Ground Resistance Meters

Ground resistance meters are a class of instruments designed to test the resistance of soil to the passage of electric current. Generally, ground resistance is tested to determine the adequacy of the grounding of an electrical system. Though soil is usually a poor conductor of electricity, if the path for current is adequately large, resistance can be quite low providing a path for fault currents. This is an indispensable component of a safe, properly functioning electrical system.

Generally, the lower the ground resistance, the safer the electrical system. Regulatory agencies set forth the maximum allowable ground resistance. The National Electrical Code requires electrical systems to have a ground resistance of 25 ohms or less. The Mine Safety and Health Administration requires the ground to be 4 ohms or better. Electric utilities design their grounding systems to keep resistance at large stations to below a few tenths of an ohm.

Though the sheer abundance of earth usually provides a suitable path for fault currents, a limiting factor in grounding systems is how well the grounding electrodes contact the earth. The resistance of the soil/grounding rod interface, as well as the resistance of the grounding conductors and the connections, must be measured by a ground resistance meter.

Why measure ground resistivity?

By knowing the resistivity of soil, understanding its affects, and being able to “read” the results, ground resistivity measurements can provide important information across a number of diverse applications.

Since the composition of soil affects its resistivity, ground resistance measurements can be used to conveniently make sub-surface geophysical surveys. This enables the identification of ore locations, depth to bedrock and other geological phenomena.

The resistivity of soil also has a direct impact on the degree and rate of corrosion in underground pipelines for water, oil, gas, gasoline, etc. A decrease in resistivity generally relates to an increase in corrosion activity. Ground resistance meters can help identify this problem as well as help dictate where cathodic protection is needed.

The primary use of ground resistance meters, however, is in the design and verification of ground electrodes. Properly installed ground electrodes provide a path for fault currents making them important elements for improving safety, preventing equipment damages, and minimizing downtime. When designing a grounding system, ground resistance measurements are useful for finding the area of lowest soil resistivity in order to achieve the most economical grounding installation.

Grounding Systems

A “ground” is defined as a conductor that connects an electrical circuit or equipment to the earth. The connection is used to establish and maintain, as closely as possible, the potential of the earth on the circuit or equipment connected to it. Typically, a grounding system consists of a grounding conductor, a bonding connector, its grounding electrode(s), and the soil in contact with the electrode.

There are good reasons why grounding an electrical system is necessary. Primarily, grounds provide a safe route for unexpected electric current caused by faults in an electrical system. By providing low resistance fault current paths, grounds are able to disperse the current as quickly as possible-- before personnel are injured or equipment damaged.

There are many types of electrical faults caused by many issues. Many faults are short duration, often caused by a lightning strike or brief contact, such as from a tree or animal momentarily touching a wire. Degraded wire insulation, rodent damage, broken insulators, and improper wiring can all cause faults of a transient or persistent nature.

With electrical systems becoming increasingly complex and electrical instruments becoming ever more sensitive, good grounding is more important than ever to prevent costly damage and downtime due to service interruptions and inoperative surge protection caused by poor grounds.

Grounding rods and their connections are subjected to environmental hazards such as high moisture content, high salt content, and high temperatures in the soil all of which can cause decay of the system over time, potentially reducing its effectiveness. Grounding systems should be checked once per year as part of a predictive maintenance schedule.

Measuring Ground Resistance

Ground resistance meters are fairly straightforward instruments. Like most instruments, they are available in a variety of ranges and accuracies while offering a range of options to tailor the instrument to an application.

Ground resistance meters are generally available in two styles. The more traditional style includes stakes which are inserted into the ground with the positioning of the stakes determined by the type of resistance test being run. With the stakes attached to the unit via leads, a current is applied through one of the stakes. When the current reaches the other stake(s), it is measured and compared to the voltage generated, whereby the instrument calculates and displays the system resistance.

For simpler ground resistance measurements, clamp-on style ground resistance meters have been developed which allow spot measurements of grounding system components without the need for setting stakes or disconnecting the grounding rod.

Factors Affecting Soil Resistivity

The resistivity of the surrounding soil is the key component determining what the resistance of a grounding electrode will be, and to what depth it must be driven to obtain low ground resistance. The resistivity of soil varies widely from place to place due to differences in soil composition and environmental factors.

Soil resistivity is determined largely by the amount of moisture, minerals and dissolved salts contained within. The greater their concentration, the lower the resistivity of the soil. Conversely, dry soils with few soluble salts and minerals have a high resistivity. The resistivity of soil containing 10% moisture by weight will as much as five times lower than soil containing 2.5% moisture. The temperature of the soil also helps determine its resistivity with higher temperatures resulting in lower resistivity. The resistivity of soil at room temperature will be as much as four times lower than that at 32 degrees.

As moisture content and temperature has such a direct effect of soil resistivity, it stands to reason that the resistance of a grounding system will vary, perhaps considerably, from season to season. Since both temperature and moisture content become more stable at greater distances below the surface of the earth, their effects on resistivity can be mitigated by setting grounding electrodes deep into the earth. Best results are obtained if the ground rod reaches the water table

Soil Resistivity Measurement Techniques

Depending upon which aspect of the grounding system is being measured and the equipment available, there are a handful of measurement techniques at the disposal of the technician. Each varies somewhat in complexity, accuracy and the applicability of results.

2-Point Method: The 2-point method simply involves measuring the resistance between two points. Two stakes are placed in the ground with current running through one and measured by the other. The difference is converted into a resistance reading. 2-point tests are commonly used in urban environments where proper placement of the auxiliary electrode can be hampered by obstructions. Measurements are referenced against a good local ground conductor.

4-Point Method: In most circumstances, the 4-point testing method is the most accurate soil resistivity test. As the name implies, the 4-point method involves placing four test stakes in the ground, in-line and equally spaced. A known current from a constant current generator is passed between the outer electrodes. The potential drop (a function of the resistance) is then measured across the two inner electrodes.

4-point resistivity measurement should be done ahead of the actual ground system installation. This test informs the engineer where the most conductive soil is and at what depth this occurs.

Fall-of-Potential Method (3-point): For the Fall-of-Potential test method, the grounding electrode is disconnected from the electrical system and connected to the tester. Two test stakes are inserted into the ground in-line, at equal distances, and away from the grounding electrode. A known current is generated and applied and the resulting resistance is measured. The inner stake is then moved to either side in increments with measurements accompanying each move. When these additional measurements are in agreement with the original measurement, the distances between the three points are considered correctly positioned and resistivity can be determined by averaging results. The fall-of-potential method is best for existing ground systems that don’t cover a wide area.

62% Method: The 62% method is a variation of the fall-of-potential method and suitable for areas considered too large for fall-of-potential measurements. Whereas with the fall-of-potential method the stakes are placed evenly and adjusted to find the optimal position, with the 62% method, the inner stake is placed at 62% of the distance between the grounding electrode and the outer stake. When voltage is applied the difference in potential between the stakes is converted into a resistance reading.

Selective Testing Method / Clamp-on: Clamp-on ground resistance meters offer the ability to test without disconnecting the ground making them very convenient for checking the bonding and overall connection resistances of grounding systems. This will verify the integrity of the individual grounds and determine of the grounding potential is uniform throughout a grounding system.

Things to Consider When Purchasing a Ground Resistance Meter:

• What type of test is most appropriate for your application?
• What accessories (electrodes, stakes) are required?
• Is memory or communication needed?
• What measurement range is desired?
• Are agency approvals or environmental ratings needed?

If you have any questions regarding ground resistance meters please don't hesitate to speak with one of our engineers by e-mailing us at sales@instrumart.com or calling 1-800-884-4967.