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Purpose of Directional inverse Over current & earth fault relay; type- CDD.

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Directional phase or earth fault protection of ring-mains, parallel transformers, transformer feeders, parallel feeders etc., employing the time graded principle.; This relay comprises an inductive disc over current unit with wound shading coils and a directional high speed induction cup unit. The cup-unit contact is wired across the shading coils so that no torque is exerted on the disc of the over current unit until the cup unit contact closes. The inductive disc unit is thus directionally controlled and it operates only when the current flows in the tripping direction. The directional unit is a high speed, low inertia four pole induction cup movement designed to give a high, steady and non-vibrating torque. its current coil is connected in series with the operating coil of the induction disc unit. The directional unit is normally provided with voltage polarising coils.

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