LAB MANUALExp. 5- Earth Electrode Resistance MeasurementAim: To understand the working of an earth-tester and measure the Earth resistance and Apparatus Required S. No.DescriptionRangeTypeQuantity1 Hand driven Earth Tester (1–100 )Ohms AC 1 2 Digital Earth Tester 3 ½ Digital 1 3 Spikes - - 4 4 Hammer - - 1 5 Wire stripper - - 1 6 Connecting Wires -- -- As Req. Operating principleThe principle used in measuring the earth resistance is based on simple ohms law. 4electrodes A, B, C, D are buried in the earth, the resistance of which is to be tested at thedistance of 20m from each other electrodes is shown in the figure.A.C. signal is applied toelectrodes A and B and the voltage developed across B and C due to flow of current throughthe earth is measured by ammeter M. if current is constant, the voltage measured will bedirectly proportional to the earth resistance. Principle of measurementTo find out resistance of earth connection, the 3 terminal method is to be used. A current ispassed through the EARTH PLATE to an auxiliary electrode A in the earth at a distance awayfrom the earth plate. A second auxiliary electrode B is inserted between earth Plate and A, thepotential difference V between E and B is measured for a given current I, so that the resistanceof earth connection is V/I. the placing of the auxiliary electrodes is however important.Following is the curve which gives a plot of distance B from E verses measured 11| Page
resistance. When earthing resistance is low, the spacing between the earth plate and theauxiliary electrode may be 20 to 30m. The exact value can be decided by actual experiment. How to operate the instrumentTo operate the instrument, first turn the range selection switch to 10Ωposition. Thedigital display reads zero. Connect the test leads to the four terminals as per procedure fortesting the given earth electrode. Press the TEST switch the LCD display will indicate theresistance. if “LO BAT indication appears on the upper side of the display while pressing thetest switch, it indicates that the battery needs charging. After completing the testing, move theselector knob to OFF position. Procedure for testingThe digital earth testes has four terminals marked as E1, P1, P2 and E2. It is suitable for measuring Earth resistance as well as earth resistivity.Measurement of the earth resistanceTo measure earth resistance, it should be used as three terminal type. Terminals E1 and P1 is to be12| Page
shorted and connected to the Earth Electrode whose resistance is to found. Connect as per figure andtake the reading by pressing the TEST switch. Note down the reading displayed on the LCD display ofthe instrument.