What is an Earth Tester ? Explain the principle of working and also method of measuring earth resistance With the help of a neat sketch. How earth resistance is brought down ?
Earth Test
Ans. Earth Tester is a meter used for testing earth. It gives resistance of earth in ohms directly. Earth tester has the following main parts :
(1) Hand-driven generator
(2) Rotary current reverser
(3) Synchronous rotary rectifier
(4) Ohmmeter.
Hand Driven Generator : It produces direct current voltage when the handle is rotated.
Rotary Current Reverser : It is mounted on the same shaft of the generator. It is used to change the DC. into A.C. supply as only alternating current is used through soil for testing the earth.
Synchronous Rotary Rectifier : The alternating current in the soil will produce an alternating potential drop in the soil. But potential to be applied across the moving coil must be direct because ohmmeter is a moving coil instrument, so for changing the alternating drop into direct drop, a synchronous rotary rectifier is also attached on the same shaft as shown in Fig.
Ohmmeter : It consists of two coils (current coil and potential coil) mounted at a fixed angle to each other on common axle. The current coil carries current proportional to the current flowing in the test circuit, while the potential coil carries current proportional to potential across the resistance under test. Since the deflection of the needle is proportional to the ratio of the current in two coils, it gives resistance directly and so there is no controlling device.
Testing (N. C. V.T.) : For measurement of earth resistance, two spikes acting as current and potential electrodes are driven into the ground at suitable distance (usually at a distance of about 25 m) from earth electrodes under test. The P1 and C1 terminals of meter are shorted and are connected to the earthed electrode under test and resistance is determined by direct reading.
SOLVE :
EARTH TESTER
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