Answer / pawar shyamkant sitaram

It depends upon the system power frequency voltage before
the fault appearing on first pole to the volage appearing
across the heathy poles after clearing the fault. The value
of first pole to clear factor is mainly decided by the
sytem grounding & ratio of zero seq.impdence to positive
sequence impedance.

Answer / ajit

iF THE THREE PHASE FAULT NOT INVOLVING THE EARTH THE
VOLTAGE ACROSS THE CIRCUIT BREAKERPOLE. FIRST POLE CLEAR
105 TIMES THE PHASE VOLTAGE IN 3 PHASE AC CIRCUIT BREAKER
ARC EXTINTION IN THE THREE POLE IS NOT SIMULTENEOUSLY AS
CURRENT IN THREE PGASES ARE MUTUALLY 120 DEGREE OUT OF
PHASE. HENCE THE POWER FREQUENCY RECOVERY VOLTQAGE OF THREE
PHASE IN WHICH ARC GETS EXTINGUISHED FIRST IS ABOUT 105
TIMES OF PHASE VOLTAGE.FIRST POLE TO CLEARE FACTOR IS THE
RATIO OF VOLTAGE BETWEEN HEALTHY PHASE TO FAULTY PHASE TO
NORMAL PHASE VOLTAGE.

Answer / sudipt jena

Actually the voltage rise due to TRV is 1.2-1.5 times that of phase voltage.And this is happened due to the 120 degree phase shift of 3 phase current leading to interrupt of current at different instants.

we have taken HV test of 2.5 k and Megger test on 11 kv HT cable.here it shows no fault but when we are charging the same cable with 11 kv supply why the VCB 11 kv gets tripped can anybody define the possible cause of fault ?

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