Touch and step potential phenomenon

When fault current flows into the earth through an electrode, tower or substation grid, the soil near it rises in potential. The potential is highest at the electrode and falls with distance, so the ground surface has a voltage gradient. A person in this area can bridge part of this voltage.

Step potential

Step potential is the voltage difference between two points on the ground surface one step (about 1 m) apart, experienced between the feet of a person walking near the electrode. Current flows foot to foot (through legs), so it is less dangerous than touch, but a fall can turn it into a worse contact.

Touch potential

Touch potential is the voltage between an earthed metal structure that a person touches with the hand and the ground surface where the person stands (about 1 m away). Current flows hand to feet, through the heart, so it is more dangerous. The mesh potential (largest touch voltage in a grid mesh) and transferred potential are special cases.

 V (potential)
 |\
 | \   GPR at electrode
 |  \___
 |      \____
 |  Et       \______ Es
 +--|---|-----|-|------> distance
    hand feet  foot foot
   (touch)     (step, 1 m)
       structure at GPR

Equivalent circuits

 Touch:  structure--Rb--[feet in parallel: Rf/2]--earth
 Step:   foot--Rf--Rb--Rf--foot (feet in series)

With foot resistance $R_f \approx 3\rho_s$ and body resistance $R_b = 1000\ \Omega$, the tolerable limits (IEEE 80, 50 kg person) are:

$$E_{touch} = (R_b + 1.5\rho_s)\frac{0.116}{\sqrt{t_s}}, \qquad E_{step} = (R_b + 6\rho_s)\frac{0.116}{\sqrt{t_s}}$$

A surface layer of crushed rock (high $\rho_s$) and a closely spaced earth grid keep these potentials safe.