Answer / ronik

1)The reason when a double line fault occurs on secondary
side then if the transformer is of DY11 type then if 2
over-current relays are used on primary side instead of 3
O/C relays then the phase in which no relay is placed will
carry double the fault current and the in current in the
either phase will be low enough that a relay can pick
up...and meanwhile the damage will be done to the system...

2)Moreover the ground fault on the secondary side will not
have any effect on the earth fault relay on the primary side...

3)on secondary side each and every fault will be detected by
the 2 O/C and 1 earth fault relay easily...

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