Interview Questions and Answers of earth resistance measurement

Question.1. What are the various methods of earth resistance measurement or testing?

Answer:- The methods of measuring and testing the ground resistance and soil resistivity are as follows:

• Two-point method
• Three-point method
• Fall-of-potential method
• Ratio method
• Four-point method
• Touch potential measurements
• Clamp-on method

Question.2. When the two-point methods for measuring earth resistance is used?

Answer:- The two-Point Method may be used to measure the resistance of a single driven ground rod. It uses an auxiliary ground rod whose resistance is either known or can be measured. The resistance value of the auxiliary ground rod also must be very small compared to the resistance of the driven ground rod so that the measured value can be assumed to be wholly contributed by the driven ground rod.

Question.3. What are the advantages of the two-point method?

Answer:- This method might be applicable in the measurement of resistance of the single driven ground rod for a residence or in congested areas where finding room to drive two auxiliary rods may be a problem.

Advantages and Disadvantages of the Fall-of-Potential Method
The main advantage of the fall of potential method is its accuracy. No other way of measuring ground resistance is more accurate. You may change the span D at will and compare the resistance outcomes until achieving reasonable figures.

In this case, the metallic water supply line can be assumed as the auxiliary ground rod whose resistance value is approximately 1 Ω or less.  This method is usually adequate where a go, no-go type of test is required.

Question.4. Discuss the Limitation of the two-point earth resistance measurement?

Answer:- This method is not accurate as the three-point methods & four-point methods. This method is mainly affected by the dead ground or water pipe and the distance between the tested electrode. This method should not be used as a standard procedure, but rather as a backup in tight areas.

Question.5. Explain the three-point method for measurement of earth resistance?

Answer:- The three-point method is similar to the two-point method except it uses two auxiliary rods. To obtain accurate values of resistance measurements, the resistance of the auxiliary electrodes should be approximately equal to or less than that of the electrode under test.

Question.6. The supply used in the three-point method for measurement of earth resistance is?


The three-point method can either use AC or DC supply. The advantage of using AC is that it minimizes the effects of stray currents on measurement readings. However, if stray currents happen to be of the same frequency, an error will be introduced in the readings.

The use of DC for making this test will totally eliminate the AC stray currents. However, stray DC and the formation of gas around the electrodes will introduce errors in the readings when using DC for this test. The effect of stray DCs can be minimized by taking readings with the current in the opposite direction.

Question.7. The three-point method is also known as

The three-point method is also called as fall of potential method.  This is a low current method and is the main technique used for the measurement of grounding resistance of electrodes as well as grounding grids. This method is based upon the principle of the potential drop across the resistance. It uses two auxiliary electrodes (a current electrode and a potential electrode) that are placed at a sufficient distance from the grounding electrode(s) under test.

Question.8. What are the advantages of the three-point method or the fall of the potential method?

  • The main advantage of the fall-of-potential method is that the potential and current electrodes may have substantially higher resistance than the ground system being tested without significantly affecting the accuracy of the measurements.
  • Fall of potential method is the quicker and simpler, with one less lead required for string Spacing of current Probe
  • This test is the most suitable test for large grounding systems.

Question.9. What type of error was encountered while using the fall of potential method?

Several problems and errors may be encountered with the fall of the potential methods, such as

(i) Stray currents in the earth may cause voltmeter readings to be either high or low

(ii) The resistance of auxiliary electrode and electrical leads may introduce errors in the voltmeter reading.

This error can be minimized by using a voltmeter of a high impedance value.

Question.10. What are the limitation or Disadvantages of the fall of the potential method?

  • To perform this test, the grounding rod must be disconnected (electrically isolated) from the neutral bus in the service panel. Failure to do so will completely invalidate the test. 
  • This method time consuming and labor-intensive.
  • There are situations where disconnection is not possible.
  • Knowledge of the location of the center probe is necessary. Measurement of resistance becomes Ineffective when the electrical center is unknown.
  • The accuracy of measurements greatly depends upon the distance between the ground bed and the auxiliary current electrode. The larger this distance, the less the error introduced.
  • If less measurements are being made then less accuracy is less.

Question.11. Explain the direct connection method for the measurement of earth resistance.

