The instrument or device used to find the resistance of earth is known as Earth Tester or Earth Resistance Tester. Every equipment in the power system is earthed to ensure protection for both the system and personnel. Earthing is the process of establishing electrical connectivity between noncurrent bearing parts of an electrical device and earth throw a low resistance path.
This protects the person working with the electrical device from leakage current flowing through its body. By maintaining all the equipment of the power system at earth potential, unexpected electrical charges due to lightning or static electricity are dissipated to the earth through earthing, thus protecting the system.
The resistance of the electrical connectivity provided between earth and electrical device to dissipate these leakage currents and unexpected electrical charges should very low. So that leakage or fault currents, static charge build-up in the conductors, the stray voltage in the line, sudden excessive induced voltages due to lightning surges get discharged directly to earth without affecting the device and personnel.
Hence, it is important to know the resistance of the earth before earthing any electrical device. The determination of earth resistance can be done with the help of an earth tester. It is recommended to maintain an earth resistance of less than 5Ω for domestic installations and less than 1Ω for power stations.
Construction of Earth Tester :
The construction of the earth tester is similar to the Megger with small modifications adding additional features. It consists of a dc generator, a rotating current reverser, rectifier, and a PMMC instrument to measure resistance. The schematic diagram of the earth tester for measuring the resistance of earth soil is shown below.
The current reverser is used to convert dc to ac so that the earth resistance test can be done with alternating current. The ac supply from the current reverser is given to the earth electrode that is under test. Here the use of ac supply for measuring the earth resistance is, the undesirable effects due to back emf produced in the soil is eliminated.
The rectifier is used because the tester is equipped with a moving coil instrument to measure resistance. Thus the pressure and the current coil of the instrument needs dc supply. The rectifier converts alternating current coming from the electrode to direct current and supplied to the current coil.
The pressure or potential coil is connected across the generator and it measures voltage applied. The current coil is connected in series with the dc generator and it measures current through the electrode. Both current reverser and rectifier are connected with the help of two commutators mounted on the shaft of the generator. Two pairs of fixed brushes are provided with each commutator to transfer power from the rotating shaft to the stationary parts.
Working of Earth Tester :
The earth tester is provided with four terminals C1, C2, P1, and P2. The terminals C1 and C2 are the current terminals while terminals P1 and P2 are known as potential terminals. The electrode under test i.e., earth electrode E is connected to the terminals C1 and P1 by shortening them. The two auxiliary electrodes P (potential electrode) and C (current electrode) are connected to the terminals P2 and C2 respectively.
The potential and current coils of the PMMC instrument are connected such that, the voltage drop between electrodes E and P is applied across the potential coil and current through the current coil depends upon the resistance of the earth.
When the generator is driven at its rated speed, the pointer of the moving coil instrument deflects and the earth tester indicates the resistance of the earth. The deflection of the pointer is given by the ratio of voltage across the potential coil and current of the current coil.
Positioning of Electrodes :
While measuring the earth resistance the positioning of electrodes plays a virtual role. An incorrect position of electrodes can decrease the degree of accuracy and invalidate measurements are taken.
In order to obtain the highest degree of accuracy, the earth electrode E and current electrode C are placed at a distance from each other that the resistance areas of two electrodes in the ground should not overlap each other.
The area to which the potential gradient of an electrode exists in the ground is said to be known as the resistance area of that electrode. To ensure that the resistance area of electrode C is placed outside the resistance area of electrode E. The potential electrode P can be moved to either left or right and resistance is measured at each position.
If there is a significant difference in the resistance values with electrode P at different positions. The distance between the earth electrode E and the current electrode C must be increased. Until the measured values remain closely constant with electrode P at different positions between E and C. The repositioning of E and C is done.