How to Measure Ground Resistance? – Testing Earth Resistance

Electrical Technology

How to Measure the Earth – Ground Resistance using Different Methods?

Grounding, also known as earthing, is a safety practice and technical technique that involves connecting electrical systems and equipment to the Earth’s conductive surface. The primary purpose of grounding is to provide a path for electric current to flow safely into the ground, preventing the buildup of excess voltage and reducing the risk of electrical shock, fires, and equipment damage. For proper protection, it is necessary to maintain, test, and measure the ground resistance in accordance with local area codes to ensure the system’s integrity and stability.

Grounding also helps stabilize voltage levels, enhance equipment performance, and protect against lightning strikes by providing a low-resistance path for electrical energy to dissipate into the ground.

Good to Know:

According to NEC 250.56, the maximum grounding resistance is 25 ohms, and 50 ohms for sensitive applications. An additional ground rod/earth electrode needs to be installed if the ground resistance exceeds 50 ohms.
According to IEC/BS EN 62561-2:2012, good earth resistance is 5 – 10 ohms.
Recommended grounding resistance per IEEE and NFPA standards is typically < 5 ohms, also applicable to telecommunications.

In the following tutorial, we will explain how to measure, check, and test ground / earth resistance using different methods, including a multimeter, megger, and digital earth/ground resistance testers such as Fluke 1625-2 geo earth ground tester.

Related Post: How to Test the Earth Fault Loop Impedance – Various Methods

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Different Methods of Testing Ground Resistance

Measuring ground resistance is important for ensuring the safety and effectiveness of electrical grounding systems. Ground resistance is the resistance encountered by an electric current flowing through the ground. It is typically measured in ohms. Here are several methods commonly used to measure ground resistance:

Two-Point Method

The Two-Point Method is basic technique for measuring ground resistance. It is a simplified method that involves using two electrodes: one for injecting current into the ground, and the other for measuring the voltage drop. Here’s how the Two-Point Method works:

Setup:
- Drive two electrodes into the ground at a certain distance apart. The distance between the electrodes depends on the specific requirements and standards, but it is typically between 20 and 50 feet.
- Label one electrode as the “current” electrode (CE), and the other as the “potential” electrode (PE).
Injecting Current:
- Connect a known current source, such as a ground tester, to the current electrode (CE). This source will inject a known amount of current into the ground.
Measuring Voltage Drop:
- Connect a voltmeter to the potential electrode (PE) to measure the voltage drop across the ground.
- Ensure that the voltmeter is set to the appropriate range and is properly calibrated.
Calculate Resistance:
- Use Ohm’s law to calculate the ground resistance. The formula is: $R = V \div I$ where $R$ is the ground resistance, $V$ is the voltage drop, and $I$ is the injected current.
Considerations:
- The Two-Point Method is a simple and quick way to estimate ground resistance, but it may not provide as accurate results as more sophisticated methods like the Fall-of-Potential Method 3-Wire or the Wenner 4-Wire Method.
- The accuracy of the measurement can be influenced by the soil type and other environmental factors. It is essential to follow relevant standards and guidelines for the specific application.
Interpretation:
- Compare the calculated ground resistance with the acceptable limits specified by relevant standards or regulations. If the measured resistance is within the acceptable range, the grounding system is considered effective. If not, further investigation and improvement of the grounding system may be necessary.

While the Two-Point Method is simpler than some other techniques, it can be suitable for quick assessments or in situations where more advanced methods may not be practical. If precise measurements are crucial, consider using more accurate methods such as the Fall-of-Potential Method (3-Wire) or the Wenner (4-Wire) Method.

Related Post: How to Test and Measure the Ground Resistance using a Megger?

Fall-of-Potential Method (Three-Point Method):

The Fall-of-Potential Method, also known as the three-point, is a widely used technique for measuring ground resistance. This method involves using a ground tester, which usually consists of three electrodes: a current electrode, a potential (voltage) electrode, and a ground electrode.

The current electrode injects a known current into the ground, and the potential electrode measures the voltage drop across the ground. By dividing the voltage drop by the injected current, you can calculate the ground resistance using Ohm’s Law (R = V/I).

