:
{"sf1.cmd": 0, "sf1.fb": 1}Alarm rule on the profile, in JSON mode since the form only does one comparison per severity:
"severities": [
{ "severity": "MAJOR",
"condition": { "any": [
{ "all": [ { "key": "sf1.cmd", "op": "==", "value": 0 }, { "key": "sf1.fb", "op": "==", "value": 1 } ] },
{ "all": [ { "key": "sf1.cmd", "op": "==", "value": 1 }, { "key": "sf1.fb", "op": "==", "value": 0 } ] } ] } }
],
"debounce": { "mode": "duration", "raise": 5, "clear": 5 }That covers welded (off but running) and failed to pull in (on but stopped). The 5 s duration debounce covers the contactor’s pick-up and drop-out time. Duration debounce only moves forward when readings arrive, so the PLC has to keep reporting; with change-only reporting the alarm would wait for the next change.
Simpler alternative: let the PLC compute a discrepancy bit with its own timer (it needs it for the local interlock anyway) and alarm on sf1.discrepancy == true. Less JSON, one source of truth. Conditions are explained on Alarm conditions reference.