Measurement of Earthing Resistance

Measurement of Earthing Resistance: Clamp on Method

Measurement of Earthing Resistance-Clamp on MethodThis measurement method is innovative and quite unique. It offers the ability to measure the resistance without disconnection the ground. This type of measurement also offers the advantage of including the bonding to ground and the overall grounding connection resistance.

The Induced Frequency test or commonly called the “Clamp-On” test is one of the newest test methods for measuring the ground resistance of a grounding system or electrode. This test uses a special transformer to induce an oscillating voltage (often 1.7 kHz) into the grounding system. Unlike the 3-point Test which requires the grounding system to be completely disconnected and isolated before testing, Induced Frequency method requires that the grounding system under test be connected to the electric utilities (or other large grounding system such as from the telephone company) grounding system (typically via the neutral return wire) to provide the return path for the signal. Induced Frequency test is the only test that can be used on live or ‘hot” systems. However, there are some limitations, primarily being:

  1. The amount of amperage running through the tested system must be below the equipment manufacturer’s limits.
  2. The test signal must be injected at the proper location, so that the signal is forced through the grounding system and into the earth.
  3. This instrument actually measures the sum of the resistance of the grounding system under test and the impedance of the utility neutral grounding, including the neutral wiring. Due to the high frequency used, the impedance of the neutral wiring is non-negligible and can be greater than the ground resistance of a very low resistance grounding system, which can therefore not be measured accurately.
  4. The ground resistance of a large grounding system at 60 Hz can be significantly lower than at 1.7 kHz.

Many erroneous ground resistance tests have been conducted where the technician only measured metallic loops and not the true resistance-to-ground of the grounding system. The veracity of the Induced Frequency Test has been questioned due to testing errors, however when properly applied to a small to medium sized, self-standing grounding system, this ground resistance test is rapid and reasonably accurate.

Measurement of Earthing Resistance-Fall of Potential Method

There are several methods and instruments deployed for the measurement of earth resistance which is recommended by various safety standards. As all the electrical work or installation should be carried in accordance with Indian Electricity Rule 1956 and should follow Indian Standards, it is recommended to follow the guidelines given in National Electrical Code 2011 (First Revision). SP 30:2011. Bureau of Indian Standards for measurement of earth resistance.

Method: Fall of Potential Method

National Electrical Code 2011 (First Revision). SP 30:2011. Bureau of Indian Standards.

SECTION 14: EARTHING

6. Measurement of earth electrode resistance.

6.1 Fall of Potential Method

       There are three basic fall-of-potential test method.  The types of Fall-of-Potential are:

  • Full fall-of-Potential: A number of tests are made at different spaces of “P” and the resistance curve is plotted.
  • Simplified Fall-of-Potential: Three measurements are made at defined distance of “P” and mathematical calculations are used to determine the resistance.
  • 61.8% Rule: A single measurement is made with “P” at a distance 61.8% (62%) of the distance between the electrode under test and “C”.

Limitations

 6.1.1 At the time of test, where possible, the test electrode shall be separated from the earthing system

6.1.4 The auxiliary electrode usually consist consists of 12.5 mm diameter mild steel rod driven up to 1m into the ground.

6.1.5 All the test electrode and the current electrode shall be so placed that they are independent of the resistance area of each other. If the test electrode is in the form of rod, pipe or plate, the auxiliary current electrode shall be placed at 30m away from it and the auxiliary potential electrode shall be placed midway between them.

6.1.6 Unless three consecutive readings of test electrode resistance with different spacing of electrodes agree, the test shall be repeated by increasing the distance between electrodes up to 50m and each time placing the potential electrode midway between them.

Instrument: Earth tester incorporating a hand-driven generator.

6.1.1 Note:

In most cases there will be stray current flowing in the soil and unless some steps are taken to eliminate their effect, they may produce serious errors in the measured value. If the testing current is of the same frequency as the stray current, this elimination becomes very difficult and it is better to use an earth tester incorporating a hand-driven generator.

These earth testers usually generate direct current and have rotary current-reverser and synchronous rectifier mounted on the generator shaft so that alternating current is supplied to the test circuit and the resulting potentials are rectified for measurement by a direct-reading moving coil ohm-meter. The presence of stray current in the soil is indicated by a wandering of the instrument pointer, but an increase or decrease of generator handle speed will cause this to disappear.

Measurement of Earthing Resistance-Three Point method

3pointThe 3 Point Method or fall of Potential Test

The 3 point method fall of potential ground resistance test requires complete isolation from the power utility. Not just power isolation, but also removal of any neutral or other such ground connections extending outside the grounding system. This test is the most suitable test for large grounding systems and is also suitable for small electrodes.

Fall of Potential Method:

In this method two auxiliary earth electrode, besides the test electrode, are placed at suitable distance from the test electrode. A measured current is passed between the electrode “A” to be tested and auxiliary current electrode “C” and the potential difference between the electrode “A” and auxiliary electrode “P” is measured. The resistance of the test electrode “A” is given by

R=V/I

Where,

R= Resistance of the test electrode in ohms.

V= Reading of voltmeter in volts and

I= Reading of ammeter in amperes.

There are three basic fall-of-potential test method.  The types of Fall-of-Potential are:

  • Full fall-of-Potential: A number of tests are made at different spaces of “P” and the resistance curve is plotted.
  • Simplified Fall-of-Potential: Three measurements are made at defined distance of”P” and mathematical calculations are used to determine the resistance.
  • 61.8% Rule: A single measurement is made with “P” at a distance 61.8% (62%) of the distance between the electrode under test and “C”.

The 3 Point Method or fall of Potential Test

Measuring Earthing resistance for Electrical grounding system

Many equipment manufacturers now require that the grounding system for their equipment must be 2-ohms or less and that resistance-to-ground tests be conducted on the grounding system in order to validate the warrantees on the equipment. The 3-point fall-of-potential method and/or the Clamp-on Induced Frequency Test are used to measure the resistance-to-ground of existing grounding systems.

Resistance-to-ground testing requires highly trained personnel to properly conduct the tests and ensure accurate results. In the past, many of these tests have been found to be invalid due to poorly trained technicians running the tests. Don’t let the warrantees on your equipment become “null and void” due to errors on the part of the people conducting those tests.

The measurement of ground resistance for a grounding system is very important. It should be done when the electrode is first installed, and then at periodic intervals thereafter. This ensures that the ground resistance does not increase over time. There are two methods for testing an existing grounding system.

  • The 3 Point Method Fall of Potential test
  • The Induced Frequency test or clamp-on method

Ground-impedance measurement are made :

  1. To determiine the actual impedance of the ground connections
  2. As a check on calculations
  3. (3)To determine (a) the rise in ground potential and its variation throughout an area, that results from ground fault current in a power system, (b) the suitability of grounding connection for lightning protection, and the suitability of grounding connection for radio-frequency transmission at a transmitter.
  4. To obtain data necessary for the design of protection for building, the equipment therein, and any personnel that may be involved.

 

 

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