An UPS device for electric power distribution installations, the UPS device including: first electric terminals for electrical connection with an electric load to be fed by the UPS device; second electric terminals for electrical connection with a power supply source adapted to feed the UPS device; third electric terminals for electrical connection with a back-up unit adapted to feed the UPS device, when the power supply source is not available, the back-up unit having one or more replaceable batteries; a power conversion stage electrically connectable with the first, second and third electric terminals, the power conversion stage being adapted to receive a power supply voltage from the power supply source or a back-up voltage from the back-up unit, when the power supply source is not available, and being adapted to provide an adjustable load voltage to the electric load; and an interface stage to electrically couple or decouple the back-up unit with or from the power conversion stage. The interface stage includes a feeding circuit to temporary feed the controller during a transient time interval following a replacement of the batteries of the back-unit.