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An 11kv system consisting of an underground cable to a sub
station about 300 meters away was shut down for about 10
hrs. On energising, it tripped off on earth fault about
10---20 mins later 6 times but since than it is not
tripping.its already 3 weeks now. The SPAJ was found to be
fine. what could be the cause.No test had been done to
determine the cause of the trippings till now because of
demand.

Answer Posted / k.prakashchandra

If only you had replaced the breaker cubicle and test
charged you would have come to conclusion.If the all 3 poles
do not close almost at the same time and there is a time
gap, the breaker is likely to trip on EFR due to the
unbalanced current in case of 2 OCR + 1 EFR protection
scheme.In such cases it is advisable to test charge on no
load during the second time and confirm the mechanical fault
which is not consistent & does not occur always.

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