Answer / shailesh aggarwal

Basically Earth faults are caused if high fault current
flows through the neutral of the transformers. This type of
fault arises mainly in Star connected transformers.Earth
faults are specified in many categories:
1. Restricted Earth faults(Inside the zone HT/LT side of
trafo) Connection are done b/t Phase CT & Neutral CT of
tranformer)Indicated as REF(64)
2. Standby Earth Faults(Directly linked with neutral of
trafo)connection only done b/t neutral CT.Indicated as SBEF
(51N).
Both the protection give signals to the numerical relays
and cause the tripping as earth faults and directly trip
the transformer.
Thus in order to save transformers a proper earthing is
required whether solid or usong NGRs.

Answer / nilkanth tailor

EARTH FAULT REQUIRE C.T.(CURRENT TRANSFORMER)..TO SENSES
IMBALANCE IN 3 PHASE..GENERALLY 3PHASE SYSTEM IS IN BALANCE
CKT...IF SINGLE OR MULTI PHASE WAS EARTHED..AND AT THAT
TIME IMBALANCE PRODUCED..CT WILL BE DETECT FAULT CURRENT
AND CIRCUIT BREAKER WILL BE OPEN...THATS CALLED EARTH FAULT
PROTECTION OF X'MER...

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