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In resonant grounded system when single-phase earth fault occurs, for arc suppression coil cannot compensate the active current of the zero-sequence grounding current passing through the fault point, the zero-sequence active current passing through the fault line is the sum of those passing through the non-faults. Therefore in theory the zero-sequence active current passing through the fault line is greater than others in absolute value. Energy function is the integrating of product of zero-sequence voltage with zero-sequence current. The fault line's energy function value is negative, while the non-fault's is positive. If three-phase earth capacitance is imbalanced, zero-sequence voltage produces, which will produce zero-sequence current passing through all the line. This zero-sequence current is the key variable that influences the phase-angle between non-fault zero-sequence current and the zero-sequence voltage. This paper analyzes the influences in different compensation conditions in theory. Simulation results are presented to demonstrate that the validity of earth fault line detection based on energy function will be worse and worse accompanied with the increasing of earth resistance when single-phase earth fault occurs on the imbalanced phase, and indicate that energy function has its validity scope.