Step and touch potentials, shock and safety

Step and touch potentials

When fault current $I_g$ flows into the earth through an electrode or substation grid, the soil around it rises in potential, with a gradient on the surface.

• Step potential: the voltage between the two feet of a person standing on the ground, about 1 m apart, without touching any earthed object.
• Touch potential: the voltage between a person's hand touching an earthed structure and their feet on the ground, at about 1 m horizontal distance.

  GPR  ___  structure
      |   |  hand
      |   |   \o    touch: hand-to-feet
  ____|___|___/|\___ ground
              / \      step: foot-to-foot
       potential falls with distance

Why a person gets an electric shock

A shock occurs when the body becomes part of a closed circuit and a current passes through it. This happens when:

1. A person touches a live conductor and earth at the same time, or two conductors at different potential.
2. Insulation fails and a metal frame becomes live, and the frame is not properly earthed or protection does not trip.
3. During an earth fault, the ground near an electrode rises in potential and a person bridges a step or touch voltage.
4. Induced voltage on de-energised lines near live lines, or stored charge in capacitors and cables.

The current is $I = V/(R_b + R_{contact})$; human body resistance is about $1000\ \Omega$. Above about 10 mA the person cannot let go, and around 50–100 mA ventricular fibrillation can occur.

Safety precautions

1. Proper equipment and system earthing; low-resistance substation earth mat with equipotential bonding.
2. Surface layer of crushed rock (high resistivity) in switchyards to cut step and touch current.
3. Fast protective relays, fuses and RCDs (30 mA) to cut the fault quickly.
4. Isolate, lock-out, tag-out, test for dead and earth the circuit before work.
5. Use insulated tools, rubber gloves, boots, mats and helmets of proper rating.
6. Keep safe clearance; use fences, barriers and danger signs.
7. Only trained and authorised persons, with work permits for HV work.
8. Regular inspection of insulation and earthing; avoid wet conditions.

Safety regulations

• Safe step and touch voltage limits as per IEEE Std 80:

$$E_{step} = (1000 + 6C_s\rho_s)\frac{0.116}{\sqrt{t_s}},\qquad E_{touch} = (1000 + 1.5C_s\rho_s)\frac{0.116}{\sqrt{t_s}}$$

• Wiring and earthing codes (IEC 60364, IS 3043, National Electrical Code).
• In Nepal: the Electricity Act 2049 and Electricity Regulations 2050 set rules for safety, clearances and licensing, and NEA follows its own safety manual and earthing standards.