EARTHING To save human life from danger ofelectrical shock or death To protect buildings, machinery &appliances under fault conditions. To ensure that all exposed conductiveparts do not reach a dangerous potential. To provide safe path to dissipate lightningand short circuit currents. To maintain the line voltage constant toprevent over current or excessive voltage onthe appliances or equipment .
Earth Resistance Value Major power station= 0.5 Ohm. Major Sub-stations= 1.0 Ohm Minor Sub-station = 2 Ohm Neutral Bushing. =2 Ohm Service connection = 4 Ohm Medium Voltage Network =2 Ohm L.T.Lightening Arrestor= 4 Ohm L.T.Pole= 5 Ohm H.T.Pole =10 Ohm Tower =20-30 Ohm
Nos. of Earthing Single phase 1 HP motor –01 earth Socket/geyser/heater/fan/1phase appliance–01 earth Three phasemotor/generator/transformer/gadgets –02earth Metallic support of O/H line after every 03poles –02 earth Stay wire for overhead line -01 earth Neutral and frame earthing-separately
While doing Earthing Distance –from the equipment not morethan 1.5 meter Cross section of earth continuity conductor-not less than 14 SWG copper wire Cross section of earth wire-not less thanhalf of the main wire Voltage between Earth Pit conductors toNeutral of Mains Supply 220V AC 50 Hz itshould be less than 2.0 Volts.
Factors Affecting Earth Resistance Soil resistivity Soil condition Moisture Dissolved salts Climate conditions Physical composition Location of earth pit Effect of grain size Effect of current magnitude Area available
Type Of Earthing Plate earthing :where digging is easy Pipe earthing :very common, used fordomestic, small substations Strip earthing :where earth bed haverocky soil Rod earthing: done is sandy area/smalllamp post
Pipe Earthing GI pipe [C-class] of75 mm diameter 10 feet long weldedwith welded flathaving 4 numbers ofholes for connections Earth pits aregenerally filled withalternate layer ofcharcoal & salt orearth reactivationcompound.
Strip Earthing Strip earthing :whereearth bed have rocky soil Copper strip size 5 SWGor 25mmX1.6mm isburied in horizontaltrenches Earth resistance remainshigh i.e. 5 ohm
Plate Earthing Cast iron plate of size 600mm x600 mm x12 mm. OR Galvanized iron plate of size600 mm x600 mm x6 mm.OR Copper plate of size 600mm * 600 mm * 3.15 mm Plate burred at the depth of8 feet in the verticalposition and GI strip of size50 mmx6 mm bolted withthe plate is brought up tothe ground level. Earth pit are generallyfilled with alternate layer ofcharcoal & salt up to 4 feetfrom the bottom of the pit.
Earth resistance test Three pointmethod The distance ofCurrent Spikefrom EarthElectrode D =60 ft, Then,distance ofPotential Spikewould be 62 %of D = 0.62Di.e. 0.62 x 60 ft= 37 ft.
Difference between Insulation Resistance andEarth resistance Insulation Resistance :measuredbetween earth and conductor, value intens/thousand/mega ohm, in built DCgenerator Earth resistance: measured for aearthed body or neutral point by earthresistance tester, value less than 10 ohm
Specification Of Earthing as per ISISr. Capacity Size of earth wire Size Of Earth ElectrodeCu GI Cu GI1 01- 10 HP 8 SWG 8 SWG 60cmX60cmX3.18mm 60cmX60cmX6.35mm2 10 – 15 HP 8 SWG 6 SWG 60cmX60cmX3.18mm 60cmX60cmX6.35mm3 15 – 30 HP 6 SWG 4SWG 60cmX60cmX3.18mm 90cmX90cmX6.35mm4 30 – 50 HP 4SWG 25mmX3 mm 90cmX90cmX6.35mm 90cmX90cmX6.35mm
Method used to reduce earth resistance Soldering/welding nuts and bolts By pouring water Increase in plate area By pouring salt and charcoal ,ratio 1:5 Depth of plate Installation of parallel electrode
Preventive Maintenance Of Earthing System Check half yearly in dry season Check for earth pit no Repair of damage/broken pit cover Tighten the earth connections Replace broken earth leads Corroded part change immediately Pass earth conductor through GI pipe Mandatory : bi-yearly and recorded