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What is Circuit Breaker ?
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Circuit Breaker (CB)
CB is a protective switch which can make the circuit and break the circuit during normal and abnormal conditions respectively by taking trip command or signal from the relay and operates under all conditions (abnormal, full load, no-load).
The above figure shows how the relay and circuit breaker are coordinated. When the current in the circuit is high the CT steps down the current and it energizes the relay operating coil. it pulls the spindle down and thus energizing the trip coil of Circuit Breaker by connecting a DC source across it. This causes the plate of Circuit Breaker to separate from the contacts and open the circuit.
- Current transformer provides electrical isolation between power network and protective system.
- Measurement CT should have saturation current of 2-3 times full load current and power CT should have saturation current of 20-30 times of full load current.
- Protective CT should be operated at ankle point under normal operating condition such that operating point will be shifted to linear region in B-H curve during abnormal condition.
- A retraining force (Fr) has to be produced exactly in opposite direction to (F0) operating force, in order to restrict relay contact during normal operating conditions.
Fr ; produced by spring
F0 ; produced by operating coil
- Resultant force = F0 - Fr
- Electromagnetic force, F0 ∝ i^2 ; F0 ∝ Ⲫ^2
- If Fres > 0 or F0 > Fr , relay contact closed, relay operated and CB contacts will be opened i.e. CB operated.
- If Fres < 0 or F0 < Fr , relay contact are open CB contacts are still in closed position.
- If Fres = 0 , relay is on verge of operation.
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