The measurement of grounding system impedance of an energized substation is usually required. Consequently, the power system frequency interference and harmonic frequencies may adversely affect the measurement. In order to improve the signal-to-interference ratio, we can increase the intensity of the test current or the test current frequency should be slightly differ-ent from the power system frequency. In the latter case, so called Frequency Shift Method (FSM), the intensity of test current may be relatively low. In this paper, we empirically examine the suffi-cient intensity of the test current. The corresponding field test in the 220/110/10 kV substation is described