Detecting and extinguishing electric arcs ultra-fast within some milliseconds, a novel modular protective device provides efficient personal protection against the hazards of an electric fault arc particularly in case of direct exposure due to opened LV installations during live working or working in the vicinity of live parts. It is part of the electric power equipment or can be used as mobile unit connected to the electric system before starting work. The paper is reporting on laboratory tests for proving the protective system tripping safety and immunity against malfunctions. Main focus is directed to the optical detection unit of the protective system. Based on an optimized design of the optical detectors unit the modular protective device is prepared for use in the practical work. A couple of test series were directed to evaluate the system response in case of different disturbing optical sources. Among others the system immunity was analyzed when fuses are withdrawn in LV distribution centers with fuses or in-line fuse switch-disconnectors. The switching arcs occurring in such cases may not trip the protection system.