Step Voltage

When fault current flows into the earth through an electrode or grounding grid, it creates a voltage gradient on the ground surface that decreases with distance from the electrode. Step voltage is the potential difference between a person's feet, assumed to be one metre apart, standing on the ground surface. This voltage drives current up one leg, through the body, and down the other leg. While the body impedance for the leg-to-leg path is higher than the hand-to-feet path (touch voltage), step voltage can still cause involuntary muscle contractions that cause a person to fall, potentially converting the exposure to a more dangerous touch voltage scenario. IEEE 80 provides detailed calculation methods for substation grounding design, including the use of grounding grids, ground rods, and surface material (crushed rock) to control step voltage. The crushed rock layer increases the contact resistance between feet and soil, reducing the current flowing through the body for a given step voltage. Tolerable step voltage limits depend on fault clearing time and soil surface resistivity, with shorter clearing times allowing higher voltages.