When a fault occurs, the fault current flow to the ground due to which the potential gradient near the electrical equipment developed. This potential gradient may affect a person in two ways, either by step-to-step contact or by touch contact.
Step-Potential – Step potential is the potential difference between the feet of a person standing on the floor of the substation, with 0.5 m spacing between the one step, during the flow of fault current through the ground system.
Touch-Potential – Touch potential is a potential difference between the fingers of a raising hand touching the faulted structure and the feet of the person standing on substation floor. The person should not get a shock even if the ground structure is carrying fault current, i.e.; the touch potential should be very small.
The resistance of the human body is highly variable, and the current that may pass through the body depends on the resistance of the body and the position in which it touches the ground. The resistance of the body is taken as 1000 ohms.