Question

A substation is to be built on soil with resistivity of 200 ohm-m. The substation is designed to have a grid of buried bare copper conductors, each 20 mm in diameter and spaced 3 meters apart, with a total area of 400 square meters. A person standing on the ground outside the grid will touch the grid with one hand and the other hand will touch a nearby conductive object, such as a fence. Calculate the step voltage and touch voltage that can be expected under worst-case conditions, assuming the fault current is 10,000 amperes. You may assume the following:

• The ground resistance of the substation is 0 ohms.
• The grid is buried at a depth of 0.5 meters.
• The person's feet are assumed to be at a distance of 1 meter apart.
• The resistance of the human body can be assumed to be 1,000 ohms.
• The maximum permissible touch voltage is 50 volts.
• Show your calculation steps and provide your answer in volts

Answer 1

The step voltage can be expected to be around 0.05 V, while the touch voltage can be expected to be around 20 V.

Step-by-step explanation:

In order to calculate the step voltage and touch voltage, we need to consider the current flowing through the person when they touch the grid and the nearby conductive object. For both the step voltage and touch voltage, we can use Ohm's law (V = IR), where V is the voltage, I is the current, and R is the resistance.

Step voltage:

The step voltage is the voltage difference between the person's feet when they touch the grid and the ground outside the grid. To calculate the step voltage, we need to find the current through the person.

Using Ohm's law, we can calculate the current through the person: I = V/R = 50 V / 1000 ohms = 0.05 A.

Next, we can calculate the step voltage using Ohm's law: V = IR = 0.05 A * 1 meter = 0.05 V.

Touch voltage:

The touch voltage is the voltage difference between the person's hand touching the grid and the hand touching the nearby conductive object. To calculate the touch voltage, we need to find the current through the person.

Using Ohm's law, we can calculate the current through the person: I = V/R = 50 V / 1000 ohms = 0.05 A.

Next, we can calculate the touch voltage using Ohm's law: V = IR = 0.05 A * 200 ohm-m * (400 sq meters / 0.5 meters) / 400 sq meters = 20 V.