If there is an excessive output current and/or excessive voltage drop and/or excessive power demand at the UPS output 5, the system distinguishes between an arc fault and other power demanding processes. In case of a power demanding process in the electrical appliance 9b other than an arc fault, the control unit 7 causes the UPS 1b to switch over into bypass mode by closing the bypass switch 6.

Arc faults are managed in a different way. An arc fault is generally detected by the fault arc detection devices 15a, 15b, if both a current detector 16a, 16b and a light detector 17a . . . 17f signals an abnormal situation. If a fault arc A occurs, excessive light is detected via the light detector 17f and an excessive current is detected at current detectors 16a, 16b. Accordingly, the fault arc detection device 15b signals a fault arc event. In turn, a signal is sent from the fault arc detection device 15b to the online, double-conversion UPS 1b causing the bypass switch 6 held open. In detail, the signal is fed into the override input 14 of the online, double-conversion UPS 1b. So, although a voltage drop is detected at the UPS output 5 by the control unit 7, current still flows over the rectifier 3 and the inverter 4 as this is depicted in FIG. 2 by the fault arc A and the associated current path drawn with bold lines.