The key steps of the Fall-of-Potential Method are as follows:

Setup:
- Disconnect the earth electrode / ground conductor from the ground bar in the main supply panel.
- Drive a current electrode into the ground at the location where you want to measure the ground resistance.
- Place a potential electrode at a certain distance from the current electrode, forming a straight line. The distance between the electrodes (normally 65 ft (20 m)) is a critical factor in the accuracy of the measurement.
- Label the three electrodes as follows:
  - Current Electrode (CE): This is the electrode through which a known current is injected into the ground.
  - Potential Electrode 1 (PE1) and Potential Electrode 2 (PE2): These are the electrodes used to measure the voltage drop across the ground.
- Connect a ground tester instrument, such as a digital earth / ground resistance tester, to the current and potential electrodes.
Injecting Current:
- The ground tester injects a known current into the ground through the current electrode. The injected current flows through the ground and returns to the ground tester through the soil.
Measuring Voltage Drop:
- Measure the voltage drop between the potential electrode and the ground at the specified distance. This voltage drop is proportional to the ground – earth resistance. Alternatively, this can be easily done using a digital ground – earth resistance tester.
Calculate Earth Resistance:
- Use the voltage drop and the injected current to calculate the ground resistance using the Ohm’s law formula: $R = V / I$ where $R$ is the ground – earth resistance, $V$ is the voltage drop, and $I$ is the injected current. Alternatively, the digital earth resistance tester will show on screen the exact ground resistance in ohms.
Interpretation:
- Compare the calculated ground – earth resistance with the acceptable limits specified by relevant standards or regulations. If the measured resistance is within the acceptable range, the grounding system is considered effective. If not, further investigation and improvement of the grounding system may be necessary.
Considerations:
- The Three-Point Method is more accurate than the Two-Point Method because it considers the geometry of the electrodes and minimizes the effects of surface layers on the measurement.
- The accuracy of the measurement can still be influenced by soil conditions, and it’s important to follow relevant standards and guidelines.

Placing the Stakes

For the highest degree of accuracy, refer to the following table, which illustrates the recommended depth and distance between the ground rod and stakes to measure ground resistance using the 3-point Fall-of-Potential Method.

Depth of the Ground Electrode	Distance to the 1^st Stake	Distance to the 2^nd Stake
2 m (6.6 ft.)	15 m (49.2 ft.)	25 m (82 ft.)
3 m (9.8 ft.)	20 m (65.6 ft.)	30 m (98.4 ft.)
6 m (19.6 ft.)	25 m (82 ft.)	40 m (131.2 ft.)
10 m (32.8 ft.)	30 m (98.4 ft.)	50 m (164 ft.)

The Three-Point Method (Fall-of-Potential Method) is a good compromise between simplicity and accuracy and is often used in practice for ground resistance measurements. However, for more critical applications or when higher precision is required, methods such as the Wenner 4-Wire Method may be preferred.

Related Post: How to Test Grounding / Earthing System using a Multimeter?

Wenner Method – Four Point Method

The Wenner Method (named after Dr. Frank Wenner of the US Bureau of Standards in 1915), also known as the Four-Point method, is a widely used technique for measuring ground resistance which is similar to the fall-of-potential method but uses four electrodes arranged in a straight line.

In this testing method, two outer electrodes are used to inject current, and the two inner electrodes measure the voltage drop. It provides more accurate results compared to the Two-Point and Three-Point Methods by using a specific electrode configuration. Here’s how the Four-Point Method works:

