The paper analyzes the amplitude-frequency spectra of zero-sequence current in the network with neutral grounding through Peterson-coil under transient earth faults. As the assumed equivalent circuit contains elements with linear current-voltage characteristic, harmonic components of the spectrum are calculated separately for discharge current of the faulted phase, charge current of the unfaulted phases and excess charge drain current followed by the arc blowout. The resulting current spectra are calculated given that current on the faulted line at all stages of transient earth fault flow in the same direction. However, on the unfaulted line drain current flows in the opposite direction to discharge and charge current. We established that in the spectrum of zero-sequence current under transient earth faults the highest amplitude refers to harmonics at frequency near to the frequency of excess charge drain current which, in turn, is near to commercial frequency. With respect to this frequency, in amplitude-frequency spectra we can distinguish two significant areas. At frequency lower than commercial frequency, harmonic amplitudes on the faulted line exceed the harmonic amplitudes on the unfaulted line. At frequency higher than commercial frequency on the contrary: harmonic amplitudes on the unfaulted line exceed harmonic amplitudes on the faulted line. Therefore, as a sign of the faulted line we suggest using harmonic level at frequency lower than commercial frequency.