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Question
WHY IS RESTRICTED EARTH FUALT PROTECTION NEEDED?? WHY CANT 
DEIFFERENTIAL PROTECTION SUFFICE???
 Question Submitted By :: Minu
I also faced this Question!!     Rank Answer Posted By  
 
  Re: WHY IS RESTRICTED EARTH FUALT PROTECTION NEEDED?? WHY CANT DEIFFERENTIAL PROTECTION SUFFICE???
Answer
# 1
restricted earth faukt protn is for unprotected zone of
neutral to earth.
Diffetential protn will work only if the falt is with in
zone of CT so that it can get a differential current. If
there is a fault between neutral CT and earth the fault
will be feeded and no diffential current will be there as
it is balaced through primary side, secndary side & nutral.
 
Is This Answer Correct ?    19 Yes 32 No
Shipra
 
  Re: WHY IS RESTRICTED EARTH FUALT PROTECTION NEEDED?? WHY CANT DEIFFERENTIAL PROTECTION SUFFICE???
Answer
# 2
REF is a backup protection for transformer incipient
faults.
if differential fails REF comes on line.
if both protections are used , usually REF time setting
will be less than differential setting
 
Is This Answer Correct ?    15 Yes 27 No
Ram Niveth
 
 
 
  Re: WHY IS RESTRICTED EARTH FUALT PROTECTION NEEDED?? WHY CANT DEIFFERENTIAL PROTECTION SUFFICE???
Answer
# 3
mainly there is some drawbacks in differential protection.
the protection is mostly surrounding at the certain region.
for example, if the transmission line length is 10km
means,we locate differential protection in between the
transmission line we can't able to protect the outerside of
the transmission line. so that we go for earth fault
protection. earth consist the infinite resistance.
 
Is This Answer Correct ?    4 Yes 32 No
Pradeep
 
  Re: WHY IS RESTRICTED EARTH FUALT PROTECTION NEEDED?? WHY CANT DEIFFERENTIAL PROTECTION SUFFICE???
Answer
# 4
RESTRICTED EARTH FAULT PROTECTION IS GENERALLY PROTECT FROM
THE DEFFERENTIAL CURRENT WITH IN THE GENERATOR CORE , WHERE
AS DIFFERENTIAL PROTECTION IS USED FOR OVERALL DIFFERENTIAL
 
Is This Answer Correct ?    5 Yes 23 No
Erharisheee51
[Greenesol Power Systems]
 
  Re: WHY IS RESTRICTED EARTH FUALT PROTECTION NEEDED?? WHY CANT DEIFFERENTIAL PROTECTION SUFFICE???
Answer
# 5
Restricted earth fault protection is used for zonal
protection for transformer, generator etc. For earth fault
protection we set 10% or 20% earth fault current hence we
leave 10-20% of winding unprotected. By using REF we provide
protection for that remaining portion of winding.
 
Is This Answer Correct ?    12 Yes 4 No
Amit
 
  Re: WHY IS RESTRICTED EARTH FUALT PROTECTION NEEDED?? WHY CANT DEIFFERENTIAL PROTECTION SUFFICE???
Answer
# 6
REF Protection- There may be number of feeders down-
stream on the T.F.So if an earth fault occurs in one
of such feeders, that fault should not disturb the
transformer.If the transformer is not protected with REF
what happens is, everytime there is a fault some where
on it's feeders,the T/F will trip and all feeders will
be affected. This will go on happening from feeder to
feeder and consumers get in to trouble unnecessarily. But
with REF ,the transformer will trip only when there is
earthfault within T/F itself, which very very seldom
only occurs.In other words T/F tripping restriced to fault
within cases only.But the case of out-going feeders is
not so safe as to avoid earth fault.They may be in
a jungle where tree branches can touch the conductor
and cause a tripping.So T/F with REF will not trip in
such situation and power interruption will be only in
the feeder affected.
 
Is This Answer Correct ?    8 Yes 8 No
Tomson Philip
 
  Re: WHY IS RESTRICTED EARTH FUALT PROTECTION NEEDED?? WHY CANT DEIFFERENTIAL PROTECTION SUFFICE???
Answer
# 7
REF IS USED TO PROTECT THE SECONDARY WINDING (WYE
CONNECTED). IT GIVES SENSITIVE PROTECTION AS COMPARED TO
THE DIFF PROTECTION.
TO PREVENT MAL-OPERATION OF DIFF PROTN (DUE TO CT ERRORS /
DIFF IN SATURATION CONDITTIONS ETC) AS WELL AS THE
MIN.SETTINGS POSSIBLE IN THE RELAY, DIFF PROTN. CANNOT
OFFER SENSITIVE PROTN.
 
Is This Answer Correct ?    4 Yes 2 No
Sanjay
 
  Re: WHY IS RESTRICTED EARTH FUALT PROTECTION NEEDED?? WHY CANT DEIFFERENTIAL PROTECTION SUFFICE???
Answer
# 8
Differential protection operate on difference of primary
and secondary current. Practically load is not balanced and
supply system too and so there will be always current
difference in normal running condition. To avoid
malfunction (during charging also) always differential
protection setting set to protect 70 to 80% zone. 20 to 30%
zone left unprotected with only differential protection.

By using REF, it is possible to protect 20 to 30%
unprotected zone without any maloperation.
 
Is This Answer Correct ?    14 Yes 3 No
Alpesh
 
  Re: WHY IS RESTRICTED EARTH FUALT PROTECTION NEEDED?? WHY CANT DEIFFERENTIAL PROTECTION SUFFICE???
Answer
# 9
Differential protection operate on difference of primary
and secondary current. One must note down that construction
of primary and secondary winding keeps them physically
separated. Any earth fault requires to be cleared faster
than line fault/shortcircuit. Difference of current in
primary and secondary is very less in case of internal
earth fault.

So when any fault happens in the transformer say primary
side bushing, due to high set point of differential which
is used for detecting 20% unbalancing, it will trip in more
time or might not trip. To avoid any immediate damage
within the transformer we put REF to detect and trip it
quicker than differential since it works on difference of
current detected by neutral CT and common of all line CT.
Mostly it is used for Dyn11 since star of secondary is
physically grounded.
 
Is This Answer Correct ?    5 Yes 1 No
Lalit Parashar
 
  Re: WHY IS RESTRICTED EARTH FUALT PROTECTION NEEDED?? WHY CANT DEIFFERENTIAL PROTECTION SUFFICE???
Answer
# 10
Diff relay has to be set allowing for unbalanced load also
Hence, it is set at 15 to 25 % of In, making it not very
sensitive to earth fault.
 
Is This Answer Correct ?    4 Yes 0 No
K.prakashchandra
 

 
 
 
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