Discussion of Measurement Methods of Impulse Grounding Impedance

The grounding device is very important for lightning protection. However, it is difficult to measure the impulse grounding impedance because the impulse generator is cumbersome and dangerous compared with the device for power frequency test. At the same time it is difficult to generate a lightning current with a suitable waveform due to the inductance of the long current lead. In this paper, two methods to measure the impulse grounding impedance were carried out at an outdoor test site. One was the traditional fallof-potential method, another was a method with a grounding ring surrounding the grounding device as the current return pole. The test results of both methods were presented and the characteristics of each method were discussed. It was found that for the method that uses a ring surrounding the grounding device as the current return pole, the measured impulse grounding impedance was always smaller than the results by fall-of-potential method. However, it could obtain the ground potential gradients near the grounding device.