Two-Point Method for Earthing Resistance Measurement
The two-point method is an alternative technique for measuring earthing resistance, especially in compact or tight areas where installing auxiliary rods can be challenging. This method provides a practical solution when a good ground connection is already available. While not as accurate as the three-point method, it serves as a useful backup in specific scenarios.
What Is the Two-Point Method?
The two-point method involves placing two ground electrodes in series and measuring the resistance between them. This approach leverages an existing ground (e.g., a water pipe or structure with negligible resistance) to complete the circuit. However, it is important to note that this method is less accurate due to its sensitivity to distance and environmental factors.
Key Considerations for Two-Point Measurement
1. Excessive Noise
Excessive electrical noise may interfere with readings, particularly when using long leads. To manage this issue:
- Detecting Noise: Use a voltmeter to measure the voltage across terminals “X” and “Z.” Ensure the reading is within acceptable stray voltage tolerances.
- Mitigation Techniques:
- Braid Auxiliary Cables: Cancels out common-mode voltages between conductors.
- Change Alignment: Avoid running cables parallel to power lines (above or below ground).
- Use Shielded Cables: Shields capture stray voltages and drain them to the ground.
- Float shields at auxiliary electrodes.
- Connect shields together at the instrument.
- Ground the remaining shield at the test point.
2. Excessive Auxiliary Rod Resistance
High auxiliary rod resistance can inhibit the tester’s ability to input constant current and measure voltage drop. Causes and solutions include:
- High Soil Resistivity: Pour water around auxiliary electrodes to improve conductivity without affecting measurements.
- Poor Rod Contact: Stamp down the soil around the rod to remove air gaps.
3. Tar or Concrete Mat Challenges
When auxiliary electrodes cannot be easily driven due to surrounding tar or concrete:
- Use metal screens placed at appropriate distances from the test point.
- Pour water on the screens to enhance conductivity.
- Allow water to soak into the surrounding surface for optimal contact.
Advantages of the Two-Point Method
- Practical in tight spaces or urban settings where auxiliary rods are difficult to install.
- Useful for quick, non-standard tests.
Limitations of the Two-Point Method
- Less accurate than the 62% method due to reliance on external grounds and environmental factors.
- Affected by distance between electrodes and quality of the existing ground.
Step-by-Step Guide to Two-Point Measurement
- Set Up:
- Connect one terminal to the existing ground (e.g., water pipe).
- Connect the second terminal to the ground under test.
- Measure Resistance:
- Use the ground tester to measure resistance directly between the two points.
- Compensate for Lead Resistance:
- Deduct the resistance of the test leads from the final reading to improve accuracy.
- Verify Results:
- Cross-check readings to ensure they fall within an acceptable range.
Conclusion
The two-point method is a valuable alternative for earthing resistance measurement in challenging environments. While it lacks the precision of the three-point method, its practicality makes it indispensable in specific scenarios. By following best practices and mitigating common issues like noise and high auxiliary resistance, reliable readings can be achieved.
For more detailed insights, stay tuned for our future posts on advanced grounding techniques and practical applications!