This is another alternative method for earthing resistance measurement when excellent ground is already available.When we want to perform the measurement for earthing resistance in the compact room or tight area and want to put the two(2) auxiliary rod may be a problem.
So..to solve this problem,we can use the two point method measurement.This measurement concept is two point into the ground in series technique used to the reading obtained.
The earth surface or other ground must be very low in resistance so that it will be negligible in the final measurement. The lead resistances will also be measured and should be deducted from the final measurement.
What is two point method measurement?
Ground Resistance Testing Principle #2
This method is not as accurate as three-point methods (62% method), as it is particularly affected by the distance between the tested electrode and the dead ground or water pipe. This method should not be used as a standard procedure, but rather as a back-up in tight areas.
Description of Two Point Method Measurement
Here i want share a several technique to perform the two point method measurement that you must consider when performing this method.
1) Excessive Noise
– Excessive noise may interfere with testing because of the long leads used to perform a fall-of-potential test. A voltmeter can be utilized to identify this problem.
– Connect the “X”, “Y” and “Z” cables to the auxiliary electrodes as for a standard ground resistance test. Use the voltmeter to test the voltage across terminals “X” and “Z”
– The voltage reading should be within stray voltage tolerances acceptable to your ground tester.
If the voltage exceeds this value, try the following techniques:
a) Braid the auxiliary cables together . This often has the effect of canceling out the common mode voltages between these two conductors.
b) If the previous method fails, try changing the alignment of the auxiliary cables so that they are not parallel to power lines above or below the gorund.
c) If a satisfactory low voltage value is still not obtained, the use of shielded cables may be required. The shield acts to protect the inner conductor by capturing the voltage and draining it to ground.
- Float the shields at the auxiliary electrodes
- Connect all three shields together at (but not to) the instrument
- Solidly ground the remaining shield to the ground under test.
2) Excessive Auxiliary Rod Resistance
The inherent function of a fall-of-potential ground tester is to input a constant current into the earth and measure the voltage drop by means of auxiliary electrodes.Excessive resistance of one or both auxiliary electrodes can inhibit this function.
This is caused by high soil resistivity or poor contact between the auxiliary electrode and the surrounding dirt.To ensure good contact with the earth, stamp down the soil directly around the auxiliary electrode to remove air gaps formed when inserting the rod.
If soil resistivity is the problem, pour water around the auxiliary electrodes.This reduces the auxiliary electrode’s contact resistance without affecting the measurement.
3) Tar or Concrete Mat
Sometimes a test must be performed on a ground rod that is surrounded by a tar or concrete mat, where auxiliary electrodes cannot be driven easily. In such cases, metal screens and water can be used to replace auxiliary electrodes.
Place the screens on the floor the same distance from the ground rod under test as you would auxiliary electrodes in a standard fall-of-potential test. Pour water on the screens and allow it to soak in. These screens will now perform the same function as would driven auxiliary electrodes.