The Same electrode with same GCIM type and quantity behaves differently at various locations on the earth. As we have discussed earlier the factors which affect the performance and conductivity of earthing. Soil structure is one of them. Due to this variation in nature or structure of soil; we get different resistance value at various locations with the same earthing electrode. GCIM is Ground Conductivity Improving Material, which is mixed with soil and placed around the electrode during the process of earthing installation.
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When all metallic parts in electrical equipment are grounded then if the insulation inside the equipment 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. 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 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. 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. 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.
Conventional methods of earthing Generally for plate type earthing normal Practice is to use Cast iron plate of size mm x mm x12 mm. OR Galvanized iron plate of size mm x mm x6 mm. 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.
The purpose of coal and salt is to keep wet the soil permanently. The salt percolates and coal absorbs water keeping the soil wet. Care should always be taken by watering the earth pits in summer so that the pit soil will be wet.
Coal is made of carbon which is good conductor minimizing the earth resistant. It will be better to use GI Pipe of size 2. 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. 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.
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.
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.
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. Common salt is most effective in improving conductivity of soil. But it corrodes metal and hence discouraged. Climate Condition Increase or decrease of moisture content determines the increase or decrease of soil resistivity.
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 4 — ohm-meter, whereas for rocky or gravel soils, the same may be well above ohm-meter.
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. Effect of grain size and its distribution Grain size, its distribution and closeness of packing are also contributory factors, since they control the manner in which the moisture is held in the soil. Effect of seasonal variation on soil resistivity: Increase or decrease of moisture content in soil determines decrease or increase of soil resistivity. Thus in dry weather resistivity will be very high and during rainy season the resistivity will be low.
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. 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. 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. Current Magnitude A current of significant magnitude and duration will cause significant drying condition in soil and thus increase the soil resistivity.
It has a voltage source, a meter to measure Resistance in ohms, switches to change instrument range, Wires to connect terminal to Earth Electrode and Spikes. The terminals are connected by wires as in illustration. All spikes are equidistant and in straight line to maintain electrical continuity. Take measurement in different directions. Distance between the spikes in cm. These two spikes are kept in same line at the distance of 25 meters and 50 meters due to which there will not be mutual interference in the field of individual spikes.
If we rotate generator handle with specific speed we get directly earth resistance on scale. Resistance must be verified by increasing or decreasing the distance between the tester electrode and the spikes by 5 meter. Four Point Method In this method 4 spikes are driven in earth in same line at the equal distance. Now if we rotate generator handle with specific speed, we get earth resistance value of that place. In this method error due to polarization effect is eliminated and earth tester can be operated directly on A.
The resistivity of the soil and the physical dimensions of the electrode play important role of resistance of Rod with earth. The material resistivity is not considered important role in earth resistivity. Any material of given dimensions would offer the same resistance to earth.
Except the sizing and number of the earthing conductor or the protective conductor. From the above calculation the GI Pipe electrode offers a much lesser resistance than even a copper plate electrode. As per IS Pipe, rod or strip has a much lower resistance than a plate of equal surface area.
Length of Pipe Electrode and Earthing Pit The resistance to earth of a pipe or plate electrode reduces rapidly within the first few feet from ground mostly 2 to 3 meter but after that soil resistivity is mostly uniform. After about 4 meter depth, there is no appreciable change in resistance to earth of the electrode.
Except a number of rods in parallel are to be preferred to a single long rod. Amount of Salt and Charcoal more than 8Kg To reduce soil resistivity, it is necessary to dissolve in the moisture particle in the Soil.
The salt content is expressed in percent by weight of the moisture content in the soil. Considering 1M3 of Soil, the moisture content at 10 percent will be about kg. Length Vs Diameter of Earth Electrode Apart from considerations of mechanical strength, there is little advantage to be gained from increasing the earth electrode diameter with the object in mind of increasing surface area in contact with the soil.
The usual practice is to select a diameter of earth electrode, which will have enough strength to enable it to be driven into the particular soil conditions without bending or splitting. Large diameter electrode may be more difficult to drive than smaller diameter electrode. The depth to which an earth electrode is driven has much more influence on its electrical resistance characteristics than has its diameter.
The earthkng need to be the electricity board. These should be interlocked with operation with an incoming mains supply — When the the incoming mains supply circuit-breaker so that generating sets have direct or resistance earthing they are open during parallel operation of the set and are used as standby to the mains, earthing with the mains, but one is closed at all other contactors are needed if parallel running is a times see Fig. In addition the hinged design shall permit doors being completely removed when necessary. NOTE 1 — These ratings correspond to those of fuse-bases. Soil Resistivity and Corrosion Range of Soil Resistivity Class of Soil ohm-metres Less than 25 Severely corrosive Moderately corrosive Mildly corrosive Above Very mildly corrosive This following methods can be adopted to safeguard Conductor against excessive corrosion: August 31, at 6: July 2, at 8: For interconnecting switchboards protected through the earth return path. Jignesh Parmar Jignesh Parmar has completed M.
IS 3043: Code of Practice for Earthing (First Revision)
Other terminologies[ edit ] While the national wiring regulations for buildings of many countries follow the IEC terminology, in North America United States and Canada , the term "equipment grounding conductor" refers to equipment grounds and ground wires on branch circuits, and "grounding electrode conductor" is used for conductors bonding an earth ground rod or similar to a service panel. The neutral is grounded earthed at each consumer service point thereby effectively bringing the neutral potential difference to zero along the whole length of LV lines. In the UK and some Commonwealth countries, the term "PNE", meaning Phase-Neutral-Earth is used to indicate that three or more for non-single-phase connections conductors are used, i. Resistance-earthed neutral India [ edit ] A resistance earth system is used for mining in India as per Central Electricity Authority Regulations. Due to the fault current restriction it is safer for gassy mines.
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Tojazragore Alternate layer of mm of Salt and charcoal power shall be used up to 2. Operating preparation M x x x1 x tective conductor shall be not less than the room appropriate value shown in Table 7. Single phase system depicted for ease. Central monitoring room connected to the system of protective conductors.