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The transient characteristics of groundings are decided by the impulse breakdown characteristics of soil, thus, the influence of soil ionization neighboring the grounding electrodes should be considered in the accurate lightning protection analysis. The breakdown delay is an important parameter of soil impulse characteristics, which are closely related to the soil properties. In this paper, the impulse breakdown delay of soils under varied temperatures and moistures were tested and the impacts of the amplitude and polarity of the applied impulse, the temperature and moisture of the tested soil samples on the breakdown delay were analyzed. A concept of critical breakdown delay of soil was proposed. From our tests, the soil breakdown under impulse is possibly an electrical but not a thermal process. According to the electrical breakdown mechanism of soil and the impulse breakdown mechanism of gas, the main conclusions were explained.