Which grounding material requires the deepest depth among silt, clay, and course ground?

• A. Silt
• B. Clay
• C. Course ground
• D. Gravel

Answer: (C)

Grounding depth is driven by soil resistivity—the ability of the ground to carry electrical current. Soils that stay moist and contain ions, like clay and silt, generally provide a more conductive path and can reach the required earth resistance at shallower depths. Coarse ground, such as gravel or rocky material, tends to drain moisture quickly and has fewer continuous conductive paths, which raises resistivity. To achieve the same target resistance in such material, you must place the electrode deeper. So the material that requires the deepest depth is coarse ground.