UPS Ground Problem?

[Laplacian]

A while back, a large process control room UPS (20kVA ferroresonant, continuous online, 1-phase 120V output) lost it's phase relationship and dropped out of synchronizm with the maintenance bypass frequency and began freewheeling.

While freewheeling, all lighting and analog instruments in the control room began pulsating at the rate of phase difference between the utility frequency and UPS output frequency.

Is this indicative of improper/insufficient grounding for UPS/Bypass sources; or other unrelated phenomena?

 

[RonShap]

Typically the output of the UPS is either inverter based or bypass based (open transition). I'm not sure how the low would have seen "phase difference between the utility frequency and UPS output frequency". The pulsating maybe a result in the inverter output being much lower than 60hz, causing flickering.
A freewheeling inverter, should have transfered to bypass and supplied bypass power to the load.

 

[alehman]

Ferro UPS's commonly have an output transformer with separate primary windings for the AC (bypass) and inverter inputs. If a malfunction is causing the inverter to run at the same time the AC input is connected, the output voltage would be modulated (beat) at a frequency equal to the difference between the two sources.