Setup:
- Drive four electrodes into the ground in a straight line, spaced at equal intervals. The distance between each electrode should be consistent and is typically denoted as “a“. The width (diameter and depth) of the electrode’s holes should be uniform and should be less than or equal to 10% of the spacing between the buried electrodes.
- Label the electrodes as follows:
  - Current Electrode (CE): This is the electrode through which a known current is injected into the ground.
  - Potential Electrode (P1): This electrode measures the voltage drop at a distance “a” from the current electrode.
  - Potential Electrode (P2): This electrode measures the voltage drop at a distance “2a” from the current electrode.
  - Potential Electrode (P3): This electrode measures the voltage drop at a distance “3a” from the current electrode.
Injecting Current:
- Connect a known current source, such as a ground tester, to the current electrode (CE). This source will inject a known amount of current into the ground. The digital tester like Megger or Fluke 1625-2 GEO Earth Ground Tester has it as a bullet-in feature.
Measuring Voltage Drop:
- Connect a voltmeter to each potential electrode (P1, P2, and P3) to measure the voltage drops at their respective distances.
Calculate Ground Resistance:
- Use the measured voltage drops and the injected current to calculate the ground – earth resistance using the formula: $R = ρ / 2πa (V_{P 2}$ $- V_{P 1})$ where $R$ is the ground resistance, $ρ$ (rho) is the resistivity of the soil, $a$ is the distance between electrodes, $V_{P 2}$ is the voltage drop at P2, and $V_{P 1}$ is the voltage drop at P1.
Interpretation:
- Compare the calculated ground resistance with the acceptable limits specified by relevant standards or regulations. If the measured resistance is within the acceptable range, the grounding system is considered effective. If not, further investigation and improvement of the grounding system may be necessary.
Considerations:
- The Four-Point Method is more accurate than the Three-Point Method and is widely used in practice.
- Ensure that the distance between electrodes (a) is appropriate for the soil conditions and the desired measurement depth.
- The advantage of this method is that it minimizes the influence of surface layers on the ground – earth resistance measurement.

Good to Know: You may use the Megger or Fluke digital earth tester to calculate ground resistance and soil resistivity via the 4-point test (Wenner Method). We have a detailed post on it with a solved example. They have built-in features to automatically inject current, calculate the potential, and then apply Ohm’s law to determine ground resistance. This way, you can easily calculate soil resistivity by using the formula for the specific resistance of the earth as follows.

ρ_E = 2π.a.R_E

The Four-Point Method provides a reliable and accurate measurement of ground resistance and is suitable for a wide range of applications. It minimizes the influence of surface layers and is less affected by soil inhomogeneities compared to simpler methods.

Related Post: How to Measure Resistivity of Earth Using Wenner Method?

Stakeless Ground Resistance Measurement (Clamp-On Method)

Stakeless ground resistance testing is a method that allows you to measure ground – earth resistance without disconnecting the grounding system. This technique is particularly useful when working with existing installations or in situations where disconnecting the grounding system is impractical.

This method is also known as Clamp-On Method using Clamp-On Ground Resistance Tester.
This method is quicker and easier to use as it doesn’t require driving additional electrodes into the ground.
A clamp-on ground resistance tester measures the magnetic field produced by the current flowing through the grounding system.
The tester calculates ground – earth resistance based on the relationship between the magnetic field and the current.

Here’s an overview of the stakeless ground resistance testing method:

Equipment Required:

Stakeless Ground Resistance Tester:
- Specialized instruments, often referred to as clamp-on ground resistance testers or geo earth ground tester, are used for stakeless testing. These devices typically consist of a clamp that goes around the grounding electrode or conductor, eliminating the need for driving auxiliary stakes into the ground.

Procedure:

Select the Testing Location:
- Identify the point in the grounding system where you want to measure the ground resistance. This could be the connection point of a ground rod, a grounding conductor, or any other part of the grounding system.
Prepare the Ground Resistance Tester:
- Turn on the stakeless ground – earth resistance tester and set it to the appropriate measurement mode.
Position the Clamp:
- Open the clamp of the tester and position it around the grounding conductor or electrode. Ensure that the clamp is securely closed to get an accurate measurement.
Perform the Measurement:
- Activate the tester to initiate the measurement. The instrument will induce a test current into the grounding system and measure the resulting voltage drop.
Record the Results:
- The tester will provide a reading indicating the ground resistance. Record this value for analysis.
Interpret the Results:
- Compare the measured ground resistance with any applicable standards or guidelines. Ensure that the resistance value is within acceptable limits for the specific application.
Repeat if Necessary:
- If you are testing multiple points in the grounding system, repeat the process at each location.

