Tolerable Step and Touch Voltage (IEEE 80)
The IEEE Std 80 tolerable step and touch voltages a grounding grid is checked against.
Example
You enter
- Fault clearing time ts (s) 0.5
- Surface (crushed-rock) resistivity rho_s (ohm-m) 3000
- Native soil resistivity rho (ohm-m) 100
- Surface layer thickness hs (m) 0.1
- Body weight 50
You get
- Surface derating factor Cs 0.700
- Tolerable step voltage 2,231 V
- Tolerable touch voltage (governs) 681 V
Checked against published worked examples from: IEEE Std 80 (tolerable step and touch voltage).
Details, formula, and sources
ScopeWhat meeting a grid-resistance target does not guarantee. The crushed-rock surface layer raises the limit via the Cs derating factor, and the limits scale inversely with the square root of the fault clearing time (a faster relay allows more). Touch governs over step.
Formula or tableCs = 1 - 0.09 (1 - rho/rho_s)/(2 hs + 0.09); E_step = (1000 + 6 Cs rho_s) k/sqrt(ts); E_touch = (1000 + 1.5 Cs rho_s) k/sqrt(ts) (k = 0.116 for 50 kg, 0.157 for 70 kg).
Source and editionThe IEEE Std 80 tolerable step and touch voltage limits with the surface-layer derating factor Cs, by name.
Free accessIEEE Std 80 is available through the IEEE; the tolerable-voltage relations are published safety criteria.
What governsEstimate. AHJ and licensed professional govern.
API field namesclearing_time_s, surface_resistivity, native_resistivity, layer_thickness_m, body_weight, cs, e_step_v, e_touch_v
- Surface factor Cs Cs = 1 - 0.09 (1 - rho/rho_s)/(2 hs + 0.09); a crushed-rock layer raises the tolerable voltageIEEE Std 80
- Body-current constant k = 0.116 (50 kg) or 0.157 (70 kg); limits scale as 1/sqrt(ts)IEEE Std 80