PLC AC Inputs Reference

See the really rough sketch of a motor starter with H-O-A switch. Saw the PLC inputs wired this way twice recently. The AC from the starter cubicle, which has its own transformer, is powering the input signals. The reference (com) on the input card (AB 1769-IA16) if from, of course, a different neutral. Of course this works, but isn't that essentially making the bonding system the return path? I prefer to add a separate contact to monitor each switch position, using the input supply power from the PLC panel. I assume when the above method is used, it is the "cheap and dirty" solution? Thoughts, please? I know there are isolated input cards, and relays could also be used, and there are other issues with doing it this way...but I am seeking thoughts on the AC references. Thanks!!

My thought would be to add contacts to the switch and keep the two circuits seperate. This way you don't subject the PLC inputs to the inductive noise the 120VAC motor is likely to induce. Just my two cents worth.

I agree with the separate contacts; the isolation eliminates a bunch of potential power issues. Another thing to mention is that the "hand" input would come on whenever the PLC contact closes in "auto" mode if wired as drawn.

Right on RonJohn :nodi:

And Vice Versa

I had to deal with a whole MCC full of starters connected this way, but at least they ran a neutral wire down there to keep from bridging input common through ground!

In my case, they also ran a wire from between the starter coil and the OL contact to detect when it tripped. That input was only true long enough for the PLC to detect it and subsequently turn off it's output. They did not understand why sometimes the alarm showed up in the Panelview when a motor starter tripped, but most of the time, nothing appeared. Since the alarm bit was not sealed or latched, it was just a copy of the input that might be on for a scan or two, there was about a 2 percent chance the Panelview would "see" it.

I never did fix all that mess, but I did manage to trap the faults correctly, because an hour on the keyboard was so much easier than pulling fifty new cables through the plant.