Soil resistivity testing is the foundation of every grounding system design. The electrical resistance of the earth — measured in ohm-meters (Ω·m) — directly determines the resistance of the grounding electrode system, the ground potential rise (GPR) during fault events, and the step and touch voltages that personnel may be exposed to. Without accurate soil data, a grounding system cannot be designed to meet IEEE Std. 80, NEC Article 250, or NFPA 70E safety requirements.
E&S Grounding Solutions performs soil resistivity surveys using the Wenner four-pin array method per IEEE Std. 81-2025. Four equally spaced probes are driven into the earth in a straight line. AC current is injected between the outer probes and voltage is measured across the inner probes. Apparent resistivity is calculated as ρ = 2πA(V/I), where A is the probe spacing. By repeating measurements at increasing probe spacings, the survey captures resistivity at increasing depths — typically from 2 ft to three times the maximum diagonal dimension of the proposed grounding system.
Raw field data is processed into a layered-earth soil model using CDEGS RESAP software. The two-layer soil model — the minimum required for IEEE 80 grounding design — characterizes the earth as an upper layer with resistivity ρ1 and depth h, and a lower infinite layer with resistivity ρ2. This model is then used as input to the grounding system design calculations.
Soil resistivity varies by orders of magnitude across soil types — from less than 10 Ω·m in saturated clay to over 10,000 Ω·m in dry granite. A grounding system designed with assumed or borrowed soil data may fail to achieve the required ground resistance, resulting in unsafe step and touch voltages during fault events. Site-specific testing is not optional for substations, industrial facilities, or any installation governed by IEEE Std. 80.
Contact E&S Grounding Solutions at +1-310-318-7151 or request a consultation to discuss your soil resistivity testing requirements.