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What is difference between earth leakage relay and
restricted earth fault relay ?
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Answer Posted / 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.
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