When a fault occurs in an electrical system, a flow of fault current to the ground is created, resulting in a potential gradient near the electrical equipment. This potential gradient can pose a hazard to people in the vicinity, either through step-to-step contact or touch contact.
What is Step Potential?
Step Potential refers to the potential difference between the feet of a person standing on the floor of a substation, with 0.5 m spacing between each step, during the flow of fault current through the ground system. If the potential difference between the person's feet is too high, it can cause an electric shock.
What is touch Potential?
Touch Potential refers to the potential difference between the fingers of a person touching a faulted structure and their feet standing on the substation floor. Even if the ground structure is carrying fault current, the touch potential should be very small to prevent the person from getting shocked. The amount of current that may pass through the body depends on the resistance of the body, which is highly variable, and the position in which the person touches the ground.
The resistance of the human body is typically assumed to be 1000 ohms for safety calculations.
It is important to minimize step and touch potentials to ensure the safety of people near electrical equipment during a fault. Proper design of the grounding system and selection of appropriate electrical equipment and materials can help minimize these hazards.
What is Potential Gradient?
When a fault occurs in an electrical power system, it can result in the flow of current to the ground. This flow of current creates a potential difference, or voltage, between different points on the ground around the fault. This difference in voltage is called the "potential gradient".
The potential gradient is important to understand because it affects the safety of people who may come into contact with the electrical structure or ground near the fault. The potential gradient is the rate of change of electric potential (or voltage) with respect to the distance from the base of the electrical structure.
The resistance of the earth electrode (or the conductor that connects the electrical structure to the earth) is not concentrated at one point, but is distributed over the soil around the electrode. This means that the potential difference due to the fault current is also distributed over the soil, creating a potential gradient.
Difference between Touch Potential and Step Potential:
| Feature | Touch Potential | Step Potential |
|---|---|---|
| Definition | The voltage caused due to physical contact with the electrical element | The voltage caused due to the ground between the person's feet |
| Occurrence | Occurs due to faults | Occurs due to faults |
| Protection | Can be protected by a single kit | Can be protected by a single kit |
| Supply | Does not matter | Does not matter |
| Occurrence in power lines | Occurs in crowded power lines | Occurs in crowded power lines |
| Example | Potential at the touching point due to a fault is 10 kilo-volt | Potential difference between the person's legs during a fault |
| Result | Can lead to electric shock | Can lead to electric shock |