Which type of soil conditions generally indicates high soil resistivity for a lightning ground electrode system?

Sandy or gravelly soil conditions indicate high soil resistivity due to their low moisture content and coarse particle size. These characteristics hinder the soil's ability to conduct electricity effectively. In sandy soils, the large air spaces between grains create a poor pathway for electrical current, while gravel, being larger and less cohesive, similarly lacks the density needed for effective grounding. This results in elevated resistance, making such soil less effective for lightning ground electrode systems to dissipate electrical energy safely into the ground.

In contrast, clay soils typically have higher moisture content and finer particles, which contribute to better conductivity. Loamy soils, being a mixture of sand, silt, and clay, generally offer good drainage and moderate resistivity. Silty soils can also retain moisture well and provide lower resistivity compared to sandy soils. Thus, sandy or gravelly soil stands out as the correct answer due to its inherent properties that lead to high soil resistivity in lightning protection applications.