Answer / biju poulose

An earth leakage relay will sense and trip the circuit for
any earth leakage fault that may occur any where in the
system. The C.T provided at the neutral bus is used for
sensing this fault.
A restricted earth fault relay will operate only for earth
leakage that occur in a zone which is determined by a group
of C.T's connected antiparallel to the neutral C.T.

Answer / pratheesh

REF detects the earth faults between the transformer
secondary winding and upto the point where REF
protection CT placed. If any earth fault occurs within
this zone, REF relay will sense it.
It will not trip when any faults occurs after the
protected zone(after the REF CTs) as the earth fault
current produced in the secondaries of the REF CTs and
the Neutral Earthing CT is neutralized.

Answer / naren

REF realys are not meant for protecting the CT's.it is used
to protect the equipment from the fault current entering
thru its neutral.

Answer / s. malhotra

Restricted EF relay is meant for sensing earth fault/
leakage current in a "restricted zone", a zone bound by the
2 sets of Current transformers - one set at the beginning of
the zone and one at its end.

There is NO particular zone restriction for the normal Earth
leakage relay which will sense an earth fault or earth
leakage downstream of it. Though you may call the entire
downstream as a zone, in comparison to the REF it is left
unqualified

Answer / velusamy

REF RELAY DETECTS INTERNAL EARTH FAULT OF TRANSFORMER (IN
ZONE PROTECTION) . EARTH LEAKAGE RELAY DETECTS EARTH FAULT
BOTH IN AND OUT ZONE

Answer / prasun bhaumik

REF sense the residual or differential current in between
two sets of CTs installed at two sides of protected unit
i.e Tr or Gen. It is a high imedance differential
protection it will only operate when
1) There is a earth fault inside the object which will
produce sufficient differential current.
2) In case fault occures out side the zone any of the CT
may satuarate then it will mal operate.


But earth leakage relay is conncetd with a CT which is
commonly ring type and surronded the earthing path of the
instrument or cable with armour it operates on Over current
principle not differential principle so there is no
requirement of stabilizing resistance as in the case of REF

Answer / vijay

For protection of Transformers or generators normally
Differential protection is used. If the faults in the
winding are nearer to neutral (say 5% of turns from neutral
earth fault occurred), due to less fault voltage, fault
current will be less which cannot be sensed by differential
protection. Hence a separate protection called Restricted
Earth fault protection is used for detecting the earth
faults near to neutral.


For the Earth faults and faults like above Earth leakage
relay also senses the fault.

There is a difference in construction of Earth leakage relay
and REF.
In REF separate CTs are used for each phase and neutral. all
the four CTs are summed up and if there is difference it
indicates earth fault.

In Earth leakage CT core is common for all the phases i.e
in a circular core all the three wires are passed and acts
as primary. as the core is common all the currents are
summed up(vector addition)and if there is difference it
indicates earth fault.

as the core is common it used only for Low voltage applications.

Answer / ramesh baba

Both Earth Fault Relay & Restricted Earth Fault Relay works
on the same principle, i.e. when fault current flows faulty
section in to the protective relay they will sense the
fault and trips the circuit.
But the main difference between these two relays is,
*** The earth fault relay will protect the system against
any phase to earth fault. But it can't sense the leakage
currents flowing throgh the neutral.
*** For sensing of over currents or leakage currents
flowing throgh the neutral , the restricted earth fault
relay is used.

Answer / sridhar.u

REF
Windings of many smaller transformers are protected by
restricted earth fault (REF) systems. Even so, I have come
across a lot of installations which would have benefited
from this type of protection, but for whatever reason it has
not bee installed.
The illustration shows the principal of REF protection.
Under normal conditions and by application of Kirchhoff’s
laws the sum of currents in both current transformers (CTs)
equals zero. If there is an earth fault between the CTs
then some current will bypass the CT's and the sum of
currents will not be zero. By measuring this current
imbalance faults between the CTs can be easily identified
and quickly cleared. Fault detection is confined to the
zone between the two CTs hence the name 'Restricted Earth
Fault'.
REF protection is fast and can isolate winding faults
extremely quickly, thereby limiting damage and consequent
repair costs. If CTs are located on the transformer
terminals only the winding is protected. However, quite
often the line CT is placed in the distribution switchboard,
thereby extending the protection zone to include the main cable.

EARTH LEAKAGE RELAY
There is NO particular zone restriction for the normal
Earth
leakage relay which will sense an earth fault or earth
leakage downstream of it.

Answer / p c sahu

Under normal conditions and by application of
Kirchhoff’s laws the sum of currents in both current
transformers (CTs) equals zero. If there is an earth
fault between the CTs then some current will bypass the
CT's and the sum of currents will not be zero. By
measuring this current imbalance faults between the CTs can
be easily identified and quickly cleared. Fault detection
is confined to the zone between the two CTs hence the
name 'Restricted Earth Fault'
An earth leakage relay will sense and trip the circuit for
any earth leakage fault that may occur any where in the
system. The C.T provided at the neutral bus is used for
sensing this fault

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