Answer / shashi suman

OVERLOAD RELAY WORKS ON THERMAL PRINCIPLE AS IT CONSIST OF
BIMETALLIC STRIP INSIDE.WHEN THERE IS PHASE FAILURE CURRENT
OF TWO PHASE WILL PASS THRU THE THIRD PHASE WHICH IN TURN
WILL CARRY TWICE THE CURRENT OF HEALTHY PHASE.IF THE MOTOR
IS UNDERLOADED THEN EVEN TWICE OF THE CURRENT OF HEALTHY
PHASE WILL BE LESS THAN OVERLAOD RELAY RATING.AND SO TH
MOTOR WILL CONTINUE TO RUN IN CASE OF PHASE
FAILURE.THEREFORE FOR IMMEDIATE TRIPPING OF MOTOR PHASE
FAILURE RELAY IS PROVIDED WHICH MAY BE BASED ON CURRENT OR
VOLTAGE.

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