You're right to be prepared – testing ground rod resistance is an important part of ensuring electrical safety. Here's a breakdown of tools, procedure, and relevant references

Tool Recommendations

You'll need a ground resistance tester (also called a ground tester, earth tester, or ground impedance tester). These testers are specifically designed for this task. Here are some reliable brands and considerations: Brands



Fluke

Generally considered the industry standard for quality and accuracy, but they come at a premium price. Models like the Fluke 1625-2 or Fluke 1623-2 are good choices.

Megger

Another reputable brand with a long history in electrical testing. Megger DET3TC, DET4TC are popular.

AEMC

Offers a balance of performance and price. The AEMC 6416 is a common model.

Amprobe

A more budget-friendly option. The Amprobe GP-1A is a decent choice.

Extech

Also a more affordable brand, but make sure to check the specs and reviews carefully.

Features to Look For



3-Point Fall-of-Potential Method

This is the most accurate and widely accepted method for testing ground resistance. Ensure the tester supports this.

2-Point Testing

While less accurate, can be used as a comparative test with the original resistance test.

Stakes/Probes and Test Leads

The tester should come with these. Pay attention to the length of the leads; you'll need adequate separation between the ground rod and the test probes. Longer leads allow for better accuracy.

Multiple Resistance Ranges

Choose a tester that can measure low resistance values (around 0-25 ohms) accurately.

Digital Display

Easier to read than analog meters.

Battery Powered

Almost all ground testers are battery-powered. Check battery life.

Data Logging (Optional)

Some testers can store readings, which can be useful for documentation.

Clamp-On Ground Resistance Testers

These are available but usually are used on multi grounded neutral systems.

Calibration Certificate

Ensure the tester comes with a calibration certificate or has a way to be calibrated for accurate readings.

The 3-Point Fall-of-Potential Testing Procedure (The Most Common and Accurate Method)

This is the standard method for testing ground rod resistance. Always follow the manufacturer's instructions for your specific tester. Here's a general outline:1. Disconnect the Ground Rod

Crucially,

disconnect the ground rod from the electrical system before testing. Disconnect the ground rod conductor at the point where it connects to the service panel or grounding busbar. Safety is paramount. Failure to disconnect will give you inaccurate readings and could damage your equipment.2. Set Up the Tester

Connect the tester's leads to the appropriate terminals on the tester.3.

Placement of Stakes



Ground Rod Under Test (E)

This is the ground rod you're testing.

Auxiliary Current Probe (C)

Drive this stake into the ground at a distance of approximately

62% of the distance between the ground rod under test (E) and the auxiliary potential probe (P). C should be in a straight line. For example, if you use a distance of 100 feet between E and P, then C should be placed 62 feet from E. The farther apart you can place the stakes, the more accurate the readings will be, but space limitations may exist. Auxiliary Potential Probe (P)

Drive this stake into the ground along the same line, further away from the ground rod. The distance between E and P should be at least 20 ft.

Stakes should be in a straight line from the ground rod. Important Note: Avoid placing the probes near underground utilities, buried metal, or other grounding systems, as this can affect the readings. If space is limited, rotate the probes 90 degrees around the ground rod and test again.4. Connect Test Leads

Connect the "E" lead (usually green) to the ground rod being tested. Connect the "P" lead (usually yellow) to the potential probe (P). Connect the "C" lead (usually red) to the current probe (C).5.

Perform the Test

Turn on the tester. Select the appropriate resistance range. Press the "Test" button. Read the resistance value displayed on the meter.6.

Verify Accuracy (Important!)



Move the potential probe (P) 10% closer to the ground rod (E) and take another reading. Move the potential probe (P) 10% farther away from the ground rod (E) and take another reading. Compare the three readings. If the readings are within a few ohms of each other (e.g., within 2-5 ohms), the test is considered valid. If the readings vary significantly, it means the probes are too close together, the soil resistivity is uneven, or there is interference. You'll need to adjust the probe placement and repeat the test.7. Record the Reading

Note the final resistance value.8.

Reconnect the Ground Rod

Important: Reconnect the ground rod conductor to the electrical system after testing.

Do not leave the ground rod disconnected!Interpreting the Results NEC Requirement

The National Electrical Code (NEC) Section 250.53(A)(2) states that a single ground rod must have a resistance to ground of 25 ohms or less. If a single ground rod exceeds 25 ohms, a supplemental ground electrode must be installed and bonded to the first ground rod.

Multiple Ground Rods

If you use multiple ground rods, the NEC does not require you to test the resistance. Connecting a second ground rod is considered acceptable, regardless of the individual resistance of each rod. They must be at least 6 feet apart.

Lower is Better

A lower resistance value is always desirable. It indicates a better path for fault current to return to the source, improving safety.

Important Considerations and Tips



Soil Conditions

Soil moisture significantly affects ground resistance. Ideally, test when the soil is neither too dry nor saturated.

Electrolyte Enhancement

In areas with poor soil conductivity, you may need to use ground enhancing compounds (e.g., bentonite clay) around the ground rod to lower resistance.

Multiple Ground Rods vs. Other Grounding Electrodes

While two ground rods are acceptable, other grounding electrodes (e.g., concrete-encased electrodes, ground rings, metal water pipes) can often achieve lower resistance values.

Local Amendments

Check for local electrical code amendments that may have stricter grounding requirements.

Training

Consider taking a training course on ground resistance testing. Proper technique is essential for accurate results.

Safety First

Always disconnect power before working on electrical equipment. Wear appropriate personal protective equipment (PPE).

References National Electrical Code (NEC)



Article 250: Grounding and Bonding

This is the primary reference for grounding requirements. Pay particular attention to:

250.4(A)(1) and (B)(1)

General requirements for grounding.

250.53(A)(2)

Grounding electrode system; grounding electrode requirements (25 ohms requirement).

250.56

Resistance of Rod, Pipe, and Plate Electrodes.

IEEE Standard 81: Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Ground System

This is a more in-depth technical standard for ground system testing. It's helpful for complex grounding systems but may be overkill for typical residential or commercial installations.

Example Scenario

Let's say you're inspecting a new residential service:1. You visually verify the presence of a ground rod.
2. You disconnect the ground rod conductor from the service panel's grounding busbar.3. You set up your ground resistance tester using the 3-point fall-of-potential method, placing the probes at appropriate distances.
4. You perform the test and get a reading of 32 ohms.5. Since the reading exceeds 25 ohms, you note that a supplemental grounding electrode (another ground rod) is required to meet NEC requirements.6. You tell the electrician to install a second ground rod at least 6 feet away from the first and bond it to the first ground rod. Testing after that may not be required (although it is good practice).
7. Once the second ground rod is installed, you visually verify the proper bonding of the two rods.8. You reconnect the grounding electrode conductor back to the grounding busbar.Remember to consult the specific instructions for your ground resistance tester. Good luck, and stay safe! Flag for review