How to check earth resistance with megger?

Earth resistance is measured with a Megger (earth tester) by driving auxiliary electrodes into the ground and applying a test current through them. The Megger then measures the voltage drop and calculates resistance (Ohm’s Law: R = V/I).

Detailed Technical Explanation:

1. Principle:

Earth resistance testing involves passing a known current through the soil between the earth electrode under test and an auxiliary electrode, then measuring the potential difference between them.

Using Ohm’s law (R = V/I), the resistance of the earth electrode system is determined.

2. Setup with Megger (3-point or 3-terminal method):

- Disconnect the earth electrode under test from the installation.

-Place two auxiliary electrodes (rods) in a straight line:

• Current electrode (C): Driven into the ground about 30–50 meters away from the test electrode.

• Potential electrode (P): Placed between the test electrode and current electrode, usually at 10–15 meters distance.

- Connect the Megger terminals:

• E (Earth): To the electrode under test.

• P (Potential): To the potential electrode.

• C (Current): To the current electrode.

3. Testing Process:

- The Megger injects a small AC current (to avoid polarization of soil) between the earth electrode and the current electrode.

- It measures the voltage drop between the earth electrode and the potential electrode.

- The instrument then calculates Earth Resistance = Voltage / Current and displays it in ohms (Ω).

4. Good Practices:

- Ensure the soil is moist for proper measurement.

- If resistance readings fluctuate, reposition the auxiliary electrodes further apart.

- For verification, move the potential electrode slightly forward and backward (10% distance each side). Consistent readings mean accurate results.

5. Acceptable Values:

- General installations: < 5 Ω

- Power stations, substations, and sensitive equipment: ideally < 1 Ω

Here's the diagram: 

Here’s a simple diagram showing how to measure earth resistance with a Megger.

E → Earth electrode under test

P → Potential electrode (placed in between)

C → Current electrode (placed further away)

Megger → Connected to all three points (E, P, C) to inject current and measure resistance.

This setup ensures accurate measurement of the resistance of the earth electrode system.

Final Summary:

To check earth resistance with a Megger, you isolate the earth electrode, drive two auxiliary rods into the soil, connect them to the tester (E, P, C), and let the Megger pass a current and measure the voltage drop. The displayed resistance indicates the quality of earthing — the lower the resistance, the safer and more effective the earthing system.