The paper presents a special case of ineffective protection with Arc Fault Detection Device. The purpose of this article was to investigate why an AFDD does not detect sparking in a circuit supplying a switch-mode power supply. Therefore, laboratory tests according to IEC 62,606 and tests under real operating conditions were performed for this purpose. The devices of different manufacturers producers were compared with each other and it was checked if a similar problem is repeated with each AFDD. The tests conclude that switching-mode power supplies as capacitive loads negatively influence the spark detection efficiency, the selection of an AFDD should always be preceded by an analysis of the minimum operating currents (2.5 A) and there are differences in the detection sensitivity between devices of various manufactures.