KYORITSU 4116A - Page 13

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Inthe event of a fault, the Fault loop impedanceshould be lowenough
(andthe Prospective Fault current higher enough) in order to havethe
automaticdisconnectionofsupplybytheinstalledprotectiondevicewithin
prescribedtimeinterval.
Everycircuitmust betested tomake surethat thefault loopimpedance
doesnotexceed thatspecified for theover currentprotectiondevice
concerned.
ForTTsystemtheFaultloopimpedanceisthesumofthefollowingpartial
impedances:
¡
Impedanceofpowertransformer'ssecondary.
¡
Phaseconductorresistancefrompowertransformertofaultlocation.
¡
Protectionconductorresistancefromfaultlocationtolocalearth
system.
¡
ResistanceoflocalearthsystemR.
¡
Resistanceofpowertransformer'searthsystemRo.
Thefigure below shows in markedline the Fault loop impedance forTT
system.
Fig.6
ForTNsystemtheFaultloopimpedanceisthesumofthefollowingpartial
impedances:
¡
Impedanceofpowertransformer'ssecondary
¡
Phaseconductorresistancefrompowertransformertofaultlocation
¡
Protectionconductorresistancefromfaultlocationtopowertransformer