EARTHING/GROUNDING SYSTEMS, AN OVERVIEW EARTHING
Earthing/GroundingSystems,anOverview
Earthing systems are installed to protect structures from the threats of fault Theresistanceofaconductordependsontheatomicstructureofthematerial.
electrical currents. Electricity has the capacity to kill people instantly and Resistivityisthepropertyofamaterialthatmeasuresitsabilitytoconductelectricity.
it is for this reason that all constructions must place great emphasis on
quality in their electrical safety systems.
Whatissoilresistivity?
Earthing systems are engineered to provide a safe, low impedance path to
Theresistanceperformanceofanearthelectrodeisheavilyinfluencedbythe
earth/groundforfaultcurrents.Earthingisalsousedtoprotectandpreserve
resistivity of the soil in to which it is driven. Soil resistivity is the resistance
equipmentandassets,whichisessentialinensuringtheongoingfunctionality
measured between two opposing surfaces of 1m³of homogeneous ground
ofasite.Withoutquestionthemostimportantcableinanyelectricalsystem
material,usuallymeasuredinΩm.Theevaluationoftheresistivityofthelocalsoil
istheearthcable,whichdirectsfaultcurrentssafelyawayfromalocationby
determinesthebestlocation,depthandsizeoftheelectrodesinanearth/ground
allowingittodissipateintothestrataoftheearth.Electricity,beitintheform
system.Soilresistivityisdeterminedlargelybyitscontentofelectrolytes,which
of a supply or lightning, is always seeking the most direct and easy path to
consistofmoisture,mineralsanddissolvedsalts.Becausesoilresistivitydirectly
groundandthroughastuteplanningasitecandictateandprovidethatroute.
relatestomoisturecontentandtemperature,itisreasonabletoassumethatthe
If a location does not have an adequate earthing provision it may lead to
resistanceofanyearthsystemwillvarythroughoutthedifferentseasonsofthe
electricityfindingalternatemethodstoground,thisisoftenthroughelectrical year. Since both temperature and moisture content become more stable at
equipment,butincertaincircumstancesitcanbeviahumanbeings.Another greaterdepthsbelowthesurfaceoftheground,itfollowsthatanearthsystem
mainobjectiveofearthingelectricalsafetysystemsistoestablishacommon tobemosteffectiveatalltimesshouldbeconstructedwiththeearthroddriven
reference potential for the power supply system, building structure, plant downaconsiderabledepthbelowthesurface.Bestresultsareobtainedifthe
steelwork, electrical conduits and instrumentation system. Performance of earthrodelectrodepenetratesthewatertable.
anyearthsystemisdictatedbysoilresistivity;stratification;typeandsizeof
electrodeused;locationanddepthtowhichtheelectrodeisburied;moisture
andchemicalcontentofthesoil,buttheseshallbeexaminedinfurtherdetail
inthefollowing.
WhatisResistance?
Anyeffectiveearthsystemmusttakeaccountofarangeofdifferingelectrical
resistances,butwhatisresistance?Resistanceisthepropertyofaconductor
whichopposeselectriccurrentflowwhenavoltageisappliedacrosstwoends.
Its unit of measure is the Ohm (Ω) and the commonly used symbol is R.
Resistanceistheratiooftheappliedvoltage(V)totheresultingcurrentflow(I)
asdefinedbythewell knownlinearequationfromOhm’sLaw:
V=IxR
Where:Visthepotentialdifferenceacrosstheconductor(Volts).
Iisthecurrentflowingthroughtheconductorin(Amperes).
RistheResistanceoftheconductorin(Ohms).
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