The invention provides a 
coal mine 
grid system insulation state monitoring method and belongs to an electrical power 
system feed line insulation state monitoring method. A 
feed line branch circuit is selected for doing single-phase 
longitude metal grounding experiment, zero sequence 
voltage of a 
bus and zero sequence current of a non-malfunction 
branch circuit are tested, zero sequence impedance of the non-malfunction 
branch circuit is obtained, namely, 
insulation resistance and ground 
capacitance, for obtaining insulation parameters of all circuits, another branch circuit is reselected to do a single-phase connection 
low resistance ground experiment or the next circuit with the non-branch-circuit number 1 is waited for generating single-phase ground malfunction, the experiment is repeated, insulation parameters of a first experiment malfunction circuit are obtained, and the insulation parameters of all 
feed line branch circuits are obtained by synthesizing results of the two experiments. The 
coal mine 
grid system insulation state monitoring method has the advantages that insulation degradation degree of the circuits is analyzed from state change of the 
bus zero sequence 
voltage and an 
arc suppression coil, the zero sequence 
voltage changes to a certain extent, switching-in situations of damping resistance change, and the degradation degree of circuit insulation is forecasted by monitoring change of transient states of a plurality of feature quantities.