Abstract: To address the dilemma of whether to prioritize ensuring power supply reliability or preventing large-scale forest fires during tree-contact fault in 10 kV overhead lines, the fault must be divided into risk intervals. By analyzing the fire mechanism during tree-contact fault, it is concluded that the range of open flame distribution is positively correlated with the third harmonic of the fault’s grounding current. Using the spread of open flames reaching 50% as the critical marker for the high-risk stage, a method for dividing risk zones based on harmonic factors is proposed. Field measurements indicate that the method has an average error of approximately 42.4 seconds in estimating the starting time of the high-risk interval. This method provides essential theoretical and methodological support for determining responsibility in forest fires.

Key words: high-impedance fault, harmonic factor, tree-contact fault, forest fires, power supply reliability