How is earthing resistance measured?

Short Answer:

Earthing resistance is measured to ensure that the connection between electrical equipment and the earth is effective and safe. It shows how easily fault current can flow into the ground. A commonly used tool for this purpose is the earth resistance tester or megger, which uses a known current to calculate the resistance between the earth electrode and the surrounding soil.

The most common method to measure earthing resistance is the fall-of-potential method, where two auxiliary electrodes are placed in the soil along with the earth electrode. The voltage drop and current flow are then used to calculate the resistance. This test helps ensure the safety and reliability of the earthing system.

Detailed Explanation:

How Earthing Resistance is Measured

Earthing resistance refers to the resistance offered by the earth electrode system to the flow of fault or leakage current into the ground. It is an important safety check in any electrical installation, especially in substations, industries, buildings, and power systems. If the earthing resistance is too high, fault currents may not safely flow into the ground, increasing the risk of electric shock, equipment damage, or fire.

To ensure the earthing system is functioning correctly, its resistance is measured using special testing instruments and methods. The aim is to confirm that the resistance is within the safe limits set by standards like IS, IEEE, or IEC.

Instruments Used

  1. Earth Resistance Tester (Megger Type)
    • This is the most commonly used device. It applies a known voltage and measures the resulting current to calculate resistance using Ohm’s law (R = V/I).
  2. Digital Earth Resistance Meter
    • Offers accurate readings, digital display, and multiple testing ranges.
  3. Clamp-On Earth Tester
    • Measures earthing resistance without disconnecting the earth rod.
    • Useful for systems with multiple interconnected earth electrodes.

Common Methods of Measurement

  1. Fall-of-Potential Method (Three-Point Method)

This is the standard and most widely used method to measure earth resistance.

Steps:

  • Three electrodes are used:
    • E (Earth electrode under test)
    • P (Potential electrode)
    • C (Current electrode)
  • The earth tester sends current between E and C.
  • Voltage is measured between E and P.
  • Resistance is calculated using:
    R = V / I

Note: The distance between the electrodes must be suitable (usually 20 to 50 meters between E and C) to avoid interference.

  1. Dead Earth Method
  • Used when there is a known and tested low-resistance earth available.
  • The unknown earth electrode is compared with this known “dead earth”.

Advantage: Simple in areas with a solid existing earth connection.

  1. Clamp-On Method
  • Uses a clamp-on meter to measure earth resistance without driving additional rods.
  • Works only if the earth system is part of a loop (i.e., multiple paths to earth).

Advantages:

  • Fast and does not interrupt the circuit.
  • No need for auxiliary electrodes.

Disadvantages:

  • Cannot be used on isolated earth rods.

Important Considerations

  1. Soil Conditions
    • Resistance can vary with soil moisture, temperature, and composition.
    • Dry or sandy soils have higher resistance than moist clay soils.
  2. Testing Distance
    • For fall-of-potential, spacing between electrodes must be carefully chosen to avoid overlapping resistance zones.
  3. Time of Measurement
    • Early morning or after rain gives more stable and reliable results.
  4. Multiple Readings
    • Take readings at different points and plot a curve to find the accurate value at the stable region.
  5. Standard Values
    • For safety, earth resistance should ideally be:
      • < 1 ohm for power stations and substations
      • < 2 ohms for general installations
      • < 5 ohms for small electrical systems
Conclusion

Measuring earthing resistance is essential for confirming the effectiveness of an earthing system. The most widely used technique is the fall-of-potential method, which uses an earth tester and auxiliary electrodes to measure voltage and current. Other methods like the clamp-on method and dead earth method are also used depending on the setup. A good earthing system with low resistance ensures fault current flows safely, keeping the system and people protected from electrical hazards. Regular testing ensures ongoing safety and system health.