KYORITSU 6050 - Page 15

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(andtheprospectivefaultcurrenthighenough)toallowautomatic
disconnectionoftheelectricalsupplybythecircuitprotectiondevicewithina
prescribedtimeinterval.
Everycircuitmustbetestedtoensurethattheearthfaultloopimpedance
valuedoesnotexceedthatspecified or appropriate for theover-current
protectivedeviceinstalledinthecircuit.
ForaTTsystemtheearthfaultloopimpedanceisthesumofthefollowing
impedances;
● Impedanceofthepowertransformersecondarywinding.
● Impedanceofthephaseconductorresistancefromthepowertransformer
tothelocationofthefault.
● Theimpedanceoftheprotectiveconductorfromthefaultlocationtothe
localearthsystem.
● Resistanceofthelocalearthsystem(R).
● Resistanceofthepowertransformerearthsystem(Ro).
ForTNsystemstheearthfaultloopimpedanceisthesumofthefollowing
impedances.
● Impedanceofthepowertransformersecondarywinding.
● Impedanceofthephaseconductorfromthepower transformer tothe
locationofthefault.
● Impedanceoftheprotectiveconductorfromthefaultlocationtothepower
transformer.
Fig.9