Ground rods should be long enough to reach which moisture level of the soil?

The correct choice is that ground rods should be long enough to reach the permanent moisture level of the soil. This is because effective grounding relies on establishing a stable and low-resistance connection to the earth. The permanent moisture level is the point in the soil where moisture is consistently available, ensuring that the electrical connection remains effective over time, regardless of seasonal changes or temporary weather conditions.

Ground rods that do not reach this permanent moisture level may not provide sufficient conductivity, leading to unreliable grounding. This could potentially compromise safety and system operation in the event of a fault or surge. While surface moisture or seasonal variations can affect grounding at times, the permanent moisture level is the depth that consistently provides reliable grounding year-round.