Answer. The direct method is the simplest way to make an earth-resistance test. With this method, the resistance of two electrodes in series is measured by the driven rod and the water system.

Question.12. What are the limitations of the direct connection method?

Answer. There are three important limitations.

1. The water-pipe system must be extensive enough to have negligible resistance.

2. The water-pipe system must be metallic throughout, without any insulating couplings or flanges.

3. The earth electrode under test must be far enough away from the water-pipe system to be outside its sphere of influence.

In some locations, the earth electrode may be so close to the water-pipe system that the two cannot be separated; under these circumstances, if conditions I and 2 are met, the water-pipe system can be used as the ground. As a precaution against any possible future changes in the water-pipe system, an earth electrode should also be installed.

The main drawback with the Four Potential methods is that, like with the Fall of Potential method, it can require excessive electrode separation distances if the earthing system being measured is large.

Question.13. What is the four-point method for the measurement of earth resistance?

Answer. To measure earth resistivity, a four-terminal instrument is used, along with four small-sized electrodes driven down to the same depth and equal distances apart in a straight line. Four separate lead wires connect the electrodes to the four terminals on the instrument, as shown. Hence the name of this test: the four-terminal method.

Question.14. What are the advantages of the Wenner four-pin method?

Answer:- (A) The Wenner four-pin method obtains the soil resistivity data for deeper layers without driving the test pins to those layers.

(B) No heavy equipment is needed to do the four-pin test.

(C) The results are not greatly affected by the resistance of the test pins or the holes created in driving the test pins into the soil. Because of these advantages, the Wenner method is the most popular method.

(D) In the Wenner method, the whole electrode array is expanded about the center point, the fall off of potential gradient with increasing a is automatically partly compensated by the increase in potential electrode separation.

Question.15. What are the limitations of Wenner’s four-pin method?

Answer:- Wenner’s four-pin method’s most serious limitation arises from the fact that it is not water per se that conducts electricity, but rather the ions dissolved in the water or associated with the surface charges on the soil mineral particles. It requires, therefore, a great leap of faith to use to determine water content. 

With the Wenner array, all four electrodes have to be moved and long lengths of cables between the current electrodes and the potential electrodes may be needed.

Another disadvantage of the Wenner method arises when five electrodes are used, the ‘ dead ‘ one being the trailing electrode. This is leap-frogged over the four ‘ live ‘ electrodes and then becomes the leading electrode, and so on.

Question.16. How will you recognize a Megger and a Megger Earth tester?

Answer. A Megger has only two points while the Megger Earth tester has three points. One point of the Earth tester is connected to the earth electrode.

Question.17. Why do we measure the resistance of the earth electrode by Megger Earth tester but not with the simple Megger?

Answer. The resistance of the earth electrode or earth wire is not only meaningful, but we wish to measure the resistance of the earth’s vicinity, up to the length of 150 feet. The charge accumulated on the equipment must go to the earth immediately. It is not possible with simple megger hence megger earth tester is used.

Question.18. When there was no earth tester, how was earth resistance measured?

Answer. The earth resistance was measured by supplying 500 volts between the earth electrode and one peg of iron, 150 feet apart, by a generator. The current was measured by an ammeter flowing through the peg. A voltage was measured by a voltmeter between the earth electrode and a second peg at 75 feet away.

Question.19. Why do we measure the earth resistance of an electric pole stay wire and stay peg in summer, winter, and rainy seasons?

Answer. To check the poles, whether the resistance is zero or nearly zero. If the resistance of the pole is not zero, install an earth wire and ground it with a coil. If the earth resistance of the pole is more than 1000 ohm, it must be reduced or dismantle it. The possibility of shock hazard is more if the insulator fails. In all three seasons the earth resistance of the poles and stay wires must be recorded, for the safety of human beings and animals.

Question.20. What type of supply voltage is provided to the Megger earth tester?

Answer. In megger earth tester current and potential coils of the ohmmeter are supplied with direct current and the testing circuit is supplied with Alternating current.

Direct current from the generator passes through the current coil of the ohmmeter to a rotating current reverser driven from the generator handle. The alternating current is thus delivered to the current terminals of the instrument which are connected to the contact under test and to the “current” temporary earth connection. 

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