Earthing in electrical network purpose, methods and measurement

Transcription

1 Earthing in electrical network purpose, methods and measurement Earth / Ground Conductive ground whose electrical potential at each point is set equal to zero, It is a conductor which provides a low impedance path to prevent hazardous voltages during abnormal and fault conditions Earthing in electrical network The main reason for doing earthing in electrical network is for the safety. When all metallic parts in electrical equipment s are grounded then if the insulation inside the equipment s fails there are no dangerous voltages present in the equipment case. If the live wire touches the grounded case then the circuit is effectively shorted and fuse will immediately blow. When the fuse is blown then the dangerous voltages are away. Purpose of Earthing 1. Safety for Human life / Building /Equipment To save human life from danger of electrical shock or death by blowing a fuse i.e. To provide an alternative path for the fault current to flow so that it will not endanger the user To protect buildings, machinery & appliances under fault conditions. To ensure that all exposed conductive parts do not reach a dangerous potential. To provide safe path to dissipate lightning and short circuit currents. To provide stable platform for operation of sensitive electronic equipment s i.e. to maintain the voltage at any part of an electrical system at a known value so as to prevent over current or excessive voltage on the appliances or equipment. 2. Over voltage protection Lightning, line surges or unintentional contact with higher voltage lines can cause dangerously high voltages to the electrical distribution system. Earthing provides an alternative path around the electrical system to minimize damages in the System. 3. Voltage stabilization There are many sources of electricity. Every transformer can be considered a separate source. If there were not a common reference point for all these voltage sources it would be extremely difficult to calculate their relationships to each other. The earth is the most omnipresent conductive surface, and so it was adopted in the very beginnings of electrical distribution systems as a nearly universal standard for all electric systems.

2 Conventional methods of earthing 1. Plate type Earthing Generally for plate type earthing normal Practice is to use Cast iron plate of size 600 mm x600 mm x12 mm. OR Galvanized iron plate of size 600 mm x600 mm x6 mm. OR Copper plate of size 600 mm * 600 mm * 3.15 mm Plate burred at the depth of 8 feet in the vertical position and GI strip of size 50 mmx6 mm bolted with the plate is brought up to the ground level. These types of earth pit are generally filled with alternate layer of charcoal & salt up to 4 feet from the bottom of the pit. 2. Pipe type Earthing For Pipe type earthing normal practice is to use GI pipe [C-class] of 75 mm diameter, 10 feet long welded with 75 mm diameter GI flange having 6 numbers of holes for the connection of earth wires and inserted in ground by auger method. These types of earth pit are generally filled with alternate layer of charcoal & salt or earth reactivation compound.

3 3. Copper coated solid rod earthing Copper coated solid rod of diameter 17mm, 19mm, 20mm,25mm Low carbon steel rod bonded with copper for contact surface and corrosion resistance Stainless steel clamp to connect flat or cable Factors affecting on Earth resistivity 1. Soil Resistivity It is the resistance of soil to the passage of electric current. The earth resistance value (ohmic value) of an earth pit depends on soil resistivity. It is the resistance of the soil to the passage of electric current. It varies from soil to soil. It depends on the physical composition of the soil, moisture, dissolved salts, grain size and distribution, seasonal variation, current magnitude etc. In depends on the composition of soil, Moisture content, Dissolved salts, grain size and its distribution, seasonal variation, current magnitude. Importance of soil resistivity: To choose the location and type of earth electrode To define the electrical specification of earth electrode and earth network Optimize the construction cost for the earth electrodes and earth networks ( required earth resistance is obtained more quickly )

4 Soil resistivity depends on: 1. Soil composition 2. Moisture content, and 3. Temperature Formula to calculate soil resistivity: ρ (Rho) ρ = 2 X π X A X R Ω m ρ = average soil resistivity, A = dist between electrodes in meter, R = Measured resistance in ohms from the earth tester 2. Soil Condition Different soil conditions give different soil resistivity. Most of the soils are very poor conductors of electricity when they are completely dry. Soil resistivity is measured in ohm-meters or ohm-cm. Soil plays a significant role in determining the performance of electrode. Soil with low resistivity is highly corrosive. If soil is dry then soil resistivity value will be very high. If soil resistivity is high, earth resistance of electrode will also be high. 3. Moisture Moisture has a great influence on resistivity value of soil. The resistivity of a soil can be determined by the quantity of water held by the soil and resistivity of the water itself. Conduction of electricity in soil is through water. The resistance drops quickly to a more or less steady minimum value of about 15% moisture. And further increase of moisture level in soil will have little effect on soil resistivity. In many locations water table goes down in dry weather conditions. Therefore, it is essential to pour water in and around the earth pit to maintain moisture in dry weather conditions. Moisture significantly influences soil resistivity.

