Soil resistivity is a critical input to step and touch voltage calculations under IEEE Standard 80. The tolerable step voltage and touch voltage limits for a person standing on the earth's surface depend directly on the surface layer resistivity.
IEEE 80 defines tolerable touch voltage as E_touch = (1000 + 1.5ρs) × 0.116 / √t, where ρs is the surface layer resistivity in ohm-meters and t is the fault clearing time in seconds. A crushed-rock surface layer with high resistivity (typically 2,500 to 10,000 Ω·m) dramatically increases the tolerable voltage limit, making it one of the most cost-effective safety measures in substation design.
Without a high-resistivity surface layer, the tolerable touch voltage for a 70 kg person on native soil (100 Ω·m) with a 0.5-second fault clearing time is approximately 157 V. With a 3,000 Ω·m crushed-rock layer, the tolerable touch voltage rises to approximately 1,000 V — a six-fold improvement in safety margin.
E&S Grounding Solutions designs grounding systems that meet IEEE 80 step and touch voltage limits for all fault clearing times and soil conditions. Contact us at +1-310-318-7151 or info@esgrounding.com.