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Chapter 1 of 12 тАв Page 1 of 248ЁЯФТ Protected PDF тАв Watermarked
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The function of protective relay in a circuit breaker is
To each any stray voltages
To close the contacts when the actuating quantity reaches a certain predetermined value
To limit arcing current during the operation of circuit breaker
To provide additional safety in the operation of circuit breaker
To close the contacts when the actuating quantity reaches a certain predetermined value
A protective relay is an intelligent sensing device that continuously monitors power system parameters such as current, voltage, or frequency. When the monitored quantity exceeds a predefined threshold (pickup value), the relay closes its internal contacts to complete the trip circuit, triggering the circuit breaker to isolate the faulty section.
A protective relay is an intelligent sensing device that continuously monitors power system parameters such as current, voltage, or frequency. When the monitored quantity exceeds a predefined threshold (pickup value), the relay closes its internal contacts to complete the trip circuit, triggering the circuit breaker to isolate the faulty section.
IpickupтАЛ=KтЛЕIratedтАЛ тАФ where IpickupтАЛ is the setting at which the relay operates, K is the multiplier, and IratedтАЛ is the nominal current.
The relay acts as the 'brain' of the protection system. It receives inputs from Current Transformers (CTs) or Potential Transformers (PTs). Once the input quantity crosses the set threshold, an electromagnetic or solid-state mechanism operates to close the relay output contacts. This closure energizes the trip coil of the circuit breaker, causing its mechanical contacts to open and disconnect the faulty load.
Protective relays are decision-making devices, whereas circuit breakers are power-interrupting devices.
Relays operate based on predetermined settings, often involving Inverse Definite Minimum Time (IDMT) characteristics.
The relay consumes very little power compared to the main power circuit it protects.
Ensures rapid isolation of faults to prevent equipment damage.
Provides selectivity by only isolating the faulty section of the network.
Requires regular calibration to ensure accuracy.
Prone to maloperation if settings are not coordinated correctly.
Generator protection
Transformer differential protection
Transmission line distance protection
Option A is incorrect as stray voltage mitigation is usually handled by earthing/grounding.
Option C is incorrect because arc quenching is the mechanical/physical function of the circuit breaker's extinguishing chamber, not the relay.
Option D is vague; the primary purpose is specific fault detection.
B is correct тАФ The protective relay senses abnormal conditions and closes its contacts to complete the trip circuit, thereby initiating the disconnection process by the circuit breaker.
Remember the hierarchy: Instrument Transformers (CT/PT) measure the signal, Relays process the signal, and Circuit Breakers execute the power interruption.