At work we run a lot of machinery with motors and its obvious when phase loss happens. From my office I can tell if the lights go out/dim and the usual shop "hum" becomes a buzzing grunt that is immediately identifiable. Older machines have to be manually powered down but the machines I rebuilt have phase loss protection in the PLC thanks to a power monitoring terminal in the IO (Beckhoff EL3453.) Since the PLC is on 24V DC I have a capacitor backup module fronted by a 24V PSU that takes the 480V three phase power which tolerates a phase loss. The machine safely stops the process and shuts down the pumps and any other AC loads, then waits to be manually powered off as the PSU and DC side doesn't care.