We are facing an issue where our S7-200 PLC CPU gets damaged after UPS maintenance.
During the activity, both UPS input and output breakers are switched off, and the 230 V AC supply to the PLC panel is also turned off.
However, after the UPS supply is restored, the CPU fails (no RUN LED, no communication).

It seems to happen due to transients or neutral/earth disturbance during UPS isolation and re-energization.

Please suggest the possible cause and recommended protection methods