5 4. Dissolved salts Pure water is poor conductor of electricity. Resistivity of soil depends on resistivity of water which in turn depends on the amount and nature of salts dissolved in it. Small quantity of salts in water reduces soil resistivity by 80%. Common salt is most effective in improving conductivity of soil. But it corrodes metal and hence discouraged. 5. Climate Condition Increase or decrease of moisture content determines the increase or decrease of soil resistivity. Thus in dry whether resistivity will be very high and in monsoon months the resistivity will be low. 6. Physical Composition Different soil composition gives different average resistivity. Based on the type of soil, the resistivity of clay soil may be in the range of ohm-meter, whereas for rocky or gravel soils, the same may be well above 1000 ohm-meter. 7. Location of Earth Pit The location also contributes to resistivity to a great extent. In a sloping landscape, or in a land with made up of soil, or areas which are hilly, rocky or sandy, water runs off and in dry weather conditions water table goes down very fast. In such situation back fill compound will not be able to attract moisture, as the soil around the pit would be dry. The earth pits located in such areas must be watered at frequent intervals, particularly during dry weather conditions. Though back fill compound retains moisture under normal conditions, it gives off moisture during dry weather to the dry soil around the electrode, and in the process loses moisture over a period of time. Therefore, choose a site that is naturally not well drained. 8. Effect of current magnitude Soil resistivity in the vicinity of ground electrode may be affected by current flowing from the electrode into the surrounding soil. The thermal characteristics and the moisture content of the soil will determine if a current of a given magnitude and duration will cause significant drying and thus increase the effect of soil resistivity.

6 9. Area Available Single electrode rod or strip or plate will not achieve the desired resistance alone. If a number of electrodes could be installed and interconnected the desired resistance could be achieved. The distance between the electrodes must be equal to the driven depth to avoid overlapping of area of influence. Each electrode, therefore, must be outside the resistance area of the other. 10. Obstructions The soil may look good on the surface, but there may be obstructions below a few feet like virgin rock. In that event resistivity will be affected. Obstructions like concrete structure near about the pits will affect resistivity. If the earth pits are close by, the resistance value will be high. Components of earth system Earth conductor Connection between the earth conductor and earth electrode Earth electrode

7 Spacing between earth electrodes There must be proper spacing between ground electrodes to reduce or eliminate their spheres of influence (Minimum spacing shall be > Length of the electrode). Without proper spacing, their sphere of influence will intersect and resistance will not lowered. Earth Resistance: The earth resistance of an electrode is made up of resistance of metal, contact resistance between the electrode and the soil, resistance of soil from the electrode surface in the, geometry setup for the flow of current from the electrode to the earth. Variables that affects Earth resistance: 1. Length / depth of the Copper coated earth rod Soil is not consistent in its resistivity and can be highly unpredictable, it is critical when installing earth electrode that it is below frost line. This is done so that the resistance to earth will not be greatly influenced by the freezing of the surrounding soil. By increasing the length of the electrode the resistance will be reduced additionally by 40 %

8 In areas where deeper rods are physically impossible alternate methods including earth enhancing mineral compounds are recommended 2. Diameter of the ground rod Increasing the Diameter of the rod has very little effect in lowering the resistance, By doubling the diameter of the rod, Resistance will decrease only by 10 % 3. Number of earth electrodes In this method multiple earth electrodes are used and connected in parallel., This method is effective only if the spacing between the electrode is depth of the rod., Without proper spacing, their sphere of influence will intersect and resistance will not lowered Method adapted to measure earth resistance value Fall-of-Potential Measurement The Fall-of-Potential test method is used to measure the ability of an earth ground system or an individual electrode to dissipate energy from a site. How Does the Fall-of-Potential Test Work? First, the earth electrode of interest must be disconnected from its connection to the site. Second, the tester is connected to the earth electrode. Then, for the 3-pole Fall-of-Potential test, two earth stakes are placed in the soil in a direct line away from the earth electrode. Normally, spacing of 20 meters (65 feet) is sufficient. A known current is generated by the earth tester between the outer stake (auxiliary earth stake) and the earth electrode, while the drop in voltage potential is measured between the inner earth stake and the earth electrode. Using Ohm's Law (V = IR), the tester automatically calculates the resistance of the earth electrode. Connect the ground tester as shown in the picture. Press START and read out the RE (resistance) value. This is the actual value of the ground electrode under test. If this ground electrode is in parallel or series with other ground rods, the RE value is the total value of all resistances.

9 How Do You Place the Stakes? To achieve the highest degree of accuracy when performing a 3 pole ground resistance test, it is essential that the probe is placed outside the sphere of influence of the ground electrode under test and the auxiliary earth. If you do not get outside the sphere of influence, the effective areas of resistance will overlap and invalidate any measurements that you are taking. The table is a Depth of the ground electrode guide for appropriately setting the probe (inner stake) and auxiliary ground (outer stake). Distance to the inner stake Distance to the outer stake 2 m 15 m 25 m 3 m 20 m 30 m 6 m 25 m 40 m 10 m 30 m 50 m To test the accuracy of the results and to ensure that the ground stakes are outside the spheres of influence, reposition the inner stake (probe) 1 meter (3 feet) in either direction and take a fresh measurement. If there is a significant change in the reading (30 %), you need to increase the distance between the ground rod under test, the inner stake (probe) and the outer stake (auxiliary ground) until the measured values remain fairly constant when repositioning the inner stake (probe). For further information please do call us.. Also we request you to kindly share your valuable comments, suggestions and feedback to improve us