EARTHING

To save human life from danger of electrical shock or deathTo 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 maintain the line voltage constant to

prevent over current or excessive voltage on the appliances or equipment .

Earth Resistance Value

Major power station= 0.5 Ohm.Major Sub-stations= 1.0 OhmMinor Sub-station = 2 OhmNeutral Bushing. =2 OhmService connection = 4 OhmMedium Voltage Network =2 OhmL.T.Lightening Arrestor= 4 OhmL.T.Pole= 5 OhmH.T.Pole =10 OhmTower =20-30 Ohm

Nos. of Earthing

Single phase 1 HP motor –01 earth Socket/geyser/heater/fan/1phase

appliance–01 earth Three phase

motor/generator/transformer/gadgets –02 earth

Metallic support of O/H line after every 03 poles –02 earth

Stay wire for overhead line -01 earthNeutral and frame earthing-separately

While doing Earthing

Distance –from the equipment not more than 1.5 meter

Cross section of earth continuity conductor-not less than 14 SWG copper wire

Cross section of earth wire-not less than half of the main wire

Voltage between Earth Pit conductors to Neutral of Mains Supply 220V AC 50 Hz it should be less than 2.0 Volts.

Factors Affecting Earth Resistance

Soil resistivity Soil condition Moisture Dissolved saltsClimate conditionsPhysical 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 for domestic, small substations

Strip earthing :where earth bed have rocky soil

Rod earthing: done is sandy area/small lamp post

Pipe Earthing

GI pipe [C-class] of 75 mm diameter

10 feet long welded with welded flat having 4 numbers of holes for connections

Earth pits are generally filled with alternate layer of charcoal & salt or earth reactivation compound.

Strip Earthing

Strip earthing :where earth bed have rocky soil

Copper strip size 5 SWG or 25mmX1.6mm is buried in horizontal trenches

Earth resistance remains high i.e. 5 ohm

Plate Earthing

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.

Earth pit are generally filled with alternate layer of charcoal & salt up to 4 feet from the bottom of the pit.

Insulation Resistance and Megger

Earth resistance test

Three point method

The distance of Current Spike from Earth Electrode D = 60 ft, Then, distance of Potential Spike would be 62 % of D = 0.62D i.e.  0.62 x 60 ft = 37 ft.

Difference between Insulation Resistance and Earth resistance Insulation

Resistance :measured between earth and conductor, value in tens/thousand/mega ohm, in built DC generator

Earth resistance: measured for a earthed body or neutral point by earth resistance tester, value less than 10 ohm

Specification Of Earthing as per ISI

Sr. Capacity Size of earth wire Size Of Earth Electrode

Cu GI Cu GI

1 01- 10 HP 8 SWG 8 SWG 60cmX60cmX3.18mm 60cmX60cmX6.35mm

2 10 – 15 HP 8 SWG 6 SWG 60cmX60cmX3.18mm 60cmX60cmX6.35mm

3 15 – 30 HP 6 SWG 4SWG 60cmX60cmX3.18mm 90cmX90cmX6.35mm

4 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 noRepair of damage/broken pit cover Tighten the earth connectionsReplace broken earth leadsCorroded part change immediately Pass earth conductor through GI pipeMandatory : bi-yearly and recorded

Typical Earthing In House Wiring

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