Insulation monitoring based on zero-sequence voltage amplitude is the basic guarantee for grounding fault identification of distribution network. Neutral grounding method and the mode of arc-suppression coil adjustment will affect the sensitivity under high-resistance grounding fault. This paper analyzes the variation law of zero-sequence voltage with transition resistance and capacitance current of system under different grounding modes. The maximum transition resistance that can be monitored under different grounding modes is determined (i.e. the insulation monitoring sensitivity). The results show that the sensitivity of the ungrounded system is generally above 2 000 Ω. The sensitivity of arc-suppression coil grounded system with pre-adjustment mode is above 2 500 Ω. The sensitivity of the arc-suppression coil grounded system with instantaneous adjustment mode is low, which is generally between 230 Ω and 2 000 Ω. The small resistance grounding system and the arc-suppression coil grounded system, after the input of small resistor, have the lowest sensitivity, which are only tens of ohms. Simulation and field data verify the correctness of the analysis. This work is supported by National Natural Science Foundation of China (No. 51477184) and Science and Technology Project of Jiangsu Frontier Electric Technology Co., Ltd. (No. KJXM-0164) “Small Current High-resistance Grounding Fault Detection Technology based on Low-frequency Component”.