Study on Surge Impedance of Grounding Electrode

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Surge impedances of grounding electrodes were studied in numerical calculations and in field measurements.(1) In the numerical calculations, the magnetic field induced by the ground current (current which was injected from the grounding electrode into the ground) was taken into account. It was found that this magnetic field considerably increased the inductance of the vertical electrode, but did not influence the inductance of the horizontal electrode.(2) In the field measurements, surge impedances of actual vertical linear electrodes were measured by the use of a surge-impedance tester (current of 0.4 A, building-up time of 1 <micro> sec). It was found that the grounding impedance of the vertical electrode did not much decrease even if the electrde was much lengthened, while the grounding resistance much decreased then. Namely there was an effective length of the electrode from the viewpoint of grounding impedance. For an example, the effective length of the vertical electrode was about 20 m, when the resistivity of the soil was 158-190 <OHM> m. As to the effective length, there is an effective length also for a horizontal electrode, and the effective length for each electrode's type is defferent when the resistivity of the soil is different.(3) In comparison between the calculated impedances and the measured ones, when the electrode was short (about 2 m), the calculated impedance nearly agreed with the measured one, but, when the electrode was long (10-200 m), the calculated one was much greater than the measured one. The reason for the error was sought and seemed that the effect of the attenuation in the propagation was not taken into account in the calculation. Thus a problem in the future is that an improved method of calculation needs to be proposed, where the attenuation is taken into account.