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Why tripping not occur when the transmission lines (11KV &
above) breaks? Doesn't
we provide earthfault protection for it?
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Answer Posted / k.prakashchandra
All the tr lines should be provided with EF protection.11
and 33 KV lines are provided with 2 OC and 1 EF relays or 3
OCR and 1 EFR.The unbalanced current passes through the EFR
which is usually set at 20% of OCR setting.The EFR is
connected between star points of CT's and relays. For
higher voltages distance relays are provided with OCR and
EFR as back up protection.If the line breaks i.e.a
conductor opens out without touching earth .The distance
relay does not act as there is no heavy current.The EFR
will trip only if the unbalanced current exceeds the set
value .Hence there will not be any tripping if the line is
not loaded sufficiently. OCR=over current relay, EFR =earth
fault relay
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