Abstract: The installed selective grounding protection equipment for underground high-voltage networks could not work properly owing to the application of the arc-suppression coil, by which the grounding current is compensated. On the basis of analyzing the ground faults, this paper expounded the protection principle based on the fifth harmonic detection, which is suitable for all kinds of underground high-voltage distribution networks. As the application example, a kind of selective single-phase grounding protection system for underground high-voltage distribution networks was introduced, and the result of the experiment was shown.