Power Isolation

[mikeroberson]

I know some of you have seen a PLC and HMI on a seperate disconnect usually mounted inside a panel with an outlet for programming. It is usually in an enclosure inside a panel. The power to this many times hits the "top side" of the primary disconnect. Is this a potential issue for "lockout tagout". any opinions welcomed. I love the convenience of this setup, and have done it in several panel builds myself. (it was actually required for the GM Saturn plant, and I copied the Idea to other facilities). no more program losses from powerdown of disconnect is the benefit.

[TimWilborne]

This is very common and as long as it is well labeled that it does remain powered it really poses no risk. Most will have a special wire color only used for this. The only thing is most do have a second disconnect on the outside of the main panel to lock this out also. You will also see the panel lights on this circuit

[gravitar]

We've got a few panels here that are built to "automotive spec" and have what you've described.. a small enclosure mounted inside the main panel with a rotary disconnect, fuses, and a small control transformer. Seems like a nice touch, albeit a bit overkill for a 1 or 2 door panel. The main benefit I've seen is the light stays on when the main disconnect is off.. I still haven't run into a situation where I wished I could keep the PLC rack running but the rest of the panel electricity shut off. I suppose if I did, I could just wire in a temporary extension cord just like I do for the overhead light :)

[TimWilborne]

It's nice on assembly lines where lots of data is being sent to and from the Plc. You don't have to worry about missing data and lets you know the cell power is shutdown instead of a bunch of communications errors