What is Earth Resistance? How to measure it?
The Earth faults are hazardous and hence need proper earthing to prevent fault current from entering into anybody or metallic object. The purpose of earthing is to minimize the effect of transient voltage that occurred due to a strike of lightning.
How is earth resistance measured?
Earthing connections are made by driving the earth electrode into several places. An earth electrode consists of a metal pipe or conducting plate connected to the earth.
There are different materials used in the making, such as copper, aluminium, steel or galvanized iron. Various factors affect the earth resistance, like the composition of the soil, temperature, moisture content and depth of electrode. Earthing enables leakage current to flow away safely and is linked to an automatic cut off device (which ensures power supply). There are different components included in an earthing system like earth electrodes, main earth terminals or bars, earthing conductors, protective conductors, equipotential bonding conductors, electrically independent earth electrodes (for measurements), termination fittings, bonding, welding kits and other materials.
What are the methods of measuring earth resistance?
There are different earth resistance measurement methods used depending on the type of neutral system, the type of installation (residential, industrial, urban environment, rural environment, the possibility of cutting off the power supply. Four variables affect the earth resistance of a grounding system which includes:
1. The composition of the soil
2. The moisture content of the soil
3. The temperature of the soil
4. The depth of the electrode
The resistance of the earth electrode depends upon the resistivity of the soil in which the electrode is inserted. Therefore, it is crucial to measure the resistivity during the design of any earthing installations.
Earth resistance is the resistance of the ground electrode measured to perform a resistance check. With additional measurements like the voltage, the test electrode moved 10% of the original voltage electrode to the earth system, separating it from the initial position and 10% closer than its original position. When both of them agree with the measurement within the required level of accuracy, the test stakes are placed in a correct position, and the resistance can be obtained by averaging all three results.
Before starting any earth resistance measurements, the maximum value for correct earthing needs to be measured. There are six basic test methods to measure the earth resistance:
1. Four-point method (Wenner method)
2. Three terminal methods (falloff potential method/ 68.1% method)
3. Two-point method (dead earth method)
4. Clamp-on test method
5. Slope method
6. Star delta method
One of the most commonly used methods for measuring earth resistance is the fall of potential method. It is based on IEEE standards and is suitable for transmission line structures. This method comprises an earth electrode and two electrically independent test electrodes. The electrodes are (P) potential and (C) current which must be electrically independent.
Fig: Fall of potential method
Source - Electrical Engineering Portal
It considers three points of ground contacts, 1) an earth electrode, 2) a current probe 3) a voltage probe. Hence the digital earth tester injects current into the tower footing earth electrode under test. An alternating current (I) is passed through the outer electrode (C), the voltage is measured by the inner electrode (P) at an intermediary point between the inner and outer electrodes. The current flows from the earth to the remote current probe and returns to the tester. As the current flow, a voltage drop takes place. This voltage drop is proportional to the amount of current flow and the resistance of the earth electrode.
Several locations calculate resistance by moving the voltage probe at regular intervals (each is equal to 10% distance) under test and current. The display of the digital earth tester shows the resistance value. The earth resistance is calculated by simply using ohms law R=V/I. For earth resistance, the crucial factor is to position the auxiliary test electrode C far away from the earth electrode, under test to ensure that the (auxiliary test electrode) P will lie outside the resistance areas of both the earth system and the other test electrode.
Slope method for large earthing systems such as power stations. In this method, it is possible to calculate the actual resistance. The star-delta method is well suited in areas with large systems or rocky terrains, where there would be difficulties placing test electrodes. In the star delta methods, three test electrodes are at the corners of the equilateral triangle with the earthing system in the centre. Measurements are taken for the total resistance between adjacent electrodes, between each electrode and the earthing system. The four potential methods or Wenner method is similar to fall of the Potential, except that the number of measurements is with the voltage electrode at different positions, and a set of equations calculate the theoretical resistance of the system. Hence different methods are applicable depending upon the area.
The earthing testers are troubleshooting tools to help you maintain uptime. All the ground and ground connections need to be checked at least annually as a part of a predictive maintenance plan. The earth resistance would be increased to more than 20% during the periodic checks to ensure an investigation at the source of the problem and make the correction to lower the resistance by replacing or adding ground rods to the ground system. The Earth Resistance profile varies between 10 Ohms and 20 Ohms. The soil identifications, earthing, and intensive field measurements show that the soil resistivity values depend on the soil type. In rocky areas, the resistance could be lowered by a buried network of (well-designed) earth mats or by a network of the buried counterpoise earth wire to reduce the effect of lightning stroke. For effective earthing of electrical systems, soil resistivity should be up to the mark.
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