What is an Earth tester or an Earth resistance tester?
A device used for the measurement of the earth resistance value is called earth tester or earth resistance tester.
All the equipment like a current transformer, potential transformer, lightning arrester, power transformer etc. are connected to the earth through the earth electrodes. During fault in this equipment, the heavy fault current can flow into the earth through earth electrodes which protect the operating personnel from the heavy fault current and also protect the power system equipment from damage.
Earthing is the process of connecting non-current-carrying parts of electrical equipment to earth through a low resistance path. The earthing resistance should be as low as possible.
What is the need of earthing?
1. The earthing is protect human life from the leakage current which is flowing in non-current-carrying parts of equipment due to fault.
2. The earthing is used to save the human life from the danger of shock death in case it comes in contact with charged frame due to any fault.
3. The earthing provides safety to electrical appliances or devices.
4. The earthing is used to maintain the line voltage constant.
The overvoltages produced by the lightning strokes are effectively discharged into the earth without affecting power system equipment.
So it is necessary to know the value of the earth electrodes before earthing any electrical equipment. Before providing the earthing to electrical equipment, it should be known the resistance of the particular area where the earth pit can be dug. The Resistance value of the earth electrodes is measured by the earth resistance tester.
It is necessary to maintain the earth electrodes resistance value below 5 ohms for domestic earthing and less than 1 ohms for power stations.
Construction of earth tester
The parts of the earth tester are shown in the
following fig. with detail.
following fig. with detail.
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| Earth Tester |
The earth tester consists following parts
1. D.C generator
2. Rotating current reverser
3. Rectifier
4. Permanent Magnet moving coil instrument
5. Earth electrode
1. D.C generator
A hand-driven D.C generator is used to produce D.C voltage to supply the Earth merger. The D.C generator is used as a source for the Earth merger. The D.C generator supplies the voltage to the rotating current reverser.
2. Rotating current reverser
The Rotating current reverser is used in the earth tester to convert the Dc voltage of the DC generator to AC voltage because the earth resistance measurement test is done with alternating current. The supply from the rotating current reverser is connected to the earth electrodes under test.
3. Rectifier
The rectifier is used in earth tester because earth tester is consists of a permanent magnet moving coil instrument for the measurement of earth resistance. The permanent magnet moving coil instrument consists of a pressure coil and current coil which is using a DC supply.
The rectifier is a device that converts the ac current coming from the earth electrode to dc current and is supplied to the current coil of a permanent magnet moving coil instrument.
The rotating current reverser and rectifier are mounted on the shaft of the dc generator.
4. Commutator and carbon brushes
The shaft of the dc generator is rotating. So two nos of commutators are mounted on the shaft of the dc generator with the two sets of carbon brushes.
The rotating current reverser consists of one commutator with four nos of brushes and also rectifier consists of one commutator with four nos of carbon brushes.
The commutators are used for converting the direction of flow of current and carbon brushes are used to transfer the power from the stationary parts to moving parts of equipment.
5. Permanent magnet moving coil instrument
The PMMC instrument is used for the measurement of earth resistance in earth tester. The PMMC instrument consists of two current coils and two pressure coils. Each pressure coil and current coils have two terminals.
The one pair of the current coil and pressure coil is placed across the permanent magnet of the PMMC instrument. Another pair of the current coil and pressure coil is short-circuited and connected to the auxiliary earth electrodes.
One end of the Current coil is connected to the DC generator and another end of the Current coil is connected to the earth electrode.
One end of the pressure coil is connected to the dc rectifier and another end is connected to the earth electrode. The pressure coil or potential coil is placed between the permanent magnets and the pressure coil is connected to the pointer. The pointer is connected to calibrated fixed dial. The pointer indicates the measured value of earth resistance on the dial.
Working of Earth tester
The earth tester consists of two potential terminals P1 and P2 and two current terminals C1 and C2 as shown in fig.
The E is an earth electrode whose earth resistance will be measured by the earth tester. The P and C is the auxiliary earth electrode which is used as a reference for earth resistance measurement.
The Potential electrodes P1 and current electrode C1 are shorted by the one link and connected to the earth electrode whose earth resistance will be measured.
The potential electrode P2 is connected to the auxiliary electrode P and current electrode C2 is connected to the auxiliary electrode C.
The voltage drop across the earth electrode E and potential electrode P is applied to the potential coil and current coil which will depend upon the resistance of the earth. Keep the distance between E and P is 15 to 25m and from E to C is 30 to 50m.
When hand driven generator is rotated at rated speed, the pointer of the PMMC instrument rotates and the earth tester indicate the value of the earth resistance.
The deflection of the pointer is proportional to the ratio of voltage across the pressure coil and current across the current coil. In this way, we can measure the value of earth resistance.
Advantages of earth tester
1. For a good and effective earthing system, it is necessary to measure the value of earth resistance from time to time. If found low earth resistance value, then it is increased by adding water to the earth pit. By adding the water into the earth pit, water increased the moisture content and increased the connectivity with the soil.
2. With the help of an earth tester we know the condition of earthing from time to time and we can take some steps to improve it. all things are possible with the help of earth merger only.