When a fault occurs,the flow of current to ground results in a voltage gradient on the ground surface in the vicinity of the grounding system. This voltage gradient may affect a person in two ways: through step, or foot-to-foot contact, and through hand-to-both-feet, or touch contact.
The resistance of the human body is highly variable, and the current that may pass through the body depends upon the resistance of the body and the position in which it is bridging the ground and the grounded structure. For calculation purposes, the resistance of the body may be taken as 1000Ω. The resistance of footwear, which is highly variable, is negligible. It will be observed that the resistance of the ground under the foot affects the magnitude of the current that may flow through the body. Assuming the ground under one foot is a circular plate electrode with a diameter of 8 cm, the resistance of such an electrode can be determined by:
while calculating the length of the conductor, touch potential from a grounded structure to the center of a rectangle of the grid mesh is used. This method is also known as mesh potential and is given by:
Touch potential: Touch potential is a potential difference between the fingers of a raised hand touching the faulted structure and the feet of the person standing on the Substations floor. The person should not get a shock even if the grounded structure is carrying a fault current i.e the touch potential should be very small.
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 two feet(one step) during the flow of earth fault current through the grounding system.
References
- https://circuitglobe.com/step-and-touch-potential.html
- https://electricalbaba.com/step-potential-and-touch-potential-must-read/
- https://incident-prevention.com/blog/understanding-step-and-touch-potential/