Non-isolated flexible interconnected distribution networks as the research object are explored, focusing on the adaptability issue of typical grounding fault detection methods when single-phase grounding faults occur in AC systems due to zero-sequence transfer characteristics. Firstly, The zero-sequence equivalent topology network for non-isolated soft open point (SOP) is established, with transmission equations for zero-sequence components proposed to quantitatively analyzed their magnitude when transmitted to the non-fault side. The suppression effects of zero-sequence components are evaluated in various scenarios. Then, an adaptability analysis of typical protections is conducted for interconnected systems with low-current, low-resistance connections and hybrid low-current/low-resistance grounding methods. Results show that in non-isolated flexible interconnected systems, zero-sequence components transmission to the healthy side may cause protection maloperation or false alarms. Finally, a typical non-isolated flexible interconnected distribution network model is established in PSCAD to verify the adaptability analysis of protections in different interconnected systems, and the theoretical analysis is confirmed to be correct. The findings provide a theoretical basis for protection configuration in non-isolated flexible interconnected distribution networks.