Earthing is most important part of the electrical technology that provides protection of personnel and equipments by the operation of the protective control gears like (ELCB) earth leakage circuit breakers and isolate the faulty circuit when insulation failure and accidental contact between high and low voltage.The earthing is the electrical connection between neutral or non current carrying electrical equipments part to the earth (Ground).The main part of the earthing is earth pit,earth electrode,connection wires and soil treatment materials like salt, charcoal etc.Earthing is two types system and equipment earthing. In system earthing the electrical connection between neutral and earth in generating station transformer used in power house.The heavy strip earth electrode use in mesh arrangements.The equipment earthing is electrical connection between non current carrying metal parts of electrical equipments and general mass of the earth like breakers,CT and PT etc. used in sub stations.In other word the earthing that restrict the voltage of live conductor with respect to the potential of earth.In practice there are three types of earthing method adopted a direct earthing system,earth neutral system and (ELCB) earth leakage circuit breakers system. In direct earthing system the all installation directly connected to earth not through the trip coil of the an earth leakage circuit breaker.In earth neutral earthing system the all installation connected to earth in addition connected within the installation to the neutral conductor of the supply system.In (ELCB) earth leakage circuit breakers earthing system the part of an installation which are sought to be protected are connected to an earth electrode through the coil of ELCB which control the supply to that part of the installation also in addition to the equipement earth.The value of earth resistance kept low including earth electrode ,connection wire etc.for the operation of protective devices i.e. maintain the voltage between neutral and earth potential should be zero for good earthing, practically it is not possible due to presence of some resistance.It is must to maintained low earth resistance when a circuit with large current carrying capacity is protected by a high capacity fuse. If the value of earth resistance is high than the fault current not flowing to the earth cased fire as heating and arcing at point of high resistance which can set any inflammable object in the vicinity on fire.So in dry season to reduced the resistivity of soil by pouring the water in earth electrode and maintain the moisture. The value of earth electrode resistance depends upon the electrical resistivity of soil in which the electrode is installed.The wet marshy soil,clayey or loamy mixed with small quantity of sand ,closely packed soil with fine grain and vegetation indicates soil with good moisture content is best for installation of earth electrode for getting low resistance path to flow of fault current.If moisture fall below 20% the resistivity of soil increases sharply so it is necessary to keep up the moisture level periodic addition of water.Where the soil is dry the artificial treatment of soil required done by adding of sodium chloride (common salt) calcium chloride and sodium chloride.The salt and charcoal added for the same purpose.The other method of treatment of soil by adding the chemical in soil.The material is suitable for earth electrode should be able to resist corrosion for a long time e.g. Galvanize iron,copper,and cast iron.Based on shape the earth electrode are Rod or Pipe, Strip and Plate. The minimum size of earth Rod electrode 16 mm for steel and 12.5mm for copper rod in 2.5meter in length.In case of pipe earthing electrode minimum inner dia 40mm for galvanize iron and 100mm for cast iron pipe in 2.5meter in length.In case of strip minimum cross sectional area for copper 20X3mm and 25X4mm for galvanize iron. When we used conductor for the earthing diameter 4mm(8 SWG) for copper and 6mm for galvanize iron.The length of buried conductor shall not be less than 15mtr.In case of plate electrode the minimum size of plate 60x60x3mm for copper and 60x60x6mm for iron and steel.Plate earthing is suitable for generating station.The driven electrodes earthing is suitable for soil free from stone , rock and undistributed soil so no excavation is necessary and cost of installation is low.Generally GI pipe are suitable for incomer breaker up to 200 ampere and GI plate earth electrode suitable for incomer breaker more than 200 ampere, neutral earthing of transformer as well as DG set up to 500 KVA.Copper plate earth electrode suitable for neutral earthing of transformer as well as DG set above 500 KVA. The size of earth bus 50mmX5mm of two copper strip in 11KV sub station and neutral of transformer, generator shall not be connected to this earth bus. The earth electrode fixed by welding ,bolting and clamped joints.All the surfaces where connections were made shall be free from grease,paint,dirt or any other foreign materials. All plugs and three pins sockets ,the earth pin connected to the earth wire.It is must to earthing of domestic appliances accepting those double insulation.The special earthing provided for data processing machine known as 'Clean Earth'. For protection against static charges such as hospital and operation theatres the static earthing suitable. The earth tester used for the measurement of earth resistance it contains three electrodes, one under measurement other two is potential electrode and current electrode.The size of the potential and current electrode is 15mm diameter and length is 400mm long bars.The value of earth resistance should not more than two ohms another method to measure the earth resistance by voltmeter and ammeter method the electrode arrangement usually as the earth tester and find the earth resistance V/I reading shown in voltmeter and ammeter. The instrument used for measurement of resistivity contains four spikes two are current electrode and two are potential electrode driven into the ground at equal interval 'A' .The earth resistivity is given byρ=2ㄫAR where R is the steady reading on the instrument. The testing of insulation resistance between an insulated conductor and earth with the help of 500 volts megger. The phase conductor connected Line marked on megger and earth continuity conductor connected to an earth marked on megger.The handle is turned to indicate the steady reading on the instrument.The protection of all building and structure against lighting strokes like power house,large warehouses and monuments is done by installing lighting arresters at the top of the structure and connecting the same to the general mass of earth through a connecting conductor and earth electrode so that the structure is not subjected to the dangerously high potential of the cloud discharge and system should be designed and maintained with low ground resistance.The conductor laid around the building near the top to the interconnected the various air termination called roof conductors protection against lightning strokes. The earthing system used for lighting protection must be independent of the equipment earthing system.
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