The Lineman's and Cableman's Handbook Practice Test

Grounding effectiveness comes from the ability of the rod to access conductive earth, and moisture is the key factor that makes soil a good conductor. A ground rod should be driven deep enough to reach soil that remains moist year-round, because damp soil provides a much lower resistance path to earth than dry soil. If you stop at a shallow depth, especially in dry conditions, the resistance can stay high and the grounding system won’t perform reliably. The idea isn’t a fixed depth for every situation; it’s about reaching perpetually moist earth so the electrode has a dependable connection to the ground. The other depths listed don’t guarantee moisture in all soils and conditions, so they’re less reliable as a general rule.