# 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.