In case of a fault arc event, the fault arc detection device 15b and/or the online, double-conversion UPS 1b can also output an alarm signal to inform personnel responsible for the operation of the electrical appliance 9b.

It should also be noted that even in case of an event that mistakenly is interpreted as a fault arc A, the online, double-conversion UPS 1b simply stays in normal mode and supplies the electrical consumers via the rectifier inverter combination. Possibly, a superfluous alarm signal is output. However, this is a negligible impact on system reliability in critical applications.

Concluding, the energy of a fault arc A depends on arc current level and the operation time of circuit protective devices 13a . . . 13h. The arc current in traditional electrical appliances 9a (FIG. 1) is much higher than in the proposed electrical appliances 9b (FIG. 2). In the electrical appliance 9a, the fault arc A is fed from power grid 10 via the bypass switch 6, whereas in the electrical appliance 9b, arc current is limited by the current limit of the inverter 4.