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.

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An 11kv system consisting of an underground cable to a sub station about 300 meters away was shut d..

Answer / 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.

Is This Answer Correct ?    1 Yes 0 No

An 11kv system consisting of an underground cable to a sub station about 300 meters away was shut d..

Answer / pillay

thanks but I had managed to ON the system and remained
energized with some load for a few minutes (10 -20 mins)
before it tripped again on earth fault , it happened 6 times
on (4/6/2011) and till today (13/07/2011) no problem. we
are using ABB spaj oc/ef relay.

Is This Answer Correct ?    0 Yes 0 No

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