In normal operation, the appliance sections 11a, 11b with their electrical consumers are powered by the online, double-conversion UPS 1a via its rectifier 3 and its inverter 4, that means that the bypass switch 6 is open in this state. During normal operation, also the circuit breakers 13a . . . 13h are closed (unless they are intentionally and manually opened to switch off certain sections 11a, 11b of the electrical appliance 9a). By means of the battery 8, fluctuations such as voltage spikes and sags occurring in the power grid 10 are compensated so that the electrical consumers are powered by a constant voltage.

In case of an excessive output current and/or excessive voltage drop and/or excessive power demand at the UPS output 5, the control unit 7 closes the bypass switch 6 so as to suspend the power limitation caused by the inverter 4. Typically, the maximum inverter current is 2 to 3 times the nominal current. However, even this maximum inverter current is substantially lower than the maximum current available from the power grid 10. By switching over to the bypass, the electrical consumers can be powered, even in case of excessive power demand.

However, a short circuit or a fault arc is also interpreted as an excessive power demand, which may be detected by the control unit 7 by detecting a voltage drop at the UPS output 5. Accordingly, the control unit 7 switches over to bypass mode also in case of a short circuit or a fault arc. In FIG. 1 such a case is illustrated by the fault arc A and the associated current path drawn with bold lines.