How to Check Earth Resistance of Transformer

Checking the earth resistance of a transformer is a critical safety and performance test. Proper testing ensures the transformer is adequately grounded to prevent dangerous fault currents and overvoltages.

The following steps outline the process for measuring transformer earth resistance using the fall-of-potential method, the most common and accurate testing approach.

Step by Step to Check Earth Resistance of Transformer

Step 1: Gather Required Equipment

To perform an earth resistance test on a transformer, you will need the following equipment:

• Earth resistance tester capable of the fall-of-potential method (e.g. Megger DET2/2 or Fluke 1625-2)
• Auxiliary earth spikes (minimum of 2)
• Test leads and clips (minimum of 3)
• Hammer for driving earth spikes
• Tape measure (at least 100 ft or 30 m)
• Safety gear (insulated gloves, boots, etc.)

Ensure the earth resistance tester is in good working condition and has been calibrated according to the manufacturer’s recommendations. Check that the test leads are free of any damage and make a strong connection.

Step 2: De-energize and Isolate the Transformer

Before conducting any testing, the transformer must be completely de-energized and isolated from the power system. Follow proper lockout/tagout procedures per safety protocols.

Confirm the transformer is de-energized using a voltage detector at the transformer terminals and any connected points. Discharge any residual voltage in the windings to earth using a grounding stick.

Disconnect any existing ground connections from the transformer tank or neutral bushing. The resistance measurement should be of the transformer grounding alone, without influence from other grounded equipment.

Step 3: Set Up the Fall-of-Potential Test

The fall-of-potential test involves passing a current between an earth electrode and a probe, then measuring the voltage between the electrode and a second probe. Multiple voltage measurements are taken as the second probe is moved in a straight line away from the ground electrode under test.

To set up:

1. Position the current probe (C) at least 100 ft (30 m) from the transformer in a straight line. Drive it firmly into the earth.
2. Place the potential probe (P) in line between the transformer ground (X) and current probe (C). Start with it near the transformer, about 10% of the total distance to C.
3. Connect the test leads:

• X terminal to the transformer tank or neutral ground
• C terminal to the current probe
• P terminal to the potential probe

1. Measure and record the distance from the transformer to each probe.

Keep the test leads separated to avoid electromagnetic interference. The current probe should be far enough to be outside the transformer’s sphere of influence, which is related to the grounding system design and soil resistivity.

Step 4: Perform the Earth Resistance Test

With the fall-of-potential test set up, you are ready to take resistance measurements at various probe spacings. Most digital ground meters will automatically calculate resistance after running the test current.

1. Turn on the earth resistance tester and select the 3-pole fall-of-potential test mode. Set the test current to the highest available frequency, typically 128 Hz, to minimize interference.
2. Press the test button to inject the current and measure the resistance. Record the resistance reading and the potential probe (P) distance from the transformer.
3. Move the potential probe to several more locations in a straight line between X and C. Space the test points evenly, such as every 10% of the total distance. At each point, measure and record the resistance and distance. Take at least 5-6 readings.
4. Plot the recorded resistances (Y-axis) against the distances (X-axis). The resistance should rise, peak, then decrease in an arc or parabola shape. The peak value where the curve flattens is the true resistance of the earth electrode.

If the resistance continually increases with distance and does not peak, the current probe (C) is likely too close and should be moved farther away. Retest with a wider probe spacing if needed.

Step 5: Evaluate the Earth Resistance Results

Compare the measured transformer earth resistance to the maximum allowable values per applicable industry standards, such as:

• IEEE 142: 1 ohm or less for large substations, 1-5 ohms for smaller systems
• NFPA 70: 25 ohms or less for a single electrode, 10 ohms or less for multiple electrodes
• NESC: 25 ohms or less in general, 1 ohm or less for very high voltages (>230 kV)

The grounding system should be designed to meet the most stringent requirements for the installation. If the resistance exceeds acceptable limits, further investigation and mitigation is needed, such as:

• Inspecting the grounding connections for corrosion or loose hardware
• Adding more ground electrodes to lower overall resistance
• Treating the soil to reduce resistivity
• Utilizing ground enhancement materials like conductive concrete

Document the final earth resistance measurements for the transformer. Repeat the test periodically per maintenance schedules or after any significant changes to the grounding system.

Step 6: Restore Transformer to Service

After completing the earth resistance testing, restore all grounding connections to their original configuration. Remove any temporary test leads or equipment.

Recheck that the transformer grounding matches the system drawings. Ensure all connections are tight and protected from corrosion.

Confirm the transformer is ready to be put back into service. Close up any enclosures and remove lockout/tagout devices per safety procedures. Re-energize the transformer from its power source.

Monitor the transformer during initial re-energization and note any unusual sounds, smells or readings. Conduct any other required commissioning tests before returning the transformer to full operation.