Considerations:

Soil Conditions: Soil resistivity can affect the accuracy of stakeless measurements. It’s essential to be aware of the soil characteristics at the testing location.
Interference: Be cautious of nearby conductive objects that may interfere with the measurements. This can include other grounding conductors or metallic structures.
Instrument Calibration: Ensure that the stakeless ground resistance tester is properly calibrated according to the manufacturer’s instructions.

Stakeless ground resistance testing provides a convenient way to assess the integrity of a grounding system without the need for extensive setup or disconnecting components. However, it’s important to be aware of the limitations and considerations associated with this method. Always follow safety procedures and adhere to relevant standards when performing ground resistance testing.

Related Post: How to Test Earthing – Grounding System using a Light Bulb?

Selective Grounding Method:

Selective earth ground testing is a method used to measure the resistance of specific components, (such as grounding electrodes or connections.) within an earthing (grounding) system using one clamp and two stakes. Rather than testing the entire grounding system, this approach allows for the targeted assessment of individual grounding elements. It involves disconnecting certain components and measuring the resistance of the remaining grounded system.

This can be useful in identifying and addressing potential issues with specific components of the grounding system, such as grounding electrodes or connections. Here’s an overview of the selective earth ground testing process:

Equipment Required:

Earth Ground Tester:
- A specialized ground resistance tester (Earth Ground Clamp) or multimeter capable of measuring ground resistance.
Test Leads:
- Test leads to connect the earth ground tester to the specific grounding element being tested.

Procedure:

Identify the Grounding Elements:
- Identify the specific grounding elements within the system that you want to test. This could include ground rods, grounding conductors, bonding connections, or any other components connected to the earth.
Disconnect Components (if necessary):
- Depending on the testing objectives, you may need to disconnect specific components from the overall grounding system for individual testing.
Connect the Earth Ground Tester:
- Connect the earth ground tester to the component you are testing using the appropriate test leads. Ensure secure and proper connections.
Perform the Measurement:
- Activate the tester to initiate the measurement. The instrument will induce a test current and measure the resulting voltage drop, allowing it to calculate the ground – earth resistance of the specific component.
Record the Results:
- Record the earth resistance value obtained from the measurement.
Interpret the Results:
- Compare the measured ground resistance with any applicable standards or guidelines specific to the type of grounding element being tested.
Repeat for Other Components:
- If testing multiple components, repeat the process for each one.
Address Issues:
- If the measured ground or earth resistance is higher than acceptable limits, investigate and address potential issues with the specific grounding component. This may involve improving connections, adding additional grounding elements, or addressing soil conditions.

Selective earth ground testing provides a targeted approach to assess and maintain specific components within a grounding system. Regular testing and maintenance help ensure the overall effectiveness of the grounding system, contributing to electrical safety and equipment performance.

Related Post: How to Size the Earth Conductor, Earthing Lead & Earth Electrodes?

Soil Resistivity Measurement:

- Before measuring ground resistance, it’s important to measure the resistivity of the soil.
- The Wenner method (4-Wire) for soil resistivity measurement technique can be used to understand the characteristics of the soil that influence earth resistance. We have published a separate and detailed step by step tutorial on it.

Guidelines:

Safety Precautions: Always follow safety procedures when working with electricity and grounding systems.
Instrument Calibration: Ensure that the earth ground tester is properly calibrated according to the manufacturer’s instructions.
Soil Conditions: Soil resistivity can impact ground resistance measurements, so consider the soil characteristics at the testing location.
Potential Hazards: Be aware of any potential hazards in the area where you are conducting the measurements.
Maintenance: It is advisable to conduct regular testing and maintenance to ensure proper earthing and ground / earth resistance in accordance with the guidelines.
Codes: It’s also crucial to comply with relevant standards and regulations in your region (NEC/IEC/NFPA).
Licensed Electrician: If you’re not familiar with these measurement methods, it may be advisable to consult with a qualified electrician or engineer.
Use of this Information: The author will not be liable for any losses, injuries, or damages resulting from the display or use of this information or any attempt to implement a circuit in the incorrect format. Therefore, exercise caution, as working with electricity poses inherent risks.