Neutral earthing resistors play a crucial role in electrical systems, ensuring the safety and stability of the network. These resistors limit the fault current that flows through the neutral point of a power system, protecting the equipment and personnel from potential damage and harm. If you’re looking to calculate the appropriate size of a neutral earthing resistor for your system, you’ve come to the right place. In this guide, we’ll walk you through the process step by step.

## Step 1: Determine the System Voltage

The first step in calculating the neutral earthing resistor is to determine the system voltage. This is the voltage at which your electrical system operates. It is typically expressed in volts (V) and can range from low voltages like 110V to high voltages like 33kV or even higher. Once you have determined the system voltage, make a note of it as it will be used in the subsequent steps.

## Step 2: Determine the Fault Current

The next step is to determine the fault current that could potentially flow through the neutral point of your system. The fault current is the current that occurs during a fault condition, such as a short circuit, and it can cause significant damage if not properly controlled. To calculate the fault current, you will need to consider factors such as the system impedance, the transformer rating, and the available short circuit current. It is recommended to consult with a qualified electrical engineer or refer to relevant standards and guidelines to accurately determine the fault current.

## Step 3: Determine the Desired Fault Current Limit

Once you have determined the fault current, the next step is to determine the desired fault current limit. The fault current limit is the maximum current that you want to allow to flow through the neutral point of your system. This limit is typically determined based on the equipment ratings and the safety requirements of your specific application. It is important to strike a balance between protecting the system and equipment from damage and ensuring the safety of personnel. Again, consulting with an electrical engineer or referring to relevant standards and guidelines can help you determine the appropriate fault current limit.

## Step 4: Calculate the Resistance

With the system voltage, fault current, and desired fault current limit in hand, you can now calculate the resistance of the neutral earthing resistor. The resistance can be calculated using Ohm’s Law, which states that resistance (R) is equal to voltage (V) divided by current (I). In this case, the voltage is the system voltage and the current is the difference between the fault current and the desired fault current limit. The formula to calculate the resistance is as follows:

R = V / (I – IL)

Where:

R = Resistance of the neutral earthing resistor (in ohms)

V = System voltage (in volts)

I = Fault current (in amperes)

IL = Desired fault current limit (in amperes)

By plugging in the values for the system voltage, fault current, and desired fault current limit into the formula, you can calculate the resistance of the neutral earthing resistor.

## Step 5: Select the Appropriate Resistor

Once you have calculated the resistance, the final step is to select the appropriate neutral earthing resistor. The resistor should have a resistance value that is equal to or as close as possible to the calculated resistance. It is important to choose a resistor that is capable of handling the power dissipation and temperature rise associated with the fault current. Additionally, the physical size, mounting options, and environmental considerations should also be taken into account when selecting the resistor.

It is worth noting that the calculation of the neutral earthing resistor is a complex process that requires a thorough understanding of electrical systems and safety requirements. It is always recommended to consult with a qualified electrical engineer or seek professional advice to ensure the accuracy and reliability of the calculations.

By following these steps and consulting with experts, you can calculate the appropriate size of a neutral earthing resistor for your electrical system. Remember, the safety and stability of your system depend on the proper implementation of neutral earthing resistors, so it’s essential to get it right.