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