So I've been tasked to document and review the status of the UPSes for my current employer and identify any changes that need to be made. Generally easy stuff, however I came across a few units that are plugged into both regular power and a UPS outlet. I've done similar before but it reminded me of an electrician I used to work with who said doing this could damage the equipment.
I've been looking online to figure out what the specifics of this warning was about, but I haven't found much. From memory, I think it had something to do with using circuits from different sources provided by Three-Phase power. I was working on industrial sites at the time, and some power was provided to my servers by three-phase transformers. Not all was, however.
My question is: Are there any instances where running a system with redundant power supplies could be damaged by running them on different power circuits, assuming both circuits are clean (eg, running both PSU on either circuit alone would cause no damage).