The fault arc detection device beneficially comprises a current detector and/or a light detector. Accordingly, the trip signal used for circuit breakers can be caused by excessive current, excessive light or a combination of both. Using both the current and the light to classify an event as an arc fault reduces the risk of accidental tripping of the circuit breaker.

However, even if the signal from the fault arc detection device is erroneous and based on the misinterpretation of an event (i.e. the fault arc detection device outputs a fault arc signal although in fact there is no fault arc), the UPS simply stays in normal mode and supplies the electrical consumers via the rectifier inverter combination. Possibly, a superfluous alarm signal is output. However, this has just a very low impact to system reliability in critical applications.

By combining the fault arc detection device with an online, double-conversion uninterruptible power supply and a circuit protection, a fast and reliable possibility to significantly reduce the arc energy and associated risks and damages is provided. This is very beneficial for many electrical installations used in hospitals, data centers and other mission-critical applications due to improved safety and reduced downtime in case of a fault.