What is Earth Resistance? How to measure it?

Electrical systems can fault at any time due to equipment failure, power surges caused by lightning strikes, or variations in the power grid. These faults can be hazardous for individuals handling electrical equipment, as they become an instantaneous pathway for current to flow. It’s here that the concept of proper earthing becomes crucial in preventing the fault current from entering the body or any metallic object. This blog provides a comprehensive guide on earth resistance measurement. When an earth electrode is prepared for a structure, the subsequent step involves measuring the earth resistance offered by it.

How is Earth Resistance measured?

The process begins with earthing connections made by burying earth electrodes at several places near the structure needing to be earthed. These earth electrodes are typically made from materials like copper, steel, or galvanized iron. The type of electrode selected depends on the requirements of the electrical system and the soil resistivity of the location.

Various factors affect the earth resistance, like the composition of the soil, temperature, moisture content and depth of electrode. Earthing enables leakage current to flow away safely and is linked to an automatic cut off device (which ensures power supply).

What are the methods of measuring Earth Resistance?

There are different earth resistance measurement methods used depending on the type of neutral system, the type of installation (like residential, industrial, urban environment, or rural environment). Four variables affect the earth resistance of a grounding system which includes:

1. The composition of the soil

2. The moisture content of the soil

3. The temperature of the soil

4. The depth of the electrode

As a result, earth resistance can vary anywhere from a single ohm to thousands of ohms. Soil identification, earthing, and intensive field measurements show that soil resistivity values depend upon the soil type. For instance, in rocky areas with high resistivity, resistance can be lowered using a network of well-designed earth mats.

Before starting any earth resistance measurements, the maximum value for correct earthing needs to be measured. There are six basic test methods to measure the earth resistance:

1. Four-point method (Wenner method)

2. Three terminal methods (falloff potential method/ 68.1% method)

3. Two-point method (dead earth method)

4. Clamp-on test method

5.  Slope method

6.  Star delta method

We will discuss the three-terminal method, the four-point method, the slope method, and the star-delta method in detail.

 

 

A) Three-Terminal Method (Fall of Potential Method)

This method is the most commonly used for measuring earth resistance and is based on the IEEE 81-2012 standard. It’s suitable for transmission line structures and calculates earth resistance using Ohm’s law. The fall of potential method is reliable and especially suitable for large grounding systems.

 

                        Fig: Fall of potential method

Source - Electrical Engineering Portal

 

B) Four-Point Method (Wenner Method)

Similar to the fall of potential method, the Wenner method utilizes four electrodes driven into the ground along a straight line at equal intervals. The current flowing into the earth produces an electric field proportional to its density and the resistivity of the soil, providing an efficient measure of earth resistance.

C) Slope Method

The slope method is often employed for measuring earth resistance of large and complex earthing systems such as power stations. This method includes taking a number of resistance measurements at various earth systems to the voltage electrode separations, allowing for the calculation of the optimal resistance.

D) Star-Delta Method

The star-delta method is advantageous in areas with large systems or in rocky terrains where it’s challenging to place test electrodes. This method utilizes three test electrodes at the corners of an equilateral triangle with the earthing system at the center.

Once the earth resistance measurement is complete and found to match the tolerance level, the earthing system is deemed safe. However, regular inspections are needed to ensure the efficiency of the earthing system. All ground and ground connections should be checked at least annually as a part of a predictive maintenance plan.

 

How to Reduce Earth Resistance?

Finally, let’s look at a few strategies you can employ to reduce earth resistance:

1. Maintain the moisture content of the earth: Regularly water the earthing point to maintain soil resistivity.
2. Increase the contact area of the electrode: You can maintain earth resistance by increasing the thickness or surface area of the rod used for earthing.
3. Increase the depth of the electrode: Earth resistance can be reduced by placing the earth electrode at a greater depth.
4. Use of an earthing compound: The application of compounds like salt, coal, marconite, and bentonite can help maintain soil resistivity.

 

We hope this blog post has provided you with a solid understanding of earth resistance measurement. The proper management of earth resistance is a critical aspect of maintaining safety within electrical systems. Stay tuned for more content around earthing and lightning protection.

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