About the Step & Touch Potential
Step and touch potential calculations verify that the voltage gradients on the soil surface within and around a solar substation earthing grid do not exceed safe thresholds during a fault — preventing electric shock to persons walking (step potential) or touching a grounded structure (touch potential).
Formula
E_step_limit = (1000 + 6 × ρ_s) × 0.116 / √t
- ρ_s
- Resistivity of the surface layer (Ω·m) — crushed rock, 3000–5000 Ω·m
- t
- Fault duration (seconds)
- E_step_limit
- Maximum tolerable step voltage for a 50 kg body (V)
E_touch_limit = (1000 + 1.5 × ρ_s) × 0.116 / √t
- E_touch_limit
- Maximum tolerable touch voltage (V)
How to calculate it
- 1Determine the fault duration t from the protection relay settings (typically 0.1–0.5 s).
- 2Measure or assume the surface layer resistivity: use crushed granite or washed gravel (3000–5000 Ω·m) for the surface layer within the switchyard.
- 3Calculate E_step_limit and E_touch_limit from the formulae above.
- 4Compute actual step and mesh (touch) voltages from the earthing grid simulation using IEEE 80 Annex B formulae or dedicated software.
- 5If actual voltages exceed the limits, mitigate by: reducing conductor spacing, adding a surface gravel layer, adding more perimeter conductors, or reducing fault current with a neutral-earthing resistor.
Worked example
A 33 kV substation with fault clearance 0.3 s and crushed rock surface (ρ_s = 3000 Ω·m): E_step_limit = (1000 + 6×3000) × 0.116 / √0.3 = 19000 × 0.212 = 4028 V. E_touch_limit = (1000 + 1.5×3000) × 0.116 / √0.3 = 5500 × 0.212 = 1166 V. The calculated mesh voltage from the grid analysis must be < 1166 V.
What you can calculate
- Touch voltage
- Step voltage
- IEEE 80
- Safety margin
Standards & references
Frequently asked questions
Why is crushed rock recommended as the surface layer inside a switchyard?
Crushed rock (granite or quartzite, washed, 20–40 mm graded) has a very high resistivity (3000–5000 Ω·m) compared to natural soil (50–300 Ω·m). When a fault occurs, this high-resistivity layer dramatically increases the tolerable step and touch potential limits (see the formulae above) — allowing a larger but safe ground rise voltage. It also inhibits weed growth and reduces maintenance. The minimum layer thickness per IEEE 80 is 100 mm; IS 3043 recommends 150 mm.
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