Section 8/Criteria of Tolerable Voltages
IEEE 80 Section 15 covers tolerable step and touch voltage limits for grounding system design.
The grounding system must limit step voltage (voltage between a person's feet spaced 1 meter apart) and touch voltage (voltage between a person's hand on a grounded structure and their feet) to levels that will not cause ventricular fibrillation during a ground fault. The tolerable voltage limits depend on the fault clearing time, the surface layer resistivity (crushed rock), body weight (50 kg or 70 kg), and the foot contact resistance. For a 70 kg person with 0.5-second clearing time on wet native soil, the tolerable touch voltage is approximately 116 volts. A 4-inch layer of 3,000 ohm-m crushed rock increases the tolerable voltage significantly.
Why this section exists
During a ground fault at a substation, current flows through the grounding grid and into the earth, creating voltage gradients across the ground surface. A person standing near the fault location experiences step voltage (different voltages at each foot) or touch voltage (voltage between their hand and feet if they are touching a grounded structure). If these voltages exceed the body's tolerance, current flows through the person's heart and can cause fibrillation. The grounding grid must be designed to keep these voltages within safe limits.
What plan reviewers look for
Plan reviewers check the grounding study for step and touch voltage calculations at the worst-case locations (fence corners, equipment pads, gate openings). They verify the assumed fault clearing time matches the protection system design. They check the surface material specification (crushed rock depth and resistivity). They verify that the calculated voltages are below the tolerable limits.