Earth Ground Tester: Ground Resistance Testing Explained

This page covers the basics of Earth ground testers and ground resistance testing. It also lists manufacturers of earth ground resistance testers used for ground resistance tests.

In broad terms, “earth resistance” is defined as the resistance of the soil in the path of electric current flow. Generally, Earth is a relatively poor conductor of electricity compared to typical conductors like copper wire.

However, the resistance can be very low when the area through which the current flows is large. In this condition, Earth is considered a good conductor.

Knowing earth resistance is important because it helps in finding the ideal location to ground equipment, protecting human beings. It also helps determine what lies a short distance below the earth’s surface.

Earth Ground Resistance Testing Using a Ground Tester

Earth ground resistance testing helps find the best earth location and depth for fixing low-resistance electrodes. The resistivity of earth indicates the degree of corrosion, which is useful in assessing underground pipelines used for oil, water, gas, etc. Typically, locations with low resistivity values will have higher corrosion rates.

In ground resistance testing, the earth tester generates an AC signal, which is fed to the SUT (System Under Test). By measuring the current flow and generated voltage, we can determine the system resistance. The earth resistance can be measured in the range between 0.001 to 20 kOhms.

Figure: Concepts of ground resistance testing employed in earth tester equipments.

As shown in the figure, Rod-3 is moved, and voltage and current measurements are recorded at various points between Rod-1 and Rod-2. Using Ohm’s Law ($R = E/I$), Earth resistance can be determined at various points. The location with low resistance can then be easily derived from the plot of earth resistance versus the distance of Rod-3 from Rod-1.

The following are three test methods used to measure Earth resistance:

• Fall of potential method OR three terminal test method
• Two point test method or Dead Earth method
• Clamp on test method

Manufacturers of Earth Ground Resistance Testers

Following are the popular manufacturers or vendors of earth ground resistance testers:

• Megger
• HIOKI E.E. CORPORATION
• Fluke Corporation
• Meco Instruments Pvt. Ltd.
• Global Instruments
• Yokogawa
• Cambridge Instruments & Engineering Co.
• FLIR Commercial Systems (Extech Instruments)
• Motwane Manufacturing Company Pvt. Ltd.