Answer Posted / vishal ingle

Not necessary to use only the middle phase.
our Back up protection cosist of two overcurrent elements
and one earth fault element.
under normal condition vector (IR + IY + IB) = 0
in fact middle phase is not used for earth fault prot.
Earth fault relay is connected between the star point of
all three phases (after O/C Relay) and the earth. if any
unbalance occurs, the unbalance current will flow through
the e/f relay.

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