Two ways the body can be exposed to voltage
Step voltage occurs between two points on the ground supporting the feet. Touch voltage occurs between parts a person can touch simultaneously, such as a metal structure and the ground where they stand. A fault current or lightning strike can create potential differences in nearby areas.
Related concept: Electrical risk.
Why the ground is not always equipotential
When current disperses through the ground, potential changes with position. Electrode geometry, soil resistivity and connections influence this distribution. Being near an energised point can therefore be hazardous even without directly touching the conductor responsible.
What determines the risk
Relevant factors include the voltage across the body, current path, duration and contact conditions. Protective design considers fault current and disconnection time alongside the ground and surfaces. Low earth resistance, considered alone, does not demonstrate that all accessible voltages are acceptable.
Design and verification
Earth grids, equipotential connections, surfaces and barriers are designed to control the anticipated scenarios. Measurements require suitable methods and competent personnel. Results must be related to the actual fault current and protection system. These are not measurements for a building or machine user to improvise.
Hypothetical example
A metal fence is added beside a transformer substation and connected to part of its installation. This modification may transfer potential to an area accessible from outside. Review must examine the new situation, even if the earth electrode has not moved and its resistance remains similar.
Responding to a suspected hazardous area
A fallen cable or damaged installation requires keeping people away and notifying emergency services and the responsible electrical specialist. Nobody should approach to touch, measure or attempt rescue without specialist control. This article explains the phenomenon; it does not provide universal safe distances or replace procedures for high-voltage installations or thunderstorms.
